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Instruction Manual

Audiology Clinic V2

By Parrot Software

Instruction Manual Audiology Clinic V2 Contents i

Contents

Introduction 1 Using The Audiology Clinic, Version 2 ......................................................... 1

Screen Area ....................................................................................... 1 File Menu .......................................................................................... 1 Case Menu ......................................................................................... 2 Test Menu.......................................................................................... 2 Audiogram Menu............................................................................... 4 Auto Test Menu ................................................................................. 4 Immittance Menu............................................................................... 5 Options Menu .................................................................................... 6 Case Menu revisited........................................................................... 7 Controlling the Audiometer with the Keyboard .................................. 7 Summary ........................................................................................... 7

Chapter One 8 Audiologic Screening .................................................................................... 8

Procedure........................................................................................... 8 Additional Considerations.................................................................. 8 Practice .............................................................................................. 9 Technique .......................................................................................... 9 Summary ........................................................................................... 9

Chapter Two 10 Air And Bone Conduction Threshold Audiometry ........................................10

Auditory Thresholds .........................................................................11 Clinical Audiometry..........................................................................11 Earphones .........................................................................................11 Threshold Testing Procedure.............................................................11 Modified Hughson-Westlake Method................................................12 Automatic Testing.............................................................................12 Interpreting Automatic Tests .............................................................13 Response Chart .................................................................................13 Manual Testing .................................................................................13 The Audiogram.................................................................................14 Practice .............................................................................................14 Audiogram Interpretation..................................................................14 Threshold Considerations..................................................................15

ii Contents Instruction Manual Audiology Clinic V2

Maximum Intensity Limits ................................................................15 Summary ..........................................................................................16

Chapter Three 16 Masking Air Conduction Thresholds ............................................................16

Crossover..........................................................................................16 Masking............................................................................................17 Three Masking Rules ........................................................................17 Example Of Crossover ......................................................................17 Synopsis Of Rule One.......................................................................18 Masking Strategies............................................................................19 The Plateau Method ..........................................................................19 Automatic Testing.............................................................................20 Establishing The Plateau ...................................................................20 Interpretation Of Results ...................................................................22 Four Parts Of The Masking Curve....................................................22 Determining The Real Threshold ......................................................23 Overmasking.....................................................................................23 Definition Of A Plateau ....................................................................24 Limitations To Plateau Definitions....................................................25 Summary ..........................................................................................26

Chapter Four 26 Masking Bone Conduction Thresholds .........................................................26

Interaural Attenuation By Bone Conduction......................................26 Air-Bone Gap ...................................................................................27 Occlusion Effect ...............................................................................27 Air-Bone Gaps In Only One Ear .......................................................28 Practice .............................................................................................29 Bilateral Air-Bone Gaps....................................................................29 The Masking Dilemma......................................................................30 Summary ..........................................................................................31

Chapter Five 32 Re-examining Air Conduction Thresholds ....................................................32

Retesting Only One Ear ....................................................................32 Practice .............................................................................................33 Disparity Between Air And Bone Thresholds....................................33 Realistic Clinical Responses..............................................................34 Summary ..........................................................................................35

Chapter Six 35 Variability In Listener Responses .................................................................35

Practice .............................................................................................36 Summary ..........................................................................................36

Instruction Manual Audiology Clinic V2 Contents iii

Chapter Seven 36 Aural Acoustic Immittance ...........................................................................36

Impedance Basics .............................................................................37 Resistance.........................................................................................37 Reactance..........................................................................................37 The Problem Of Timing ....................................................................37 Acoustic Admittance.........................................................................38 Static Admittance..............................................................................39 Summary ..........................................................................................40

Chapter Eight 40 Tympanometry .............................................................................................40

Tympanometry Procedure .................................................................41 Tympanometric Norms .....................................................................42 Tympanogram Classification.............................................................43 Tympanometric Screening For Middle Ear Disorders........................45 Summary ..........................................................................................45

Chapter Nine 45 Acoustic Reflex ............................................................................................45

Ipsilateral Stapedial Reflex ...............................................................46 Contralateral Stapedial Reflex...........................................................47 Normative Stapedial Reflex Behavior ...............................................47 Clinical Patterns................................................................................48 Summary ..........................................................................................55

Chapter Ten 55 Speech Audiometry ......................................................................................55

Speech Recognition Threshold ..........................................................55 Word recognition ..............................................................................56 Recorded vs. Live Voice Presentation ...............................................56 Descending Threshold Protocol.........................................................56 Calibration of the Speech Signal .......................................................57 Masking Speech Thresholds..............................................................57 Practice .............................................................................................58 Summary ..........................................................................................58

Index 59

Instruction Manual Audiology Clinic V2 Introduction 1

Introduction

Using The Audiology Clinic, Version 2 Did you ever hear of a computer program that wasn’t user-friendly and easy to use? On the other hand, did you ever try one the first time that was? The point is that most computer programs are not simple and intuitive until after you have learned how to use them. The objective of this chapter is to get you over the hurdle of learning a new program so that you can use it readily to simulate the tests used in audiology to assess hearing. It is assumed, incidentally, that you know how to manipulate Windows programs. If not, it would be advisable to get some help. You will find installation instructions at the end of the book, just before the Index.

Screen Area Version 2 of The Audiology Clinic automatically detects the screen area of your computer monitor. The numbers that are displayed represent how many pixels your screen can show horizontally and vertically. The larger the set of numbers, the more “things” can be shown on the screen. Three settings are checked for: 1) 640x480, the setting used by Version 1, 2) 800x600, and 3) 1024x768. To simplify, we will call the 1024x768 screen area high resolution, the 800x600 screen area medium resolution, and the 640x480 screen area low resolution. You can learn the screen area of your computer by starting The Audiology Clinic and clicking the Options menu. The last item on that menu indicates the current screen area of your monitor. Ideally, your computer should have high resolution because this permits most

2 Introduction Instruction Manual Audiology Clinic V2

windows used in The Audiology Clinic to be displayed on the screen simultaneously without any overlapping. Furthermore, if your computer is set to high resolution, the program can be run on a portion of the screen so that it looks exactly like Version 1 by clicking Options and then Run in Window. This cannot be done with low or medium resolution. Using Run in Window permits the remainder of the screen to be used for other programs. It is possible to switch back and forth between full-screen and partial screen by clicking Run in Window on the Options menu. If a case is currently displayed, it must be closed (on the menu click Case, then Close) before the display can be changed. Remember that high resolution is the recommended setting because more windows are visible at one time. The screen area setting can be changed on most computers, so you may want to experiment with the different settings to ascertain the one that is best for you. If you are working in a lab, check with the lab supervisor first. However, if you are using your own computer, the screen area can be adjusted by clicking Start, Settings, and Control Panel. Then double click the Display icon. In that window click the Settings tab. Finally, drag the arrow in the Screen Area box to the desired setting. Finally, click OK. If you have made a change, a second window will appear, so click OK again. The screen area setting can be changed on most computers, so you may want to experiment with the different settings to ascertain the one that is best for you. If you are working in a lab, check with the lab supervisor first. However, if you are using your own computer, the screen area can be adjusted by clicking Start, Settings, and Control Panel. Then double click the Display icon. In that window click the Settings tab. Finally, drag the arrow in the Screen Area box to the desired setting. Finally, click OK. If you have made a change, a second window will appear, so click OK again.

File Menu After starting The Audiology Clinic, the first step is to open a data file. Data files come in two varieties: the Standard data file and Extra data files. The former contains all the cases discussed in this text/manual, while the latter enable you to access additional cases that your instructor may want you to test. Extra data files are not accessible from the Lite edition of the program.


Figure 1-1. File menu. To open a data file, on the menu bar click File, and a submenu will appear. Pick the type of file you want on this submenu (see Figure 1-1).

Case Menu The Case menu now becomes available, so the next step is to select the Case you want to test. On the menu bar click Case, followed by Select Case on the submenu, as revealed in Figure 1-2.

Figure 1-2. Case menu This action will open the Case Selector window (Figure 1-3). You may type in the number of the case you wish to test if you know it. Alternatively, you may click the down arrow to reveal a list of all available case numbers, using the scroll bar to move among the choices. Click the case desired. Finally, click OK.

Figure 1-3. Case selector.

Test Menu


The last step is to select the type of test you want to conduct. The Test item on the menu has now become enabled. Click Test to see the submenu, which is shown in Figure 1-4.

Figure 1-4. Test menu. If your immediate objective is to begin pure tone or speech testing, then click Pure tone audiometry; however if you want to do immittance first, then click Immittance. Selecting one type of testing protocol does not prevent your performing the other test because you may switch at any time. Just return to the Test menu and click on the other name to change testing modes. If you choose Pure tone audiometry, your screen will look like Figure 1-5 on a monitor that has high resolution. Notice that there are five parts to the display, which are identified in the figure below.

Figure 1-5. Screen display for high resolution. On the other hand, if you are using the program with low or medium resolution or have selected the Run in Window option, then the screen will appear like the representation shown in Figure 1-6. Notice that the screen is divided into three major parts. On the left the listener is in the upper window, and the audiometer is in the lower window. On the right is the audiogram. Behind the audiogram is the immittance form, which also reveals the case history.


Figure 1-6. Screen for pure-tone audiometry. On the other hand, if your choice on the Test menu was Immittance, your screen will look like the picture below (Figure 1-7). You can see that the immittance instrument is at the left and the immittance and case history form at the right with the audiogram behind it.

Figure 1-7. Screen for immittance. It is important to understand that to switch back and forth between these two forms click on the title bar of the one that is underneath. Refer to Figure 1-8. In this instance the audiogram is active, and the immittance/history form is mostly hidden underneath. Only its title bar is visible above the audiogram. To make the entire immittance/history form visible, click on its title bar. The immittance/history form then moves to the front, and the audiogram goes behind. To view the whole audiogram again, click on its title bar.


Figure 1-8. Audiogram and immittance title bars. Remember that you can easily alternate testing modes between audiometry and immittance. Simply click Test and then click the type of test desired. For instance, if you are doing pure tone testing and you want to switch to immittance, click Test and then click Immittance.

Audiogram Menu Assume that from the Test menu you picked Pure tone audiometry. This activates the Audiogram menu and affords the choices presented in Figure 1-9 below. Most importantly, you can plot the audiogram of the person represented by the current case. This grants you the opportunity to check your results against the correct results. Recognize, however, that it is possible for your instructor to revoke this privilege. Another choice on the Audiogram menu is Erase. This will delete all pure tone thresholds and speech and immittance results. After an audiogram is plotted, it may be printed by clicking Print. The final choice on the Audiogram menu is Symbols. Clicking this opens a window that reveals the explanation of the symbols used on the audiogram.

Figure 1-9. Audiogram menu.

Auto Test Menu Again assume that from the Test menu you selected Pure tone audiometry. Observe that Auto Test has become available on the menu. It offers five alternatives (Figure 1-10) A primary feature of The Audiology Clinic is its capacity to find pure tone unmasked and


masked thresholds automatically. The first item on the submenu, Modified H-W, measures the unmasked pure tone threshold at the current audiometer setting (Left or Right ear, Air or Bone conduction, at the selected frequency). Clicking this item will display a new window, the Response Chart. This chart will appear on top of the Immittance instrument, if operating in high resolution mode (Figure 1-5) or on top of the Audiogram and Immittance windows at the right of the screen, if operating in low or medium resolution mode or Run in Window (Figure 1-6).

Figure 1-10. Auto test menu. It should be emphasized that during auto testing using low resolution or Run in Window mode there are three overlapping windows on the right half of the screen. It is possible to view any one of them in its entirety by clicking on its title bar. This action will bring that window to the front and place it on top of the others. The unmasked threshold can be obtained manually by repeatedly clicking the Next button on the Response Chart or automatically by setting the Speed to a value between 1 and 9 by clicking the arrows above the Speed label. Setting 1 is the slowest and 9 is the fastest. The process can be interrupted at any time or reset when completed by clicking the Reset button. These controls are revealed in Figure 1-11.

Figure 1-11. Top part of Response Chart.


The masked threshold can be acquired by choosing Standard masking or Complete masking from the Auto Test menu. These procedures are described in Chapter Three. The Complete masking operation reveals the masking curve at all masking levels and is intended to illustrate theoretical concepts useful for learning purposes. It is not the method that would be used clinically. Normal clinical protocol is illustrated by the Standard masking selection. When you are performing tests or having the computer do automatic Standard masking or Complete masking, the Crossover diagram option becomes available (for bone conduction testing the non-test ear must be occluded - see Chapter 4). Choosing this menu item opens a window on the screen that depicts a schematic of the head showing the amount of hearing loss in the conductive mechanism, in the sensorineural mechanism, and the total loss for each ear. It also reveals the signal and masking levels at each ear and any crossover that may be occurring. Using this diagram permits thorough examination of the variables involved in obtaining correct thresholds when masking. The Crossover window is depicted in Figure 1.12.

Figure 1-12. Crossover diagram. Lastly, it is possible to click Close auto test and remove the Response or Masking chart and the Crossover diagram from the screen.

Immittance Menu The Immittance menu item is accessible only when there is not an active case. To close the current case, click the Case menu and then click Close Case. Now Immittance can be chosen. Clicking this item opens the submenu displayed in Figure 1-13 below.


Figure 1-13. Immittance menu. The Probe tip presents an animation of tympanometry in the normal and pathological ear. The Ipsilateral reflex arc and the Contralateral reflex arc demonstrate the neural pathways involved when eliciting acoustic reflexes. Impedance opens a submenu that demonstrates instances of the mathematics involved in calculating acoustic impedance. Finally, Examples accesses illustrations referred to in Chapter 9.

Options Menu Last of all, the Options menu item provides nine alternatives. These are shown in Figure 1-14 on the next page. All choices are visible in this figure, although when the program is running, not all choices are available at all times. These operations are described below.

Figure 1-14. Options menu.


1.Transducer allows you to switch between standard circumaural earphones and the newer insert earphones (unless your instructor has authorized only one kind of earphone to be used). This choice must be made after choosing Pure tone audiometry as the type of test but before any further action is taken. Otherwise said, the kind of earphones in use cannot be changed once a test is begun (by clicking the mouse or pressing any key on the keyboard). 2.Audiometer lets you pick either a clinical or a portable audiometer to test with. This item is only available when there is no active case. Close the current case to obtain access to this option. 3.Listener enables you to opt to test a particular listener; otherwise one of four listeners is selected randomly when a case number is picked. 4.Non-test ear occluded prepares for the measurement of masked bone conduction thresholds by placing a circumaural earphone on or an insert earphone in the non-test ear (Chapter 4). 5.Calibrate speech is only available when Speech has been selected as the mode on the audiometer. It opens a window with a pointer that lets you calibrate the level of the speech signal prior to doing speech audiometry (Chapter 10). 6.Sound on determines whether the program produces sound. If your computer has a sound card, then the Sound on item has a check mark before it, and the program produces sound. Sound output can be defeated, however, by clicking this item to remove the check mark. This may be preferable in a laboratory. If there is not a sound card in the computer then this item is unchecked and cannot be selected, and there is, of course, no sound output. 7.Show privileges informs you whether for the current case you can view the correct audiogram, the immittance results, the speech results, and the case history It also notifies you whether you can do the three types of auto testing. These privileges can be controlled by your instructor. When any of these privileges is denied, it will not be available for selection on the menu. 8.Run in Window permits you to run the program on a reduced portion of the entire screen, if your computer monitor has high resolution as discussed above. 9.Screen area discloses the current screen area setting of your computer monitor.

Case Menu revisited


When the Case menu was presented above, two items were not discussed: Record results and Save case(s). These two items are not accessible from the Lite edition of the program. For evaluative purposes your instructor may require you to test one or more cases and submit the results to him or her. In order to record the results of your testing procedures, click the Case menu followed by Record results before selecting the case to be evaluated. After you have finished all the tests you want to administer, click Case again, followed by Save case(s), select the disk you want to save the case(s) on, and name the file according to instructions given to you.

Controlling the Audiometer with the Keyboard When performing pure tone audiometry (Chapters One through Six), the audiometer can be controlled with the mouse or with the keyboard (your preference). When using the keyboard, the function of the relevant keys is shown in the table below. These keys are consistent with the older Pure Tone Simulation program. The audiometer window must be the active window to use the keyboard to control the audiometer (i.e., the title bar that says “Audiometer” must be blue). If the audiometer window is not the active window, click anywhere on the window with the mouse to activate it. Note: if the audiometer window cannot be activated, then the program is in a mode, such as Auto Test, which does not permit the audiometer to be activated. Exit that mode first, then click on the audiometer window.

Function Key Change Output: Left - Right HOME Change Mode: Air - Bone - Speech END Intensity - increase UP ARROW Intensity - decrease DOWN ARROW Frequency - increase RIGHT ARROW Frequency - decrease LEFT ARROW Present signal SPACE BAR Plot threshold P Erase audiogram & immittance E

Summary

12 Chapter One Instruction Manual Audiology Clinic V2

An overview of the operation of The Audiology Clinic has been presented by explaining the menu choices. Practice manipulating the features of the program until you become comfortable with it, and refer back to this chapter when needed.

Chapter One

Audiologic Screening In this chapter we will discuss the techniques used in audiologic screening using the pure tone audiometer. The procedures to be used are described by the Guidelines for Audiologic Screening published by the American Speech-Language-Hearing Association (ASHA) in 1997. “The purpose of screening is to detect, among apparently healthy persons, those individuals who demonstrate a greater probability for having a disease or condition, so that they may be referred for further evaluation” (ASHA, 1997, p. 6). These guidelines are extensive, and only the specific details that relate to pure-tone audiometry will be presented here. If you are administering a screening program, it is strongly suggested that you read these guidelines and become thoroughly familiar with their content.

Procedure The recommended screening procedure differs somewhat depending on the goal of the screening and the age group being screened. We will concern ourselves with screening for hearing impairment in three age groups: 1) 3-5 yrs., 2) 5-18 yrs., and 3) adults. In all cases the frequencies to be used are: 1000, 2000, and 4000 Hz.

Instruction Manual Audiology Clinic V2 Chapter One 13

1.The first group, ages 3 - 5 yrs., is screened at 20 dB HL. Each stimulus is to be presented at least twice at each frequency in each ear, and the criterion for passing is to respond to at least 2/3 of the presentations. 2.The second group comprises school-aged children between 5 yrs. and 18 yrs. This group is also to be screened at 20 dB HL. To pass they must respond to all the signals; otherwise they are to be rescreened after repositioning the earphones and being reinstructed. 3.The third group is comprised of adults. They are to be screened at 25 dB HL and must hear all the signals to pass the screening. Another group, 7 mo. - 2 yrs., is included in the guidelines, but screening this group involves specialized techniques. Consequently, the details will not be covered here. The guidelines provide actions to be taken, if a listener fails the screening. These differ depending on the age group. Of interest here is simply whether the listener passes or fails the screening test.

Additional Considerations Other details of the screening procedure will be mentioned only briefly. As always in audiometry the instructions to the listener are of the utmost importance. They should be as concise as possible without causing the person being tested to be confused due to insufficient information about the task expected of them. Although multiple presentations of the signal can be delivered to the listener, under no condition should the tone be presented repeatedly until the listener responds. The criterion for the 3 - 5 yrs. group serves as a good rule of thumb: present the tone at each frequency a maximum of three times and consider two or more responses as passing. In addition, as with all audiometric testing, a sufficiently quiet acoustic environment and a properly calibrated audiometer are of paramount importance. A checklist of the steps is provided in the box below.

Operational Checklist

Preparation: 1. Adjust the Intensity to 20 or 25 dB depending on the age group. 2. Confirm that the Frequency is 1000 Hz. 3. Set the Output to Right, the Mode to Air, and be sure the Masking is

off (0 dB).

14 Chapter One Instruction Manual Audiology Clinic V2

Testing: 1. Present the signal for approximately one second and observe

whether the listener heard the signal. 2. Continue by offering a second, and if necessary a third, presentation;

note whether there was a response. 3. Decide whether the signal was heard on a majority of the

presentations. 4. Repeat at other frequencies in current ear and in other ear.

Practice If you haven’t already read the Introduction to find out how to use The Audiology Clinic, be sure to do that now. Screening audiometry is typically done with a portable audiometer. The type of audiometer you want to use must be selected before a case is chosen. Immediately after starting the program click Options followed by Audiometer; finally click Portable. (If there is a case currently being tested, click Case, then Close case before going to the Options menu as previously stated.) Let’s begin with Case No. 1, so select that case. Convention dictates that you always start at 1000 Hz in the right ear, unless you have reason to do otherwise. In general, it is advisable to begin the test in the ear that is known (or believed) to have the better hearing sensitivity. The steps in the ASHA guidelines for screening are summarized in the following panel. Let’s assume that this listener is an adult; therefore the proper intensity to use is 25 dB. Now begin to screen the hearing of this listener (i.e., test at 1000 Hz). You will discover that the listener heard the 1000 Hz tone in the right ear. Now change the frequency to 2000 Hz and present the tone; once more the listener hears the tone. Continue by switching to 4000 Hz and deliver the signal. At this point you should have presented three tones and noted three responses. Next switch to the opposite ear, the left ear, and continue the test. Don't forget to reset the frequency to the starting setting: 1000 Hz. Complete the test (i.e., present tones at 2000 and 4000 Hz). You have now completed the screening text on the first listener. Note that he or she passed the test because a hand was raised after each of the six tones was presented. To continue, choose Case on the Menu, followed by Select Case, and pick Listener No. 2, another adult. Employ the same technique that you have just used to test this listener. The listener associated with Case 2, as you will discover, fails the screening test because he or she did not respond to all three tones in

Instruction Manual Audiology Clinic V2 Chapter One 15

the left ear. There are a total of ten listeners to be screened (5 in the Lite edition), so continue with Case 3. Assume that Case 3, 4, and 5 are ages 18 or younger, so set the intensity to 20 dB HL. Consider the remainder to be adults, so reset the intensity to 25 dB HL.

Technique There are two aspects of a clinician’s technique that are important to consider at all times. First, it is essential to present the signal to the listener with a duration that is sufficiently long for the listener to perceive it. The length of a tonal signal should be approximately one second. Durations shorter than one second may not be perceived, while signals longer than one second do not improve detection and are inefficient time-wise. The listeners in The Audiology Clinic will not respond to very short stimuli regardless of the intensity. The second factor is the temporal pattern with which the signals are presented. It is necessary to vary the interstimulus interval and not to present the signals with a fixed, predictable pattern. In other words vary the amount of time between one signal presentation and the next. Otherwise, the listener will consciously or unconsciously anticipate the next signal presentation and respond accordingly, since reacting to very soft signals often involves making a guess.

Summary This chapter has described pure tone screening tests for different age groups using procedures defined in the ASHA guidelines. In the next chapter the topic of air conduction threshold tests will be explained.

16 Chapter Two Instruction Manual Audiology Clinic V2

Chapter Two

Air And Bone Conduction Threshold Audiometry The purpose of this chapter is to master the technique of obtaining hearing thresholds by both air conduction and bone conduction. Furthermore, you will learn to use several new features of The Audiology Clinic. Let us begin by quickly reviewing the reasons for measuring hearing thresholds by both air and bone conduction. The objectives are primarily to quantify the individual's sensitivity at each frequency and secondarily to specify the locus of the hearing impairment. Testing performed with the earphones directs sound waves into the ear canal toward the middle ear, thus the term air conduction; e.g., the sound waves are conducted through the air to the eardrum. In contrast, testing done with the bone conduction vibrator placed behind the pinna (or, much less frequently, on the forehead) vibrates the temporal bone and stimulates the cochlea directly; this testing mode is called bone conduction. Signals presented by air conduction (AC) must pass through the entire auditory mechanism: first the outer ear, then the middle ear, next the inner ear, and finally along the auditory nerve. Air conduction testing, therefore, indicates how much hearing loss an individual has. This total loss may be comprised of abnormalities in any or all of the four sections of the auditory mechanism. On the other hand, signals introduced through the bone conduction (BC) vibrator bypass the outer and middle ears and directly quantify the amount of the sensorineural deficit; so the bone conduction thresholds unambiguously reveal the amount of loss in sensitivity due to impairment of the inner ear and/or the auditory nerve. Together the air conduction threshold and the bone conduction threshold determine what kind of hearing impairment a person has. The establishment of the kind of hearing loss is straightforward. As already stated, the bone conduction thresholds directly indicate the

Instruction Manual Audiology Clinic V2 Chapter Two 17

amount of sensorineural loss (inner ear and/or auditory nerve). Ascertaining the amount of conductive loss (outer and/or middle ear) necessitates subtracting the bone conduction thresholds (sensorineural loss) from the air conduction thresholds (total loss). To illustrate these ideas, let us consider three different people. 1.Assume that person "A" has air and bone conduction thresholds of 50 dB HL in both ears at all frequencies. Since the air and bone conduction thresholds are equivalent, this person has a sensorineural deficit. There is no conductive involvement because AC (50) - BC (50) = 0. 2.Now let’s assume that person "B" has the following thresholds in both ears at all frequencies: air conduction = 50 dB HL and bone conduction = 0 dB HL. Clearly, this individual has no sensorineural involvement because there is no reduction in sensitivity for bone conduction stimuli; however he or she does have a 50 dB conductive loss because AC (50) - BC (0) = 50. 3.Lastly, let’s suppose that person "C" has air conduction thresholds equal to 50 dB HL and bone conduction thresholds equal to 30 dB HL. This individual has a mixed hearing loss, which is to say that part of the loss is conductive and part is sensorineural. The magnitude of the conductive component is 20 dB, as AC (50) - BC (30) = 20, while the degree of the sensorineural component is 30 dB, because the bone conduction thresholds were obtained at 30 dB HL. Be sure to avoid confusing the terms: air conduction and conductive. Air conduction refers to a type of testing, namely presenting stimuli through earphones, while the word conductive implies a kind of hearing impairment, that is one involving the outer and/or the middle ear. It is not advisable to refer to an "air conduction loss"; instead refer to a "conductive loss" (vs. a "sensorineural loss").

Auditory Thresholds As was the case with Audiologic Screening, discussed in Chapter 1, guidelines for pure-tone threshold audiometry have been published by ASHA (ASHA, 1978). You are urged to become thoroughly familiar with them. There has been much discussion in the scientific literature, especially in sensory psychology, regarding the definition of auditory "threshold". In audiometry we are primarily concerned with the measurement of absolute thresholds (Humes, 1994), or the softest intensity an individual can hear 50% or more of the time. We shall use an operational definition that has been published by ASHA to quantify this threshold. Simply stated, a threshold is the lowest intensity tone


that can be heard on three (usually nonconsecutive) presentations. The thresholds for tones across the frequency range from 125 to 8000 Hz for a listener with perfectly normal hearing would be 0 dB HL. 0 dB HL thresholds are analogous to the commonly used term 20/20, when referring to vision. At the opposite end of the intensity continuum is the threshold of feeling. The extremely loud sounds that elicit this sensation are very unpleasant to the listener, and the audiometer cannot deliver such excessively intense stimuli to most people. The maximum output of most portable and clinical audiometers through the earphones is 110 dB HL in the mid-frequencies (500-6000 Hz). Tones of this intensity usually will not elicit the sensation of feeling, but they will be uncomfortably loud for individuals with recruitment. Consequently, the clinician must always be cautious when presenting stimuli at or near maximum intensity so as not to cause distress to the listener.

Clinical Audiometry Clinical audiometry involves both air conduction and bone conduction testing. The frequencies used for air conduction measurements include the octave frequencies 125-8000 Hz, in other words, 125, 250, 500, 1000, 2000, 4000, and 8000 Hz. The intraoctave frequencies, 750, 1500, 3000, and 6000 Hz, need only be tested when there is a 20 dB or greater difference between any adjacent octave frequencies. For example, according to this provision, if you got a threshold of 40 dB HL in the right ear at 4000 Hz and a threshold of 70 dB HL in the same ear at 8000 Hz, then you should subsequently obtain a threshold in that ear at 6000 Hz because the difference in the thresholds at the adjacent octave frequencies is 30 dB. The lowest frequency, 125 Hz, need be tested only when there is a low frequency hearing loss. Because the vast majority of hearing impairments are high frequency, it is usually unnecessary to test at 125 Hz. Bone conduction thresholds are typically measured at the octave intervals between 250 and 4000 Hz, that is at 250, 500, 1000, 2000, and 4000 Hz. But bone conduction thresholds may be assessed at 750, 1500, and 3000 Hz as well, if the difference between the thresholds at adjacent octave frequencies is 20 dB or greater.

Earphones There are two types of earphones commonly used in clinical audiometry. One kind is the older, larger earphone with a rubber cushion that makes contact with and surrounds the pinna. Such


earphones are referred to by several terms. The one we will use is circumaural earphones. In contrast, the smaller, newer type is called the insert earphones. This name is somewhat of a misnomer because it suggests that the earphone is inserted into the ear canal, which it is not. Rather the earphone is external to the ear, from a few millimeters to several centimeters depending on the brand. In either case a narrow tube leads from the actual earphone to an earplug which is inserted into the ear canal to deliver the sound. Insert earphones offer several advantages including ease in fitting and placement because of their small size and light weight, avoidance of closure of the ear canals (called collapsed canals), and most importantly increased interaural attenuation. This last factor is very significant and will be explained in the next chapter. The Audiology Clinic will permit the use of either type of earphone but defaults to the standard, circumaural earphones for the cases described in this text/manual. The results obtained, of course, will be identical except in a few very difficult cases. The intricacies of these cases will be the topics of future chapters. Also in the next chapter you will learn how to select between the two kinds of earphones with this simulation.

Threshold Testing Procedure The ASHA guidelines describe two phases to obtaining each threshold: familiarization of the signal and measurement of the threshold. The motive for this two-stage method is that the pure-tones used for determining hearing thresholds are not common sounds in many people's lives. Furthermore, ascertaining a threshold involves, by definition, the use of a stimulus of very low intensity. Therefore, it is of foremost importance to familiarize the listener with the nature of the signal before presenting it at low levels just below and just above threshold. For the familiarization part of the procedure, the tone is presented initially at 30 dB HL. The choice of this intensity presumes a listener with normal or near normal sensitivity. Because many individuals receiving hearing tests will not have normal thresholds, the following steps are further reported by the guidelines. If there is not a response at 30 dB HL, then increase the intensity to 50 dB HL. If there is still no response, then continue to increase the intensity in 10 dB steps, until there is a response to the tone. The first response to the tone concludes the familiarization phase of the threshold determination. If the maximum output of the audiometer at the frequency being tested is reached, then present the tone three times at this maximum level. If the listener responds to fewer than half of these presentations (i.e., none or


one), the threshold is unmeasurable at the current frequency. On the other hand, if the listener hears more than half of the tones (i.e., two or all three), then his or her threshold is this maximum intensity. Usually the listener will respond at a level less than the maximum intensity, so next the threshold measurement phase of the test commences. Actually, we would label an additional phase called a transition step, as it leads to the measurement of the threshold which begins after the listener is no longer able to hear the signal. In the transition phase, the level of the tone is decreased by 10 dB and presented to the listener. If there is a response, the tone is again decreased by another 10 dB and the signal introduced. This process is repeated until an intensity is reached at which the listener does not respond. If the listener responds to the tone at the lowest intensity the audiometer can produce, which is -10 dB HL, then the tone is again presented at that level. Finally, a third presentation is made. If the listener has responded either two or three times in a row to the -10 dB HL signal, then his or her threshold at the frequency being tested has been obtained at -10 dB HL. Moreover, this person has better-than-normal hearing at that frequency. The more normal situation is for the listener to stop responding to the tones before -10 dB HL is reached. After the first non-response, the actual threshold search (or what the ASHA document calls measurement) begins. The intensity of the tone is increased by 5 dB and presented to the listener. If he or she hears it, a note is made of this level. Initially, you should make a notation (i.e., write it down) of this intensity; after much practice and experience you will learn to make only a mental note of this level. If the listener does not respond to the 5 dB increase in intensity, again raise the signal by 5 dB and present it. Keep increasing the tone in 5 dB steps until the listener hears it. As soon as you obtain a positive response, repeat the above procedure: that is, decrease the intensity of the tone in 10 dB steps until you get a non-response, then increase the tone in 5 dB steps until you do get a response. Each time you get a response after increasing the intensity of the tone, record the level. Threshold is reached as soon as the listener has responded to the tone at the same intensity three times. This procedure is indeed cumbersome to describe in words, but fortunately it lends itself to graphical representation with great ease. Thus, we will very shortly view this process using The Audiology Clinic. To summarize the threshold-measuring process, first increase the intensity of the tone until there is a response, then decrease the level of the tone until there is not a response, then alternately increase and decrease the intensity of the tone until three responses are recorded.


Modified Hughson-Westlake Method The process just portrayed is widely known as the modified Hughson-Westlake procedure, and it was discussed in detail by Carhart and Jerger (1959). This method of measuring thresholds is known in sensory psychology as an "ascending" technique, as responses are recorded only when the signal is being increased in intensity. Unlike strictly ascending methods, however, each different frequency is first presented at a supra-threshold level to familiarize the listener with what the stimulus sounds like. Obtaining thresholds quickly and reliably using the modified Hughson-Westlake technique is the quintessence of pure-tone audiometry. The adept clinician must be able to execute this procedure with great efficiency and expertise. Both air and bone conduction thresholds are obtained using the same procedure. Ordinarily, air conduction testing is done first in both ears. Then the earphones are removed, and the bone conduction vibrator is positioned behind one pinna (and one earphone may be replaced in or on the opposite ear as we shall see in Chapter 4). Bone conduction thresholds are obtained for that ear at all frequencies, and finally, after the bone conduction vibrator is placed behind the other pinna, bone conduction testing takes place in the opposite ear.

Automatic Testing Before attempting to measure some thresholds yourself, let’s watch The Audiology Clinic obtain some. This is done by selecting Modified H-W from the Auto test menu as described in the Introduction. The case we want to examine is Case 11, so select that case now. Furthermore, let’s use the clinical audiometer from now on. A summary of the steps appears below.

Setup checklist: 1. File: Open - Standard data file 2. Options: Audiometer, Clinical 3. Case: Select case (choose Case 11) 4. Test: Pure-tone audiometry (be sure that the Masking is

set to 0 dB) 5. Auto test: Modified H-W 6. Speed: manual Action: 1. Click Next repeatedly until the threshold is obtained;

watch the explanation in the green box at the bottom


Interpreting Automatic Tests Notice that the threshold measuring process was graphed on the Response chart. This chart characterizes the intensities presented and the results of each presentation: an "R" means that the listener responded, and an "N" shows that the listener did not respond. The recurrent presenting of the tone and recording of the response on the graph will continue until the threshold is measured, or until it is found that the listener cannot hear this frequency even at the maximum intensity. The test proceeded in accordance with the rules outlined previously. There are several factors to observe. First the intensity of the tone was adjusted to 30 dB HL. You could observe the numbers change in the intensity window of the audiometer. The dialog at the bottom of the Response Chart explained each step. When the level of the tone reached 30 dB HL, the tone was sounded. The listener, who heard the signal, responded by raising his or her hand. Afterwards, the level of the tone was decreased by 10 dB HL, and presented again. The tone continued to be lowered in 10 dB steps until there was not a response on the part of the listener (at 0 dB). This was because the signal was too soft to hear, in other words, below his or her threshold. Next, the intensity of the tone was increased by 5 dB and presented; the listener heard it and raised his or her hand. And so the test continued until the listener heard the tone at the same level three times. The intensity representing threshold is 5 dB HL. Click Reset and repeat the process until you are able to follow all the steps in the modified Hughson-Westlake procedure. If you wish, change the “Speed” to a number between “1” and “9” to have the entire process completed automatically.

Response Chart When interpreting the response graph after a threshold has been automatically obtained by The Audiology Clinic, keep in mind the three phases of the threshold-measuring procedure that have been defined. First, familiarization (increase the intensity until audible); second, transition (reduce intensity to below audibility); and third, measurement (increase and decrease intensity until the tone is heard three times). As already stated, initially you should keep a written record of the listener's responses. Later, you will be able to keep track of the responses mentally.


Unless contraindicated by the case history information (the object is to test the ear having the better hearing first), air conduction testing generally begins in the right ear at 1000 Hz. Such was the case in the automatic sequence just witnessed. To experience a different threshold being measured, change the output of the audiometer to Left by clicking the Left button on the Output of the audiometer or by pressing the Home key. Again perform an automatic test by executing all the steps outlined above. Watch the screen. You will discover that the threshold in the left ear is 10 dB HL, which reflects slightly reduced sensitivity, but hearing that is still considered to be within the normal range (as will be explained later in this chapter).

Manual Testing Now that you have viewed The Audiology Clinic while it measured hearing thresholds, it is time to try it yourself. A checklist of the steps to complete before starting the test is presented in the box on the next page. Next obtain the air conduction thresholds at the remaining frequencies. Recall that if there are no differences between any two octave frequencies of more than 20 dB, you need not test at the intraoctave frequencies; i.e., 750, 1500, 3000, and 6000 Hz. The usual order is to follow 1000 Hz by 2000 Hz and then proceed to 4000 and 8000 Hz. After that 1000 Hz should be retested to verify reliability. Lastly, measure the thresholds at 250 and 500 Hz. IMPORTANT! You must use a bracketing threshold technique like the modified Hughson-Westlake procedure in order to plot your thresholds. Proceed by obtaining all the air conduction thresholds for the right ear followed by the left ear. You can verify your results by having The Audiology Clinic show the correct results. If you are using high resolution, click Audiogram followed by Show results. For low or medium resolution or Run in Window the Response Chart (if visible) mostly covers the Audiogram, so click on the title bar of the Audiogram, which will enable the Audiogram menu. Now click Audiogram followed by Show results. An alternative procedure would have been to remove the Response Chart, thus returning the Audiogram to its position on top. This is done by clicking Auto test, then Close auto test. Note: as indicated in Chapter 1, your instructor can revoke your privilege to view any or all of these results.


Operational Checklist 1. Adjust the audiometer to the initial settings. This can be done by clicking the controls

on the audiometer with the mouse or by using the keyboard as indicated in the last chapter.

2. Confirm that the Frequency is 1000 Hz., the Intensity is 30 dB HL, the Output is set to the better ear, or to the Right ear if the better ear is unknown or hearing is believed to be equivalent.

3. Present the signal for about one second by clicking the Present Signal button or pressing the Space bar on the keyboard.

4. Observe whether the Listener responds. A response is indicated by a hand-raise and the Response light at the top of the audiometer glows red.

5. Adjust the intensity and go to Step 3.

Normally, all frequencies are tested by air conduction in both ears before removing the earphones and placing the bone conduction vibrator, so next measure the bone conduction thresholds. Reset the ear to Right, as the right ear is usually tested first unless contraindicated. Finally, set the output to Left and get the left bone conduction thresholds.

The Audiogram The thresholds obtained from threshold audiometry in the clinic are recorded one at a time on the audiogram. To do this, you can use the audiogram utilized at your clinic or office, or you can employ the audiogram feature of The Audiology Clinic. To use the on-screen audiogram, first find the correct threshold using the modified Hughson-Westlake technique. Then click on the plot symbol (shown below) on the audiometer or press P on the keyboard. "P" stands for "plot", and your just-measured threshold will be plotted on the audiogram, using the correct symbol for the ear you are testing.

Practice


Ten cases are affiliated with this chapter (5 in the Lite edition), each displaying a different configuration of hearing. Practice by measuring the air conduction and bone conduction thresholds of each of the remaining cases (Cases 12 - 20). The correct results for all of the audiograms can be found by plotting the audiogram (Click Audiogram, then Show results). Of course these audiograms should only be examined after you have acquired your own results. For additional practice return to Chapter 1 and find the thresholds for Cases 1 - 10.

Audiogram Interpretation There are two aspects to the specification of every hearing loss: 1) what kind of loss, and 2) how much loss. The first of these was discussed at the beginning of this chapter. The second consideration, how much loss, is determined from the air conduction thresholds, which directly reflect the total impairment. Many audiologists describe categories of hearing, such as: hearing within normal limits, slight loss, mild loss, moderate loss, moderately-severe loss, severe loss, profound loss, and no measurable hearing. These categories, listed in Table 2-1 below, are discussed in an article by Goodman (1965) and were modified by Clarke (1981). For instance, if an individual had thresholds of 35, 40, and 50 dB HL at 500, 1000, and 2000 Hz respectively in the left ear, then the average threshold across these three frequencies would be 42 dB HL. According to Table 2-1, this person's loss would be defined as "moderate". Using these same words to describe every individual's hearing sensitivity may an oversimplification as different people with the same numeric thresholds have very different handicaps resulting from their hearing losses. A parallel situation might be to report one's visual acuity as a "little" nearsighted. As a result using a phrase, like "severe loss", must always be considered carefully. Nevertheless, many clinicians prefer to use a single word to summarize hearing test results, rather than merely reporting a set of numbers.

Table 2-1. Descriptive terms for hearing loss categories and the associated range of thresholds (after Goodman, 1965, and Clarke, 1981). Descriptive Term Average of Hearing Thresholds

at 500, 1000, and 2000 Hz Normal Limits -10 to 15 dB Slight Loss 16 to 25 dB Mild Loss 26 to 40 dB Moderate Loss 41 to 55 dB Moderately-Severe Loss 56 to 70 dB Severe Loss 71 to 90 dB


Profound Loss 91 to 110 dB

Threshold Considerations Simulation is useful for many purposes, but seldom includes all the conditions that can occur in real life. Relating this to obtaining auditory thresholds, there are several factors that you should be aware of as influencing the validity of the test. Instructions. The listener should be directed to respond to the faintest sounds detectable; furthermore, he or she should be told to respond immediately to each signal heard by raising a hand and/or pressing the signal button and to cease the response (i.e., lower the hand/release the button) as soon as the tone has ended. Transducer Placement. The proper positioning of the earphones and bone conduction vibrator is critical to a successful test. Especial care must be taken to prevent the circumaural earphones from collapsing the ear canals, and the vibrator should not touch the pinna. These and other significant considerations relating to the finding of pure-tone thresholds are discussed in considerable detail by Yantis (1994).

Maximum Intensity Limits As you have seen, there are two types of audiometers available for use with The Audiology Clinic. The default represents a clinical audiometer. This type of instrument is permanently installed and generally contains numerous features that vary among manufacturers but permit an extensive range of tests to be performed, many of which are not incorporated into this simulation. The other instrument is a portable audiometer. As the name suggests this kind of instrument can easily be transported such as from school to school, for example. The portable audiometer has fewer features than the clinical audiometer. The portable audiometer in The Audiology Clinic cannot do speech tests. In addition, it incorporates an undesirable feature often found in portable audiometers. To change audiometers, there cannot be a case in progress, so terminate the current case by clicking Case, followed by Close Case. Now click Options, followed by Audiometer and click Portable. Finally, reselect a case to test. Lastly click Test, then Pure-tone audiometry.


Look at the audiometer. A noticeable difference is the little box labeled "Max dB" in the middle of the audiometer. Due to the differential sensitivity of the human ear to sounds of varying frequencies, audiometers are incapable of supplying equivalent maximum intensities (dB HL) at all frequencies. The reason is that the decibel scale used on audiometers (that is, dB HL) represents different physical magnitudes across frequency. For instance, 100 dB HL at 125 Hz is a greater physical intensity than 100 dB HL at 1000 Hz. The net result is that audiometers are incapable of providing the same maximum intensity at the frequency extremes (125 and 8000 Hz) as they are in the mid frequency range. Even more importantly, the bone conduction vibrator cannot yield nearly as great a maximum output as the earphone. In fact, at 125, 6000, and 8000 Hz, the bone conduction vibrator does not generate any sound at all. Consequently, many of the simpler audiometers display a notation of what the maximum intensity is at each frequency for both air conduction and bone conduction signals. The number under the "A" stands for the maximum output by air conduction, and the value beneath the "B" represents the maximum intensity by bone conduction for the frequency displayed in the frequency window. Rotate through the frequency range for both air conduction and bone conduction At 125, 6000, and 8000 Hz a dash (--) can be seen for bone conduction; this means that bone conduction cannot be tested at these three frequencies. The clinical audiometer simulated by The Audiology Clinic will not permit the intensity to be increased past the maximum intensity; however, the portable audiometer will. Thus, it is the clinician’s responsibility to ensure that the intensity is not increased past the maximum level. Exceeding the limits results in varying effects and unreliable results with portables. Many older audiometers of all types do not incorporate a warning feature to indicate when the maximum intensity limit has been exceeded. To clarify, the intensity dial can always be adjusted to any intensity setting, including the maximum of 110 dB HL, regardless of whether the audiometer is capable of producing a sound of that intensity at the selected frequency. In fact, some audiometers actually have been known to attenuate the signal if the intensity dial is advanced beyond the maximum limit at a particular frequency. Therefore, on audiometers that do not automatically restrict illogical intensity settings, it is the clinician's responsibility to check the intensity limit, which is always displayed somewhere on the face of the audiometer, and increase the tone only as far as the maximum intensity limit and never beyond it. Furthermore, remember not to attempt testing bone conduction at 125, 6000, or 8000 Hz.

28 Chapter Three Instruction Manual Audiology Clinic V2

Summary This chapter has been devoted to obtaining air conduction thresholds using the procedure described by ASHA. In particular, the modified Hughson-Westlake protocol was described as the appropriate method for varying the intensity of the stimulus when measuring a particular threshold. Unfortunately, the responses the listener gives us are not always representative of the actual, or organic, threshold because of a phenomenon called crossover. Due to crossover, we must "mask" or retest some thresholds. This is the topic of Chapter 3.

Chapter Three

Masking Air Conduction Thresholds The focus of the current chapter will be on masking. This process involves retesting a threshold to confirm whether it is the actual threshold while directing a noise into the opposite ear to prevent or "mask" it from detecting the signal. The original threshold may be incorrect due to the phenomenon of crossover.

Crossover Crossover is the situation wherein a signal presented to one ear is actually perceived at the opposite ear. For example, if a signal introduced through the right earphone were really heard in the left ear, then crossover has occurred. As a result an erroneous threshold will be

Instruction Manual Audiology Clinic V2 Chapter Three 29

recorded for the right ear. Crossover occurs when relatively intense stimuli (about 40 dB or greater) are presented to an ear. Two factors contribute to crossover. First, the sound waves may leak from under the earphone cushion or radiate from a bone conduction vibrator and travel around the outside of the head to the other ear. Second, the sizable vibrations caused by comparatively intense sounds may vibrate the entire skull and thereby stimulate the contralateral cochlea because this organ is encased within the temporal bone. In either instance the listener may respond to the tone because it was perceived at the non-test ear rather than at the test ear. The resistance of the skull to vibration as a whole and thus from transmission of a sound from one ear to the other through the bony structures is called interaural attenuation. That is, a tone becomes significantly weaker as it traverses the bones of the head from one side to the other. The amount of interaural attenuation experienced when a tone is presented by air conduction depends on the type of earphone that generates the sound. With the older circumaural earphones interaural attenuation is almost always 40 dB or more. Actually, interaural attenuation varies as a function of frequency. Although it may be as little as 40 dB at the low frequencies, it increases to 50 dB or more at the high frequencies. On the other hand with the newer insert earphones interaural attenuation is usually at least 60 dB, although again it differs depending on the frequency and the manner of insertion of the earplugs (Killion, Wilber, and Gudmundsen, 1985; Munro and Agnew, 1999). In all cases interaural attenuation will vary among individuals. To promote consistency in this simulation, however, interaural attenuation will remain fixed at 40 dB for circumaural earphones and at 60 dB for insert earphones throughout all of the following lessons. Be aware, nevertheless, that your instructor may prepare cases for you to test that have different values for interaural attenuation than 40 dB or 60 dB. Crossover also occurs when testing by bone conduction as will be discussed in Chapter 4.

Masking The purpose of masking is to eliminate the possibility of an incorrect threshold due to crossover. The concern is that the signal will be perceived at the listener's other ear, not the one presently being tested. The process of masking consists of retesting any thresholds that are regarded as suspect, after obtaining the original, unmasked thresholds (as in the previous chapter). A noise stimulus is introduced into the ear opposite the one under test, to prevent, or "mask out", its participation.


Then the threshold is redetermined in the test ear. The conditions that create doubt about the validity of an unmasked threshold will be discussed momentarily. Incidentally, a frequent comment on the part of beginning students in audiology is that surely the listener can differentiate which ear he or she hears the signal in. Lateralization of the tone should not be used as a criterion for determining the cochlea in which perception has occurred. Although some individuals are able to make this distinction reliably, many of the listeners tested by audiometry are either too young, too old, or too handicapped to make this often subtle discrimination. Accordingly, it is not recommended that a listener be instructed to raise the left hand when hearing the tone in the left ear or the right hand for the right ear as the correspondence between ears and hands is just too undependable. The common usage of the term "mask" can be confusing and misleading. By way of illustration, if the left ear is under test by air conduction and the thresholds are suspect, then the convention is to say, "Mask left air conduction". This terminology means to retest the left ear while simultaneously applying the masking signal to the right ear. Understand, then, that the term "mask" as used in audiology refers to a process: namely, retest the ear currently being examined and present masking noise to the opposite ear. Likewise, "mask right air conduction" would signify that the right air conduction threshold is to be reevaluated while masking noise is delivered through the left earphone to the left ear.

Three Masking Rules Now that we have an understanding of what masking is and why it is used, let us consider when it must be done. Three rules will be introduced to indicate when masking must be performed. Each rule will be discussed in separate chapters, beginning with the first rule in this chapter. MASKING RULE NO. 1

An air conduction threshold at a given frequency in a given ear must be masked whenever it is 40 dB or more poorer than the air conduction threshold at the same frequency in the other ear when using circumaural earphones. It must be masked whenever it is 60 db or more poorer when using insert earphones.


Which ear is suspect? Which ear gets retested? It is always the ear with the poorer threshold that must be reexamined. Upon retesting, the threshold in question can either remain the same or become poorer, thus reflecting an even greater loss than that originally measured.

Example Of Crossover Let’s use a diagram to clarify the masking process. Refer to Figure 3-1. In this schematized head, two boxes are used to represent the auditory mechanisms. The two boxes are further subdivided into two parts each. The lateral (outer) boxes stand for the conductive portion of the ear, that is the outer and middle ears. The medial (inner) boxes designate the sensorineural structures (inner ear and auditory nerve). The number in each box reveals the actual, organic hearing loss attributed to that part of the auditory apparatus. To reiterate, these values show the actual loss, not the measured thresholds obtained from testing, which can be erroneous due to crossover. The numbers below the boxes show the total hearing impairment, that is the sum of the conductive loss (the lateral box) and the sensorineural loss (the medial box).


Figure 3-1. Schematic representation of the conductive and sensorineural components of a hearing loss. To illustrate the undesirable effects of crossover, let us assume as shown in Figure 3-1 the instance in which there is a profound hearing loss in the left ear; this situation is noted by the fact that the left ear's sensorineural mechanism shows a threshold denoted as 110 dB HL, meaning that there is a response only at the maximum intensity (in the mid-frequencies, such at 1000 Hz). The conductive part of the left ear, however, is normal; i.e., 0 dB HL. Furthermore, the right ear is perfectly normal (i.e., 0 dB HL thresholds by both air conduction and bone conduction), thus all 0s are shown for the right ear. Now that we know beforehand the real thresholds, we can predict what will happen when this hypothetical person is tested. When the right ear is assessed by air conduction, the results will divulge thresholds at 0 dB HL. On the other hand, when the left ear is tested, thresholds will be recorded at about 40 dB HL when using circumaural earphones and at about 60 dB HL when using insert earphones. These results are, of course, incorrect as the listener has a profound loss on the left side.


Why is there such a surprising discrepancy? It occurs because of crossover. Remember that for air-conducted signals the skull provides an interaural attenuation of approximately 40 dB for circumaural earphones and about 60 dB for insert earphones; therefore, when the tones delivered through the left earphone reach or exceed these levels, they cross the skull and/or leak from beneath the earphone cushion and are heard at the right ear. Specifically, in our example a 40 dB tone from the left circumaural earphone will create an intensity of 0 dB at the right cochlea, enough to be perceived since the right inner ear/auditory nerve (medial box) is depicted as having a threshold of 0 dB HL (bear in mind that 0 dB HL does not mean no sound; it is just a position along the sound intensity continuum, just as 0 degrees Fahrenheit is a point along the temperature continuum). If an insert earphone is being used, then an intensity of 60 dB in the left earphone will create an intensity of 0 dB at the right cochlea. To continue the illustration of crossover, if the earphone is of the circumaural type and a 50 dB HL tone were sent to the left ear, 10 dB would cross over to the right ear: 50 dB tone - 40 dB interaural attenuation = 10 dB crossover. For that matter, the same situation pertains in reverse; if a 110 dB tone were channeled into the right ear, it would cross over to the left ear as follows: 110 dB HL tone - 40 dB interaural attenuation = 70 dB crossover. Naturally, this crossed-over stimulus would not be perceived because we know a priori in this case that the listener has a 110 dB hearing loss in the left ear; nevertheless the tone has crossed the head to the contralateral side and is causing vibrations throughout the cochlea even though such vibrations are not transduced into nerve impulses. In contrast, if insert earphones are in use and a 70 dB tone were delivered to the left ear, 10 dB would cross over to the right ear: 70 dB tone – 60 dB interaural attenuation = 10 dB crossover.

Synopsis Of Rule One To recap, if circumaural earphones are being used, whenever the air conduction threshold in one ear is 40 dB or more poorer than the air conduction threshold at the same frequency in the other ear, the poorer threshold must be masked. That is, the poorer threshold must be retested to determine whether it is the actual organic threshold, or whether the real threshold is even poorer than the level originally obtained. In reality, the true threshold could be anywhere between the one first determined (the unmasked threshold) and the maximum intensity limits of the audiometer at the frequency being examined. There could even be no measurable response if the loss were greater than 110 dB HL. Remember that the crucial intensity of 40 dB is raised to 60 dB when using insert earphones.


Masking Strategies Once it is determined that a threshold must be masked, or retested, the next step is to decide how much masking noise is necessary to prevent participation by the non-test ear. Numerous methods have been advocated for deciding the intensity of the masking stimulus. They can be reduced, nevertheless, to two fundamental strategies. The first method is calculable; it involves estimating the amount of the masking stimulus required to preclude the contralateral ear from any possible perception of the tonal stimulus directed to the test ear. This technique requires computing a value influenced by numerous variables. Although only one level of masking noise need be applied, still the factors contributing to its calculation are subject to dispute; additionally, it takes valuable time to perform the calculation prior to testing each frequency. The second method is empirical. It requires the use of several masking intensities, but there need be no complex, beforehand computation of the levels to use. Furthermore, this latter strategy absolutely identifies the correct threshold, if a simple graphing method is utilized (or merely visualized mentally after sufficient experience). Because of its simplicity and validity the second rationale, called the plateau method, is currently used by more than 3/4 of practicing audiologists (Martin and Sides, 1985). A thorough discussion of various masking strategies is presented by Goldstein and Newman (1994).

The Plateau Method The so-called plateau method of masking was first discussed by Hood (1960) in an article well worth reading. The primary decision that the clinician must make is the initial level at which to set the masking noise; nevertheless, after the starting intensity is decided, the remainder of the technique is straightforward. The recommended beginning level is the air conduction threshold of the non-rest ear. This fact is predicated upon the critical assumption that the masking dial represents effective masking; in other words that a tone of x dB will be just at threshold with a noise of x dB, and if the noise were increased by 1 dB more, the tone would be masked out (be inaudible). Most modern clinical audiometers have a masking dial that is calibrated in effective masking. This is sometimes not true for portable audiometers.


In the ensuing example it will be assumed that circumaural earphones are being used. The same strategy pertains if insert earphones are used, but the number that represents interaural attenuation changes from 40 dB to 60 dB. For this reason each value that applies if using insert earphones is shown in parentheses after the corresponding value for circumaural earphones. To examine the masking process, suppose that we obtain an unmasked air conduction threshold of 60 (80) dB in the right car and 20 dB in the left ear for a pure-tone at 1000 Hz. The right ear must be masked (retested) according to the first rule of masking because the response at 60 (80) dB is 40 (60) dB greater than that in the left ear. Consequently, this response may have been due to stimulation of the left cochlea. The initial intensity of the masking noise that goes to the left ear should be 20 dB, the level of the threshold in that ear. Introducing noise at a lesser intensity, cannot be perceived by the listener; while starting with masking at a suprathreshold level may, in some instances, lead to an erroneous result. This point will be illustrated later. Once the initial masking intensity is applied, then a simple strategy prevails. The tone is again introduced into the test ear at the same level as the original, unmasked threshold. Referring to our current example, the intensity would be 60 (80) dB HL. If the listener responds, the masking noise is increased by 5 dB. On the other hand, if the listener does not respond, then the pure-tone signal is raised by 5 dB in the test car and presented to the listener again. When obtaining a masked threshold, unlike the situation when measuring an unmasked threshold, the convention is to present the tone only once, unless, of course, there is a legitimate reason to presume that the listener was distracted, inattentive, etc. Note especially that the intensity of the tone is not alternately decreased and increased. The interchange between increasing the level of the tone and increasing the level of the noise continues in 5 dB increments until either a plateau is reached, or overmasking is a risk, or the masking signal cannot be increased any further. Distinguishing among these alternatives is critical and will be discussed presently. It is necessary to digress momentarily to argue for the use of 5 dB increments when searching for a masked threshold. Some clinicians use 10 dB increments, maintaining that such a strategy is quicker but yields equally accurate results. It is conceivable, nevertheless, that the correct threshold may not be found when using 10 dB changes in the masking noise. Ultimately, there may be numerous shortcuts that you employ; but for the present time we will endeavor to support the most complete and accurate technique for instructional purposes; e.g., 5 dB increments. If a case is currently active on the screen, click Case and then Close Case. Next click Case again, followed by Select case. Pick Case 21


and use the default (circumaural) earphones. Click Test and Pure tone audiometry. At 1000 Hz manually obtain right and left air conduction thresholds. You will find a right air conduction threshold of 15 dB HL and a left air conduction threshold of 55 dB HL. Plot them on the audiogram so that you do not forget them. According to the first rule of masking, stated above, the result in the left ear satisfies the criterion requiring masking; that is, the threshold in the left ear is suspect, as it is 40 dB poorer than the threshold in the right ear. Thus, the left threshold could be the result of crossover with the sound having stimulated the right cochlea. On the other hand 55 dB HL could be the correct, organic threshold (the listener could have actually perceived the sound at the left cochlea when responding at 55 dB HL to the tone. To clarify which situation pertains, this threshold must be retested, or masked. Now repeat this case using insert earphones instead. Close the current case and reselect Case 21 again. Click Test and Pure tone audiometry. Before doing anything else, click Options, Transducers, and Insert. Notice that the earphones on the listener change from circumaural to insert. Now proceed as before. This time the left unmasked air conduction threshold will be 75 dB HL due to the greater interaural attenuation provided by the insert earphones.

Automatic Testing Before you endeavor to obtain your first masked threshold, allow The Audiology Clinic to demonstrate the process to you. This simulator is capable of automatically performing the masking procedure and displaying the results by plotting a masking curve, a graph that has a horizontal axis labeled "masking level" (noise) and a vertical axis labeled "hearing level" (tone). Make sure that the output of the audiometer is set to the Left ear, which is the ear to be retested. Also make certain that the Frequency = 1000 Hz and Mode = Air and that the listener is wearing circumaural earphones. With our simulated audiometer, as is the case with most actual audiometers, the masking stimulus is automatically channeled to the non-test ear (i.e., the ear opposite the one designated in the output box on the audiometer), in this instance the right ear. To invoke automatic testing, click Auto test, then click Standard masking. Repeatedly click Next and watch the screen. Observe the narrative description that appears in the box at the bottom of the masking chart. If the points plotted were connected, they would look like the solid line at the left side of in Figure 3-2. The significance of the shape of


this curve will be explained in detail soon. Remember that this process can also be executed automatically by selecting a speed between 1 and 9. If the arrow buttons to the upper right of the Speed label are not enabled, click Reset. Then these buttons can be clicked to vary the speed.

Establishing The Plateau This illustration assumes 40 dB of interaural attenuation (i.e., that circumarual earphones are in use). Now plot your own masking curve on the same ear at the same frequency. Begin by erasing the existing masking curve by clicking Reset. Next close the Masking curve window: click Auto test, then Close auto test.

Figure 3-2. Masking curve that reveals the plateau. To execute the plateau masking procedure, first adjust the intensity of the masking signal to the level of the threshold in the non-test ear. Since the threshold in the right car was 15 dB, set the masking intensity to 15 dB. It should be understood that the masking procedure will work correctly even if the masking level is begun at 5 dB, in other words, at the lowest masking intensity. Because this level is below the air conduction threshold of the non-test ear, no masking can possibly occur. To repeat, beginning the masking process at the lowest masking level will not cause any errors, it will just take longer to establish the correct threshold. Therefore, to maximize efficiency, always initially


set the masking stimulus to the level of the non-test ear's threshold (unless otherwise instructed to demonstrate a particular point). Your previously plotted unmasked Left air conduction threshold should still be at 55 dB on the audiogram. To proceed, with the masking intensity at 15 dB, adjust the test tone to the original threshold (55 dB HL); then present the tone. Watch the screen: The listener responds, and the first point is plotted on the masking curve. Next follow the steps listed in the box below and confirm the results indicated.

Adjustment Action Result 1. Increase noise to 20 dB Present signal No response 2. Increase signal to 60 dB Present signal Response 3. Increase noise to 25 dB Present signal No response 4. Increase signal to 65 dB Present signal Response 5. Increase noise to 30 dB Present signal No response 6. Increase signal to 70 dB Present signal Response 7. Increase noise to 35 dB Present signal No response 8. Increase signal to 75 dB Present signal Response 9. Increase noise to 40 dB Present signal No response 10 Increase signal to 80 dB Present signal Response 11 Increase noise to 45 dB Present signal Response 12 Increase noise to 50 dB Present signal Response

Note that the listener responded at the same tonal intensity while the noise level was increased twice beyond 40 dB. In other words the listener responded three times to the same level of the tone, namely 80 dB HL (steps 10 - 12). The important part of this lesson occurred when the magnitude of the tone reached 80 dB HL. At this point further increases in the masking noise beyond 80 dB did not result in any change in the level of the signal necessary to elicit a response. Reexamine Figure 3-2; the above steps formed the solid part of that curve. Now continue increasing the magnitude of the masking stimulus in 5 dB increments to its maximum: 110 dB. Present the tone after each advance in the level of the noise. Do this as an experiment to develop the masking curve that resembles Figure 3-2. The listener continues responding to the tone at 80 dB HL, thus creating the dotted portion of the curve in Figure 3-2. Observe the masking curve: it has leveled off or reached a plateau, thus the name of the masking procedure: the plateau method. You have found the plateau, which denotes the real threshold of the ear under test. To reiterate, when further increases in the masking level do not demand additional increases in the signal level for the listener to respond, then the actual threshold has been measured.


In this example the masked threshold (80 dB HL) was 25 dB poorer than the unmasked threshold (55 dB HL), leading us to conclude that the original, unmasked threshold was indeed the result of crossover. The masking curve you generated should look exactly like that drawn by the computer using the automatic testing mode described above. When you were instructed previously to view the automatic masking procedure, you could select either the Standard masking or the Complete masking curve. You were instructed to choose the standard method, which duplicates normal clinical procedures. The other option, the complete curve, results in the masking noise being increased in 5 dB steps from 5 dB all the way to 110 dB. Understand that the latter choice, Complete curve, does not replicate clinical methodology, but rather is available to illustrate what happens at masking levels other than those generally used, in other words to draw the masking curve that would result from the utilization of all masking levels for use as a learning tool. There are occasions where the complete masking curve represents important points about the masking process. To verify the results you just obtained, click Auto test, Complete curve and set Speed to 9 (click the right-facing arrow). Click Start and watch the plotting of the curve you just obtained, which is very similar to Figure 3-2. Now complete the manual testing of this listener. First click Reset, Auto test, Close auto test. Since clearly the right ear is the ear with the better hearing thresholds, switch the output to the right ear and measure the thresholds at the other frequencies. Afterwards return to the left ear and obtain the remaining thresholds in that ear. Each time that a threshold is found at a different frequency, the masking rule is applied; and if need be, the masked threshold is measured before advancing to the next frequency. It is important to plot each threshold on the audiogram as it is obtained. Notice that the symbol for a masked threshold is plotted. You will discover that every frequency in the left ear when tested by air conduction requires masking. If you want confirmation of your results, return to the automatic masking demonstration discussed previously. Furthermore, you may use the automatically-drawn curves to check the accuracy of the curves that you derive. Switch to insert earphones and repeat the above illustration. Of course because of the greater interaural attenuation afforded by this type of earphone, the initial unmasked left air conduction threshold at 1000 Hz will be 75 dB, so this value will be used as the initial intensity. Accordingly, the correct masked threshold will be derived much more quickly, as there will only be a 5 dB shift between the original unmasked threshold and the final masked threshold.


To see the correct, completed audiogram, click Audiogram and click Show results.

Interpretation Of Results The two questions always to be answered when doing audiometry are: 1) how much loss, and 2) what kind of loss. In the example just completed only the first issue can be addressed. The amount of loss expressed numerically is 80 to 85 dB HL in the left ear and 10 to 15 dB HL in the right ear. Using the categories specified in Chapter 2, these thresholds would be described as a severe loss in the left ear and hearing within normal limits in the right ear. It is impossible to stipulate the kind of loss because both air conduction and bone conduction thresholds are required to make this determination. It is worthwhile considering only the air conduction results, nevertheless, because the air conduction thresholds do quantify the overall amount of the hearing deficit.

Four Parts Of The Masking Curve The yet unanswered question is, “how much masking is necessary to get the correct threshold?” The answer to this issue can be found by examining the masking curve, specifically the plateau: as soon as the plateau can be defined, the true threshold has been measured. A masking curve can theoretically have four (4) parts. These divisions will be found when you test Case 22 with circumaural earphones. With Case 22 you will discover that with circumaural earphones the right ear has unmasked air conduction thresholds of 45 to 55 dB HL across the frequency range from 250 to 8000 Hz. Furthermore, the thresholds in the left ear are 5 to 15 dB HL, or precisely 40 dB better than the corresponding thresholds in the right ear. Recall Masking Rule No. 1: all thresholds in the right ear, the ear with the poorer hearing, are in doubt and must be masked. Begin by obtaining the unmasked thresholds in the right and left ears at 1000 Hz; they are 50 and 10 dB HL, respectively. Next apply masking where required. Realize that the right air conduction threshold (50 dB HL) is equivocal, for it is 40 dB poorer than the corresponding threshold in the other ear. Either acquire the masked thresholds manually, if you are comfortable with the masking protocol, or have The Audiology Clinic draw the curve. If you use Auto test, select Complete curve as we want to plot the entire masking curve. If you choose to plot the curve yourself, set the masking noise initially to 5 dB and then increase in 5 dB increments. Understand that this is not


normal clinical practice but it being employed here to demonstrate that there can be four possible components to a masking curve. The resulting curve will resemble that shown in Figure 3-3. Inspect the four (4) parts of this masking curve. Proceeding from left to right, the first part is a horizontal line that represents undermasking. We know it is the undermasking portion of the curve because the masking level remains lower than the threshold for that ear. If we follow standard procedure and initially set the masking intensity to the threshold of the non-test ear, this portion will not appear on our graph. In this example we purposely started masking below the threshold of the non-test ear so we could see this part of the curve. It must be emphasized that an incorrect result will not materialize if masking is begun at 5 dB, but it is a waste of time to do so except for demonstration purposes. The second part of the curve is called the shifting segment. The questionable threshold is being shifted from its crossed-over (unmasked) value to its correct (masked) value. The third division of the curve is the plateau and denotes the true threshold. By extending the plateau leftward until it intersects the vertical axis of the graph, we can observe the dB value of the actual threshold (60 dB HL).

Figure 3-3. The four parts of the masking curve. The fourth and final element of the masking curve is the overmasking section. At this point so much noise has been delivered to the non-test ear that it has crossed over and has masked the test ear. The dominant part of the masking curve depicted in Figure 3-3 is the plateau. Be


advised that many masking curves will have a much narrower plateau, or perhaps none at all. Additionally, not all hearing configurations will lead to the development of a masking curve having all four parts, as we shall see with future examples. One such example has already been presented: review Figure 3-2, as that masking curve had no overmasking component. When this case is repeated using insert earphones, there is no crossover, that is the original unmasked air conduction thresholds in the right ear (55-65 dB) are the correct thresholds because there is not a 60 dB or greater difference between the air conduction thresholds in the two ears. There was no crossover due to the increased interaural attenuation offered by insert earphones. We should point out in closing that our nomenclature, that of labeling four parts to the masking curve, is different than that used in some texts.

Determining The Real Threshold Establishing the exact masked threshold is straightforward: it is indicated by the plateau on the masking graph. The only possible confusion is whether a horizontal segment represents the undermasking part or the plateau of the graph. (Recall from Figure 3-3 that there were two horizontal sections on that graph). The section representing undermasking will not ordinarily be obtained; it was developed in the previous example solely for illustrative purposes. Good clinical practice dictates that the beginning masking level be equal to the air conduction threshold in the non-test ear. Thus the possibility that the horizontal part of the curve is due to undermasking is impossible if masking were begun at the proper level. This being the case, the only horizontal segment generated when using standard clinical procedures is the plateau. Now complete Case 22 using both circumaural and insert earphones by finding the masked thresholds for the remaining frequencies in the right ear. They are similar, but not identical, to the result at l000 Hz. Clearly with one type of earphone masking is necessary but not with the other.

Overmasking For the most part the danger of overmasking arises when masking bone conduction thresholds; this is the topic of the next chapter. As an introduction to this situation, however, there are a few instances wherein overmasking can happen with air conduction testing. Inspect


the situation portrayed by the audiogram segment at the right side of Figure 3-4. According to Masking Rule No. 1, the right ear must be masked when using circumaural earphones. Hence, noise is applied to the left ear. To simulate this using The Audiology Clinic, select Case 23 with circumaural earphones. Be sure the frequency is set to 1000 Hz. First find the unmasked air conduction thresholds. They are: 40 dB HL in the right ear and 0 dB HL in the left ear. The former ear, therefore, requires masking. Since the left air conduction threshold is 0 dB HL, the initial masking intensity in this ear will be 0 dB, while the beginning intensity of the test signal will be 40 dB HL, the level of the unmasked threshold. Again plot the masked threshold automatically (click Auto test, Complete curve); then click Next. Pay attention to the fact that as you deliver tones to the right ear and increase the noise in the left ear, a plateau is found at 40 dB (i.e., the level of the unmasked threshold. Notice that as you proceed, overmasking occurs, and a warning message is displayed in the text dialog box. Unlike the previous examples, there is no shifting part on this masking curve. Such is the case when the original unmasked threshold is the real organic threshold. The plateau is realized as soon as the initial masking intensity is introduced because the unmasked threshold is the true threshold. As the intensity of the masking sound increases, the listener continues to respond until it reaches a value of 85 dB. At that point the listener no longer responds. Accordingly, the intensity of the tone is increased: to 45 dB HL. Next the masking level is increased to 90 dB, and the tone is subsequently raised to 50 dB HL in order to elicit a response. What happens to the masking curve at this high masking intensity? The threshold of the tone is shifted to 50 dB HL. This threshold is incorrect; as can be seen by inspecting the head in Figure 3-4, the proper threshold is 40 dB HL. The 10 dB shift of the threshold in the right ear is the result of overmasking. Overmasking is the situation when so much masking noise is delivered to the non-test ear that it crosses the head to the cochlea of the test ear and masks out the signal being delivered there. Why overmasking occurred can be understood by considering Figure 3-4 once more. What will the amount of crossover be when a 90 dB masking noise is sent into the left ear? As always, the intensity of the crossed-over signal will be the applied signal (90 dB) minus the interaural attenuation (40 dB), or 90 - 40 = 50 dB. The intensity of the masking noise in the test (right) ear will be 50 dB; this will shift the true threshold by 10 dB because the masking noise will have crossed over into the test ear and prevented it from responding to a 40 dB tone any longer.


Figure 3-4. Configuration in which overmasking can occur with air conduction testing. Now repeat the above process again, and this time display the Crossover diagram on the screen. Click Reset on the Masking Chart, if necessary, then click Auto test followed by Crossover diagram. (This diagram represents both circumaural and insert earphones the same way--as a gray rectangle.) The process described above can be seen dynamically as the masking process occurs. Proceed slowly and study carefully the signal and noise levels and the resulting crossover after each change in the signal level and the masking level. Figure 3-5 shows a snapshot of the Crossover diagram at the point that overmasking begins to occur. As discussed above this is when the masking level reaches 85 dB. As can be seen 45 dB of masking crosses over to the non-test ear, and this shifts the threshold in that ear from 40 to 45 dB. Thus, the threshold of the test ear has been changed.


Figure 3-5. Crossover diagram. Overmasking is a very serious circumstance and will be studied thoroughly in the next chapter when we discuss the masking of bone conduction thresholds. Close auto test and complete this audiogram by obtaining masked thresholds for right air conduction at the other frequencies. After each threshold has been masked, remember to reduce the masking intensity to 0 dB before switching to the next frequency.

Definition Of A Plateau The emphasis thus far has been to develop a complete masking curve, that is, to increase the masking signal to the maximum intensity. This has been done to familiarize you with all the component parts of a masking curve. Needless to say, this is not the procedure followed in actual clinical testing. The masking signal need only be increased far enough to define the plateau; moreover the plateau is defined when three points have been found on it. Reexamine Figure 3-2; the masking procedure is complete when the masking level reaches 50 dB, that is the part of the curve represented by the horizontal section of the solid line. Otherwise said, the masked threshold has been determined when responses are elicited to masking levels of three different intensities, while the level of the tone remains constant. The Audiology Clinic comprehends when a plateau has been defined. To experience this, remask some of the thresholds in Cases 21, 22, and 23 using the Auto test with the Standard curve selected. This causes the computer to use the standard clinical procedure, so the masking process stops as soon as the plateau has been adequately defined. Use both circumaural and insert earphones; you will observe that the need to mask and the subsequent shift of an unmasked threshold occurs less often when using insert earphones.

Limitations To Plateau Definitions Another way to view the plateau is to consider it as being the segment of the masking curve that connects the shifting and overmasking portions (Figure 3-3). Some hearing configurations can cause these two parts of the curve to intersect, that is, they are plotted as one continuous line (their slopes are identical). In other words, there is no plateau because overmasking starts immediately after the threshold has


been shifted. Examined from still another angle, identification of the plateau necessitates distinguishing between the shifting and overmasking parts of the masking curve, which both extend toward the upper right of the graph. In addition to being nonexistent it is also possible that the plateau could exist but be very narrow, only 5 dB wide in fact. A plateau could theoretically be defined as the situation in which a 5 dB increase in masking intensity does not shift the threshold of the test ear any further, that is, the threshold for the tone remains the same after the last increase in masking level. To see this idea portrayed, refer to Figure 3-6. Assuming that right air conduction is being masked, note that with 30 dB of masking introduced into the non-test ear the threshold has been shifted to 70 dB HL (the left side of the plateau); furthermore when the masking level is increased to 35 dB there is no change in the threshold; in other words the graph proceeds horizontally: this is the plateau. In this hypothetical case the next increase in masking intensity (to 40 dB) leads to overmasking. Thus, to be heard, the signal must be raised to 75 dB. This is an extremely narrow plateau. Many clinicians require at least three points on the plateau, rather than the two points just demonstrated, to feel secure in defining the actual threshold. The reason for this seemingly conservative attitude will become evident later on when we consider variability in listeners' responses. To summarize for the present, real listeners are often not as reliable in their responses as The Audiology Clinic has been representing. The reality is that many listeners, if tested and retested, will have thresholds that vary by ± 5 dB (or even ± 10 dB). These inconsistencies dictate that the plateau be unquestionably defined, a requirement that can be realized by requiring three, or even four, points on the plateau. Recognize that a narrow plateau may evade detection if a 10 dB increment in signal and noise intensity were to be used. This reality provides the rationale for recommending only 5 dB changes in both the signal and the masking noise. To gain experience with narrow plateaus, choose Case 24 with circumaural earphones. Use the following setup: Output: Left; Frequency: 250 Hz, Auto test, Complete masking, Speed = 9. Note the width of the plateau. Continue by testing at 500, 1000, 2000, 4000, and 8000 Hz. This example illustrates the fact that a demonstrable plateau may exist at some frequencies but not at all frequencies. Verify that the plateau narrows, as in Figure 3-6, and then disappears completely.


Figure 3-6. Masking curve with a vary narrow plateau. Although three points can be considered as adequately defining the plateau, at this stage experiment by increasing the masking noise to its maximum level. Although this is not an acceptable clinical method because it requires excessive time and unnecessarily loud stimuli in the listener's ear, the practice in observing complete masking curves will be most worthwhile as a learning experience. Complete Cases 25 through 30 (not available in the Lite edition) before advancing to the next chapter. Furthermore, review any concepts you are unsure of by testing a particular case over again. Of course, not all of the audiograms will require masking at every frequency. Bear in mind that the decision to mask is a frequency by frequency judgment. Remember, too, that currently we are concerned only with the masking of air conduction thresholds according to Masking Rule No. 1. In reality some bone conduction thresholds will end up being incorrect, but that situation cannot be corrected until the next chapter.

Summary Chapter 3 has introduced the need to mask air conduction thresholds and described the first masking rule. The air conduction threshold at a particular frequency in one ear must be masked whenever it is 40 dB or more poorer than the air conduction threshold in the other ear for circumaural earhones or 60 dB or more poorer for insert earphones.

48 Chapter Four Instruction Manual Audiology Clinic V2

The correct threshold is signified by the plateau, the plateau being the condition in which the signal is heard at the same level after the masker is increased at least 10 dB (two 5 dB increments). In the following chapter consideration will be given to the masking of bone conduction thresholds, which unfortunately must be retested far more frequently than air conduction thresholds. Luckily, the technique is exactly the same.

Chapter Four

Masking Bone Conduction Thresholds Although air conduction thresholds have to be masked when there is an asymmetrical loss with differences between ears of 40 dB or greater, bone conduction thresholds have to be masked much more frequently. Masking when testing bone conduction is the topic of this chapter.

Interaural Attenuation By Bone Conduction The reason for the necessity to mask bone conduction thresholds so often is due to the fact that the interaural attenuation afforded by the head for a signal presented through the bone conduction vibrator is essentially nil. In fact, some authorities indicate that there is no interaural attenuation whatsoever for bone conducted tones, while others attribute 10 dB as a realistic amount of interaural attenuation. From a clinical standpoint the magnitude of the actual interaural attenuation is chiefly a moot point because additional factors such as

Instruction Manual Audiology Clinic V2 Chapter Four 49

placement of the bone conduction vibrator on the mastoid, variability in a listener's response between tonal presentations, etc. confound the exact measurement of interaural attenuation. Another way of looking at this situation is to realize that pragmatically a tone presented from a bone conduction vibrator placed on one mastoid (either the left or the right) stimulates both cochleas essentially equally. As an example, suppose that a 30 dB HL sound is delivered from the bone vibrator, which is placed behind the right ear. The intensity of the tone at the left cochlea will also be 30 dB because 30 dB - 0 dB "realistic" interaural attenuation = 30 dB crossover. In The Audiology Clinic the interaural attenuation by bone conduction will be considered to be 0 dB. While this may be a conservative value, nevertheless guessing which bone conduction threshold has crossed over based on slight differences in unmasked bone conduction thresholds is totally unreliable due to the reasons outlined above. The net result is that it is frequently essential, when testing by bone conduction, to apply masking to the opposite ear to preclude its participation in the measurement of the threshold because the tonal stimulus so readily crosses the head to the non-test ear.

Air-Bone Gap Before proceeding to define the second rule of masking, a new term must be explained. An air-bone gap is defined as the difference between the air conduction and the bone conduction thresholds in a given ear at a particular frequency. It must be realized that this difference can only exist in one direction: the air conduction threshold can be poorer than the bone conduction threshold, but the reverse situation is impossible. An example of an air-bone gap is depicted in Figure 4-1. The magnitude of this air-bone gap is 40 dB. Remember that bone conduction reflects only the condition of the sensorineural


Figure 4-1. Example of an air-bone gap. mechanism, while air conduction tests the conductive pathway plus the sensorineural portion; therefore it would be inconceivable for a part (bone conduction) to be greater than the whole (air conduction) resulting in a bone conduction threshold that was poorer than the corresponding air conduction threshold. In reality bone conduction thresholds are sometimes measured clinically that are slightly poorer (5 dB, possibly even 10 dB) than the corresponding air conduction threshold (Barry, 1994). This seemingly impossible result can be due to any or all of the following: calibration errors, placement of the earphone and/or bone conduction vibrator, or inattention by the listener. Skilled clinicians generally disregard an occasional occurrence of this situation. We are now prepared to learn the second rule of masking. MASKING RULE NO. 2 A bone conduction threshold at a given frequency in a given ear must be masked whenever there is an air-bone gap. As was the case with air conduction thresholds in the last chapter, a bone conduction threshold, when masked (retested), can remain the same as the unmasked threshold, or it can become poorer, thus reflecting a greater degree of hearing loss. The masked threshold can never become better. In a given ear there may be air-bone gaps at just one frequency, at several frequencies, or at all frequencies. In addition, depending on the configuration of the air conduction thresholds, there may be air-bone gaps in only one ear or in both ears. Regardless of the number of air-bone gaps, each one must be masked. The one exception to this rule will be explained shortly.

Occlusion Effect There is one additional issue that must be addressed when masking bone conduction thresholds. To perform the masking task, an earphone must be placed over the contralateral ear in order to deliver the masking noise. Covering the ear canal will, with some hearing loss configurations, enhance the signal strength in the cochlea at some frequencies. This situation is known as the occlusion effect. Its dominance is seen when the non-test ear has normal hearing or a


sensorineural loss, but not a conductive loss. Its presence is noted at the low and mid frequencies up to and including 1000 Hz. The existence of the occlusion effect means that slightly different bone conduction thresholds will be obtained when the non-test ear does not have an earphone covering it (known as the unoccluded condition), compared to when the non-test ear is prepared to be masked and has an earphone covering it (known as the occluded condition). Some audiologists prefer to obtain right and left bone conduction thresholds unoccluded, so the only transducer on the head is the bone conduction vibrator (no earphones). Then they proceed to mask bone conduction later, if needed. Other clinicians recognize the frequent responsibility to mask bone conduction and configure the transducers to allow masking at the outset. To mask a bone conduction threshold when utilizing circumaural earphones, the bone conduction vibrator is placed behind the test ear, and the earphone is positioned over the non-test ear. The other earphone is not placed over the pinna of the test ear (the ear with the bone conduction vibrator). Rather it is placed against the head between the pinna and the eye. Thus, if the test ear must be masked, it can be done without having to return to the listener to place the earphones. The Audiology Clinic simulates both the unoccluded and occluded situations. The default state is for the non-test ear to be unoccluded. In order to mask, you must “place” the earphones. To change to the occluded condition, click Options, then Non-test ear occluded. Don’t forget: the default condition is unoccluded. Moreover, every time you change ears on the audiometer or select a new case, the simulation returns to unoccluded. As a result of the occlusion effect, the initial masking intensity at 250, 500, and 1000 Hz must be somewhat greater than normal. Table 4-1 shows suggested amounts by which the initial masking intensity should be increased to overcome the occlusion effect (Goldstein and Newman, 1985). To illustrate, if the air conduction threshold of the non-test ear were 30 dB HL at 500 Hz, then the appropriate intensity with which to commence masking would be 45 dB: the threshold plus the allowance for the occlusion effect at that frequency. Although it is clear when the non-test ear has normal hearing, it may be indeterminate whether a loss in that ear is conductive or sensorineural, as the non-test ear may also have to be masked. Other test results, like immittance and/or case history information, may resolve this quandary. In the case studies that follow the occlusion effect will be evident; you should attempt to account for it by elevating the initial masking intensity.

Table 4-1. Magnitude of the occlusion effect at three frequencies (after


Goldstein and Newman, 1994). Frequency Magnitude of the Occlusion Effect

250 Hz 15 dB 500 Hz 15 dB

1000 Hz 10 dB

Air-Bone Gaps In Only One Ear Air-bone gaps are more easily masked when they are present in just one ear rather than in both ears. With a unilateral air-bone gap overmasking will not be a problem, and a plateau can easily be defined that reflects the true threshold by bone conduction for the ear under test. Let us suppose a right air conduction threshold of 35 dB HL and an unmasked right bone conduction threshold of 0 dB HL, thus reflecting an air-bone gap of 35 dB. Further, suppose that there is no air-bone gap in the left ear. Such a configuration is presented in Figure 4-2 (top). It is evident by recalling the second rule of masking that the right bone conduction threshold must be masked (due to the air-bone gap). Consequently, at this point in time it is totally incorrect to describe the right ear as having a conductive loss. Why? Because the real right bone conduction threshold is unknown until after it has been masked.


Figure 4-2. Unilateral air-bone gap (top) and three possible results after masking (bottom). There are three possibilities after masking, and these are illustrated in the lower section of Figure 4-2. Focus on the right bone thresholds ( [ ). 1.The masked bone conduction threshold can remain the same as it was before being masked. In this instance the test (right) ear has a conductive loss (Fig. 4-2a). 2.The masked bone conduction threshold can shift until it is equal to the air conduction threshold at the frequency under test, thus designating a sensorineural loss (Fig. 4-2b). Therefore, the original, unmasked results, which showed normal bone conduction, were in error. The initial air-bone gap was due to crossover, so the test ear did not actually have a conductive loss as suggested by the unmasked thresholds. 3.The masked bone conduction threshold can become poorer than the unmasked threshold but not as great as the air conduction threshold (Fig.4-2c). This instance denotes a mixed loss with a conductive component of 15 dB (the difference between the air and the bone thresholds) and a sensorineural component of 20 dB (the amount of loss by bone conduction).

Practice Cases 31 and 32 will furnish the opportunity to mask audiograms of the type depicted in Figure 4-2, that is unilateral air-bone gaps. Since they portray the simplest examples of masking by bone conduction, they should be completed before progressing on to the next section. Don't forget the technique we advocate is that the initial masking intensity be equal to the air conduction threshold in the non-test ear with allowance for the occlusion effect where appropriate (Table 4-1). Also 5 dB increases in both tone and masking noise should be used. For these audiograms the masking process is completed when a plateau has been defined. For a more thorough understanding of the masking curve, you may prefer to begin masking with 5 dB of noise and alternate increases in tone and noise levels until you reach 110 dB of masking; i.e., a complete masking curve. Many examples will not reveal a masking curve with all four parts; nevertheless, the plateau will be readily identifiable. Such experimentation is acceptable on the simulator but not for clinical use due to the unreasonable duration of the test and excessive sound levels stimulating the listener's ears.


Bilateral Air-Bone Gaps An especially complex masking predicament can arise when there are air-bone gaps in both ears; and in general the greater the air-bone gaps, the more difficult it becomes to derive the real thresholds. As an introduction to bilateral air-bone gaps examine the loss symbolized by the partial audiogram displayed in Figure 4-3 (top). This configuration represents a particularly interesting situation. Three distinct possibilities exist regarding the actual bone conduction thresholds. 1.Both true bone conduction thresholds could be the same as the unmasked thresholds shown on the audiogram; thus when masked, they would portray the final effect presented in segment (a) at the bottom of Figure 4-3, which specifies bilateral conductive involvement. 2.Or the right bone conduction threshold could really be poorer than shown in the upper panel as the result of crossover. For purposes of simplification let us assume that the right ear has a sensorineural (as opposed to a mixed) loss, causing the actual right bone conduction threshold, when masked, to be equal to the right air conduction threshold, as shown in part (b). This outcome reveals a conductive loss in the left ear, and a sensorineural loss in the right ear. 3.Lastly, the left bone conduction threshold could in fact be poorer and equal to the left air conduction threshold, as indicated in panel (c), thereby presenting a conductive loss in the right ear, and a sensorineural loss in the left ear.


Figure 4-3. Bilateral air-bone gap (top) and three possible and one impossible result after masking (bottom).

Remember that in alternatives 2 and 3 above the real bone conduction threshold could have been anywhere between 0 dB HL and the air conduction threshold (30 dB HL), thus Figure 4-3. Bilateral air-bone gap (top) and three possible and one impossible result after masking (bottom). designating varying degrees of mixed loss. For simplicity, only two possibilities have chosen as illustrations: a "pure" conductive loss and a "pure" sensorineural loss. It is essential to understand that the configuration revealed in section (d) in the lower part of Figure 4-3 is impossible to derive from the unmasked audiogram presented in the top part of the figure. One cochlea (if not both) heard the tone at 0 dB HL. Thus, the significance of the unmasked audiogram is that a conductive loss does exist in at least one ear. Figures 4-3(a) through 4-3(c) have demonstrated that the conductive loss may be in the left ear, or it may be in the right ear, or it may be in both ears. But there is no way that the masked audiogram can show a sensorineural loss in both ears as in Fig. 4-3(d). It needs to be underscored, therefore, that even the unmasked audiogram with bilateral air-bone gaps reveals significant information:


because there must be a conductive loss in at least one ear (if not both), a medical referral is often appropriate. Realize, furthermore, that it is unnecessary to mask both ears, if, after masking one ear, that threshold has shifted (shows a greater loss). On the other hand, if the threshold of the first ear to be masked does not change, then masking the other bone conduction threshold is required. Which bone conduction threshold should be masked first? The one with the suspected mixed or sensorineural loss, as that threshold will shift. If it does, you do not have to mask the opposite threshold; remember only one threshold can shift (become poorer). Valuable assistance in determining which ear is likely to have a conductive (as opposed to a sensorineural) component can be attained from immittance tests, if given before pure-tone audiometry, and from case history information. To repeat, left bone (Fig. 4-3b) and right bone (Fig 4-3c) do not have to be masked according to Masking Rule No. 2. These cases demonstrate the exception to the rule, and comprehending this fact eliminates unnecessary masking and saves much valuable time for the knowledgeable clinician. The opportunity to acquire experience with bilateral air-bone gaps is afforded by Cases 33 and 34. Test these simulated listeners before progressing to a much more difficult configuration. Especial care must be devoted to the masking of bilateral air-bone gaps. If the gaps are not too large, as in these three audiograms, plateaus on the masking curve can be described, thus accurately defining the threshold. Beware, nevertheless, that the plateaus are narrow with circumaural earphones, so care must be taken to follow the masking protocol precisely. Again, plot the entire masking curve, if you want to identify the various parts of the masking function.

The Masking Dilemma The anathema of every audiologist is the masking dilemma. This predicament derives from large, bilateral air-bone gaps of approximately 40 dB in magnitude when using circumaural earphones and 60 dB in magnitude when using insert earphones. To demonstrate such a situation, look at Figure 4-4. This diagram of the head indicates the real thresholds, while the fragment of an audiogram discloses the unmasked thresholds, which are unreliable until after they are masked due to the possibility of crossover confirmed by the bilateral air-bone gaps. Although the audiogram reveals a left air conduction threshold of 40 dB and a left bone conduction threshold of 0 dB, an examination of the left ear within the head identifies the true thresholds of 80 dB for air conduction and 60 dB for bone conduction. Recall that the total


loss (the air conduction threshold) is shown below the boxes (80 dB). The discrepancy between the unmasked air and bone conduction thresholds and the authentic thresholds disclosed within the head can be explained by crossover. The left bone conduction threshold that is displayed on the audiogram resulted from the tone's being heard in the right cochlea; the same circumstance happened with the left air conduction threshold (this latter information creates a paradox that cannot be resolved until the next chapter). By inspecting the audiogram, we acknowledge that the bone conduction thresholds must be masked due to the bilateral air-bone gaps. Let us assume that we are using circumaural earphones and that we decide to retest right bone conduction first, thus directing the masking noise into the left ear. Inspection of the head reveals that this was the wrong choice, as the right unmasked bone conduction threshold is the actual threshold (0 dB HL) and consequently does not need to be masked. Realize, however, that the clinician often has only the audiogram to go by and certainly not the internal anatomy of the listener's head, and the audiogram reveals an air-bone gap on the right side (as well as the left side). The unmasked audiogram informs us that we must start masking with a 40 dB noise, because that is the level of the unmasked left air conduction threshold (consider the situation at 2000 Hz in order to disregard the occlusion effect and thus simplify this case). Be sure to observe that 40 dB of noise will cause no masking in the non-test (left) ear because the bone conduction sensitivity of the left ear is really 60 dB as revealed by the diagram of the head. Nevertheless, what does transpire as soon as we apply the noise at 40 dB? It crosses over to the right cochlea: 40 dB masking - 40 dB interaural attenuation


Figure 4-4. Example of unmasked thresholds that will lead to a masking dilemma. = 0 dB crossover. A 0 dB noise, however, will not mask out a 0 dB HL tone, so the listener responds. Since there will be a response, our masking protocol dictates that we increase the noise to 45 dB in our pursuit to define the plateau. This time 5 dB of noise crosses over (45 dB noise - 40 dB interaural attenuation = 5 dB crossover), and this 5 dB noise will mask out the 0 dB HL right bone threshold. As a result, the level of the tone must be increased to 5 dB. Alternately, the levels of the tone and masking noise are increased, attempting to find the intensity of the tone that can be heard with at least three increases in masking level (i.e., the plateau). But it never takes place. Refer again to Figure 3-3; in our present example the shifting and overmasking parts of the masking curve are united with no plateau between them. (Restart the test with 5 dB of masking in order to the plot the complete masking curve and note that a plateau never emerges.) This example demonstrates a case in which the masked threshold cannot be measured: thus a masking dilemma. Suppose, instead, that we had decided to mask (retest) the left bone conduction threshold first. The same unfortunate situation results. As we apply greater and greater amounts of masking, a plateau never materializes. This could be predicted by looking at the head in Figure 4-4. The original masking intensity must be 40 dB, the degree of the


unmasked air conduction threshold. As the level of the masking stimulus is increased above 40 dB, it overcomes the conductive component in the right ear, reaches the right cochlea, and begins to shift the crossed-over left bone conduction threshold toward its true value of 60 dB HL. Specifically, 50 dB of noise will shift the threshold to 10 dB HL; 60 dB of noise will shift it to 20 dB HL, etc. Continuing along the same line, 100 dB of noise will finally shift the left bone conduction threshold to its real value, 60 dB, which is the first point on the plateau. But greater masking levels will cross over and mask out the test tone (e.g., 110 dB masking - 40 dB interaural attenuation = 70 dB crossover, and the actual left bone conduction threshold of 60 dB HL will be shifted to 70 dB HL, an overmasked condition). The final result is a masking dilemma when retesting the left bone conduction threshold, just as was the case when masking the right bone conduction threshold. You can discover this bilateral masking dilemma for yourself by testing Case 35 with circumaural earphones. This is a very important example; explore it thoroughly. Be sure to derive a complete masking curve for both left and right bone conduction. Carefully examine the shape of the curve. There is not a plateau; therefore, the true threshold cannot have been measured. Notice that plotting the standard curve using auto test does not reveal the correct masked threshold. For the right ear, you will observe that the dots change from green to red as the signal level advances from 60 to 70 dB to denote overmasking at this level, as pointed out in the previous paragraph. The details contributing to the masking dilemma just explained can be seen by viewing the Crossover diagram available within Auto test. Incidentally, if you have The Audiology Clinic mask right bone conduction, you will discover that it shows the correct thresholds, unobtainable by you due to the masking dilemma. The computer calculates the thresholds mathematically, not based on the realities of human anatomy. What does the audiologist do when realizing there is a masking dilemma? Remember foremost that there is a conductive loss in at least one ear: that’s valuable information. Also for some individuals interaural attenuation may be greater than 40 dB for air-conducted stimuli particularly at the higher frequencies, which will make establishing a plateau possible. Furthermore, pure-tone audiometry is just one test, and other information from the case history and immittance tests must be taken into account. Clinical decisions are based on a test battery, not just one test in isolation. There is something else that the audiologist can do to avoid the masking dilemma. Recall from Chapter 2 that we said that the greatest advantage to be gained by the use of insert earphones is increased interaural attenuation. Retest case 35 using insert earphones. This time


you will discover that the masked bone conduction thresholds can be obtained in the left ear. Even though overmasking still occurs when trying to mask right bone conduction, the astute clinician can deduce that the correct bone conduction thresholds are 0 dB in the right ear and 60 dB in the left ear. This conclusion can be reached by process of elimination. Recall the unmasked bone conduction thresholds of 0 dB. Since the correct left bone threshold is 60 dB that leaves only right bone to have responded at 0 dB. This case illustrates the very situation in which insert earphones make a real difference in the results that can be obtained. Finally, complete the examples in this chapter by determining the thresholds for Cases 36 through 40 (not available in the Lite edition), which will offer additional cases like those illustrated by Figures 4-2 and 4-3. Distinguish the subtle difference between right bone results at 2000 and 4000 Hz in Case 37 with circumaural earphones. Realize in one instance you can determine three points on the plateau and thus a valid masked threshold, whereas in the other instance you can find only two points, which makes the validity of the threshold questionable. In such cases the results at other frequencies, and other information such as immittance results and case history, are of the utmost importance in leading you to the correct clinical interpretation. To gain additional practice, return to Chapter 3 and mask the bone conduction thresholds of those audiograms whenever necessary.

Summary This chapter has discussed the requirements for masking bone conduction thresholds as represented by the second rule of masking. All bone conduction thresholds must be retested when an air-bone gap is present except in one circumstance. This exception exists when there are bilateral air-bone gaps, a condition which dictates that both bone conduction thresholds are initially suspect. If, however, after retesting one of these bone conduction thresholds, it becomes poorer, then the other unmasked threshold was a true threshold and need not be reevaluated. In the next chapter we will examine an instance in which an air conduction threshold may have to be masked due to the results of masked bone conduction.

Instruction Manual Audiology Clinic V2 Chapter Five 61

Chapter Five

Re-examining Air Conduction Thresholds When the necessity to mask was first discussed in Chapter 3, only the air conduction thresholds were considered. Masking was predicated upon a 40 dB or greater difference between the left and right unmasked air conduction thresholds at any frequency for circumaural earphones and a 60 dB or greater difference for insert earphones. The fact of the matter is, however, that a crossed-over signal is transduced or "heard" by the cochlea; and the sensitivity of the cochlea is directly reflected by the bone conduction, not the air conduction, threshold. As a result the need to mask a given air conduction threshold must be based upon the opposite bone conduction threshold, not the opposite air conduction threshold. Why, then, was this not revealed earlier? Several reasons support the current organization. The first rule of masking is quite valid, and in many instances a 40 to 60 dB difference between air conduction thresholds does materialize when left and right air conduction thresholds are measured (recall that both air conduction thresholds are obtained before any bone conduction thresholds are acquired). In this situation, it is necessary when using circumaural earphones to retest immediately all air conduction thresholds that are 40 dB or poorer than the opposite threshold without ever removing the earphones and placing the bone conduction vibrator. The crucial difference would be 60 dB for insert earphones. Furthermore, there may be some instances in which only air conduction testing can be done due to time, equipment, or conditions of the testing environment, so it is important to realize the need to mask based on the air conduction results alone. Finally, most complex processes are easier to learn when subdivided into several segments; presumably this is true of masking. With this background, the third rule of masking is defined in the following box.

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MASKING RULE NO. 3

An air conduction threshold at a given frequency in a given ear must be masked whenever it is 40 dB or more poorer than the bone conduction threshold in the opposite ear when using circumaural earphones. It must be masked whenever it is 60 db or more poorer when using insert earphones. It is assumed that the bone conduction threshold in the opposite ear has already been masked, if necessary. To clarify the order of testing, consider the following sequence of events. First, air conduction thresholds are acquired in both ears, masking in accordance with the first rule, if required. Second, bone conduction thresholds are acquired in both ears, masking, if required, by the second rule. At this point the air and bone conduction thresholds on the audiogram can be examined and the third rule of masking applied. Each air conduction threshold is compared to the opposite bone conduction threshold to reveal the necessity of retesting. If needed, the bone conduction vibrator is then removed, and the earphones once again placed over the ears. It is important to emphasize that there is no way to recognize that further air conduction testing is mandatory until after the bone conduction testing is completed, including any masking of bone thresholds that may be needed.

Retesting Only One Ear A significant aspect of the third rule of masking is that it pertains only to one ear, just as was the situation with the first rule of masking. An example will help to clarify the point being made. Let’s assume that we are using circumaural earphones. Inspect the portion of an audiogram in Figure 5-1(a). Look first at the air conduction thresholds: they do not qualify for masking according to the first rule because there is not a 40 dB or greater disparity between them. Then realize that both bone conduction thresholds may need to be masked due to the second rule (remember that if the first bone threshold to be retested becomes poorer, the other threshold was originally correct and need not be masked). Let us assume that after masking bone conduction, the audiogram appears as in Figure 5-1(b). Lastly, apply the third rule of masking: the right air conduction threshold is 40 dB below the left bone conduction threshold; therefore, the right air conduction threshold must be retested. If the initial response at 40 dB HL was due to crossover, then the real right air conduction threshold will be poorer; or, if the recorded threshold was really heard in the right cochlea, then the masked threshold will remain the same. Recognize


that the crucial difference between right air conduction and left bone conduction would be 60 dB for insert earphones.

Figure 5-1. Configuration (a) in which bone conduction must be masked. After bone conduction has been masked (b), right air conduction must be masked. Because the third masking rule is applied after masked bone conduction results are finalized, you should realize that this rule should now be applied to the audiograms in Chapters 3 and 4. Some of those air conduction thresholds (e.g., Case 23) will require further testing according to this third rule, so it would be beneficial to reexamine your results and complete those tests where required.

Practice The masking procedure itself is exactly the same when applying the third rule of masking as it was for the other two rules. The object is to find the plateau, if there is one, because the plateau reveals the true threshold. But, before reaching the section of the test where the third rule can be executed, it is first necessary to find the air conduction thresholds bilaterally (masking by the first rule, if warranted), then measure the bone conduction thresholds in both ears (masking by the second rule, if required), and finally inspect the results to determine whether any of the air conduction thresholds must be retested by the third rule. Proceed with your practice by selecting Case 41. After that examine Cases 42 and 43. These three cases will reveal instances in which there are thresholds that have crossed over and other cases in which the original thresholds are correct.

Disparity Between Air And Bone Thresholds

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Up to this point there has generally been an exact association between the air and bone conduction thresholds when the audiogram represented normal hearing or a sensorineural loss. An example of this is shown in Figure 5-2, wherein the right ear has both air and bone conduction thresholds of 0 dB HL, and the left ear has equal air and bone thresholds of 30 dB HL, signifying a mild sensorineural loss. This equivalence was fabricated to permit easier learning of the different kinds of hearing losses; i.e., conductive, mixed, and sensorineural.

Figure 5-2. Equivalent air and bone thresholds. Such precise correspondence between air and bone conduction thresholds is not commonplace in the clinical situation. Rather there will frequently be slight differences between the respective air and bone conduction thresholds at nearly every frequency for both normal hearing and sensorineural losses. A typical example is presented in Figure 5-3, which depicts a listener with hearing within the normal limits in the lower frequencies, and a mild sensorineural loss in the higher frequencies. Such an audiogram is typical of may individuals in the fifth or sixth generation of life.

Figure 5-3. Typical audiogram revealing intermingled air and bone conduction thresholds.


Notice that the air and bone conduction thresholds intermingle, with a bone conduction threshold occasionally being slightly better or even somewhat poorer than the corresponding air conduction threshold. It was stressed in Chapter 4 that a bone conduction threshold can never be poorer than the respective air conduction threshold. But audiometry is somewhat imprecise due to a multitude of factors including calibration of the pure-tone signals, accuracy of the tester in following the exact modified Hughson-Westlake protocol, and several aspects of the listener's behavior, including alertness, motivation, cooperation, internal physiological noises (stomach, arteries, etc.). The consequence is that a bone conduction threshold is occasionally measured as being 5 dB or even 10 dB poorer than the corresponding air conduction threshold at the same frequency. Similarly, a bone conduction threshold can be realized as 5 or 10 dB better than the corresponding air conduction threshold and not be a true, though minimal, conductive loss. The frequent inequality of thresholds has an important implication for masking with regard to Masking Rule No. 2. Inspect the bone thresholds in Figure 5-4a. They differ by 5 dB with the left ear appearing to have the better sensitivity, but they should be viewed as equivalent due to the multiple factors that determine the accuracy of a threshold. Let’s assume that the clinician decided to mask left bone first. The result is indicated in Figure 5-4b: left bone did not shift after masking, so according to Masking Rule No. 2, right bone now must be masked. Indeed, after masking, right bone did shift as demonstrated by Figure 5-4c. Remember that differences as small as 5 dB or even 10 dB cannot be regarded as significant differences and do not necessarily represent better or poorer hearing sensitivity.

01020304050

0 10 20 30 40 50

01020304050

(a) (b) (c)

Figure 5-4. Masking Rule No. 2 and unequal unmasked bone conduction thresholds.

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Realistic Clinical Responses By now it is expected that you can find thresholds with considerable ease and readily apply the three rules of masking. Therefore, the remainder of the practice audiograms in this chapter will reveal results more representative of actual everyday clinical experience; i.e., the air and bone thresholds will not always match up exactly for normal hearing and sensorineural losses. This realization will necessitate that you inspect the entire audiogram to determine overall the kind of loss that is present, rather than considering each frequency individually. Like much of life, the audiogram reveals many "gray" areas. Only extensive real-world experience soothes the qualms that arise when not all of the air and bone conduction thresholds are equal as in the "textbook" cases presented heretofore. Select the remaining cases in this chapter, cases 44 and 45, to assess hearing losses chosen from clinical files.

Summary Chapter 5 has addressed the necessity of reexamining air conduction thresholds after masked bone conduction thresholds have been realized. In cases where the air conduction thresholds in one ear differ by 40 dB or more from the masked bone conduction thresholds in the contralateral ear, they must be retested. Chapter 6 will project you farther into the "real world" by introducing variability in the listener's responses to the tone.

Chapter Six

Instruction Manual Audiology Clinic V2 Chapter Six 67

Variability In Listener Responses All of the topics have been covered that are prerequisite to obtaining correct pure-tone thresholds. These include the modified Hughson-Westlake threshold procedure and three rules of masking which account for every type of configuration. Only one situation has not been explored: variability in the listener's responses. To facilitate rapid learning of the basic techniques used to determine pure-tone thresholds, the listener has been, in every case, totally consistent. Complete reliability is not realistic; rather in the actual clinic there will be many instances wherein the listener behaves unreliably. For instance, assume that when testing is first begun, the right air conduction threshold is obtained at 1000 Hz and found to be 35 dB HL. The ASHA protocol dictates that this threshold be remeasured after the other thresholds in that ear are recorded before proceeding to test the left ear. This being accomplished, the retested threshold is found to be 45 dB HL, a 10 dB difference. This confirms one instance of variability. Now consider a different situation. It is most easily expressed in graphical form, so refer to Figure 6-1. Notice that when the intensity of the tone was increased in the threshold measurement stage of the procedure (beginning with presentation 3), the listener responded at three different levels that are circled: 25 dB, 35 dB, and 30 dB. What is the threshold? It has not been determined yet, as there have not been three responses at the same level while increasing the intensity from below threshold to above threshold. In fact there have not even been two responses at the same level; each response has been at a different intensity. So the threshold determination must progress until three responses at the same intensity are recorded. This example demonstrates the kind of variability which some listeners exhibit. Needless to say, such behavior is very frustrating to the clinician.

1 2 3 4 5 6 7 8 9 10 11Presentation

010203040

dBHL

RN

RN

NN N

RN R

Figure 6-1. Graph of inconsistent responses. There are many reasons for variable responses. Some have to do with the current physiological state of the listener. He or she can be

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momentarily distracted by a pain or an itch. He or she can become drowsy, or too hot or too cold. He or she can have interfering tinnitus. He or she can purposely be attempting to falsify the true threshold. There are a plethora of causes, many of which can never be explained. The occurrence of variable responses makes the clinician's task considerably more difficult. Often persistence will lead to satisfying the threshold criteria, although sometimes clinical intuition must be invoked when three responses at the same intensity cannot be found. Results may be obtained that require a special message to be added to the audiogram; phrases such as "reliability: fair" or even "reliability: poor" may be included under the "comments" section. The nuisance of unstable responses becomes increasingly magnified when attempting to obtain masked thresholds. Bear in mind that the tone is ordinarily presented only once at each level during the masking process. Additionally, the plateau representing the actual threshold may be only 5 or 10 dB wide, thus variable responses can entirely obscure your observation of a plateau.

Practice Because fluctuating responses are very much a part of everyday audiometry, they have been included as part of The Audiology Clinic. The audiograms for this chapter, which includes Cases 51 - 55, will supply ample opportunity to sample the "real world" in terms of variable responses by the listener. These responses will change over a range of 10 dB, and the variation from the exact threshold for a given audiogram will take place about 50% of the time on the average. The percentage is selected randomly by the computer. Thus, the same audiogram may represent different behavior each time it is tested; that is, one time it may display minimal, or even no, variability and the next time extreme inconsistency. Practice, using all of the skills you have acquired from studying the previous chapters. These audiograms are guaranteed to be challenging!

Summary This chapter completes your training in pure-tone audiometry. Simulation offers the safe environment in which the student or clinician can experiment, repeat, and practice without worry about the listener's reaction to his or her mistakes. Therefore, redetermine the many audiograms over and over again until such time as you can record the correct results in no more than 15 minutes for even the most complex configuration. With the ability to apply the rules and

Instruction Manual Audiology Clinic V2 Chapter Seven 69

procedures presented in the previous chapters, you should have no difficulty when testing a live listener.

Chapter Seven

Aural Acoustic Immittance Use of the immittance meter allows one to contribute substantially to the diagnosis of middle ear disease, separate cochlear from retrocochlear dysfunction, and even assist in the prediction of hearing sensitivity. The immittance test battery, completed in less than 10 minutes, can provide the information to make all these clinical discriminations as well as provide a wealth of related information. The auditory system, being an exquisite sound analyzer, is not without a good bit of complexity in the manner in which its dysfunction is revealed. Thus, while data from the immittance test battery provide a quick and reliable means of assessing the entire anatomical chain of structures in the ear, the task of arriving at a defensible conclusion from the information obtained requires the integration of a considerable breadth of knowledge. With this in mind, it is the primary goal of this text and software to present the essential components of electroacoustic immittance and through the presentation of case studies to teach the integration of clinical findings into comprehensive, defensible conclusions about the state of the auditory system.

Impedance Basics The movement of mechanical systems like the middle ear is governed by physical properties common to all vibrating systems. Whether

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mechanical, electrical or acoustic, a system's impedance describes the degree of opposition to the flow of energy through that system. In the ear, we are interested in the acoustic impedance produced by the middle ear and cochlear structures. The acoustic impedance of the middle ear is comprised of two elements, resistance and reactance.

Resistance The friction created by moving structures that are coupled together produces an energy dissipation termed resistance. While the middle ear system is an efficient sound transducer, it is not perfect. The manner in which the ossicular structures are suspended in the middle ear along with the interaction between the inner ear fluids and the basilar membrane creates a resistance. This resistance may then be thought of as the byproduct of the friction created by the conversion of acoustical to mechanical vibration in the middle ear and by the conversion of hydro-mechanical to electrical energy in the cochlea.

Reactance The second component of middle ear impedance is quite unlike resistance. Whereas energy is dissipated and lost through resistance, it is stored through reactance. Systems that vibrate have a certain mass and a certain stiffness, and it is not too difficult to visualize the middle ear structures rocking back and forth under the influence of sound energy moving the tympanic membrane. That element of impedance related to the middle ear structures reacting to movement is termed reactance. The mass and stiffness of the structures each oppose separately the positional change brought about by sound energy. Thus, reactance is made up of two components: mass reactance and stiffness reactance. Mass reactance is a product of the mass and resulting inertia of the ossicular chain. Stiffness reactance is generated by the tympanic membrane, and the oval and round window membranes. See Zwislocki (1976) for a review of the resistance and reactance components of middle ear impedance.

The Problem Of Timing When sound waves set the middle ear structures in motion, the characteristics of mass and stiffness reactance will dictate how the structures move. For instance, if the tympanic membrane (TM) and


ossicular chain have a high degree of stiffness, the condensation cycle of the sound wave will displace the TM in proportion to the sound pressure exerted, and the TM's motion will follow closely the pressure cycle of the sound wave. In contrast, in a middle ear that is dominated by mass, the inertia of the structures prevents them from moving in unison with the pressure cycle of the sound wave. This is not unlike what we experience when attempting to move a heavy object. The inertia of a piano (even on coasters) prevents the initial force of our pushing from immediately creating movement, unlike the push and subsequent movement of a much lighter object where there is little time lag between the applied force and subsequent motion. The model describing how resistance and reactance interact must account for the fact that the middle ear system responds differently when it is dominated by stiffness as opposed to when the mass components dictate its movement. The stiffness and mass components of impedance affect the time lag between the sound pressure wave impinging on the tympanic membrane and its subsequent movement in a manner that allows us to consider them as opposites. They are not opposite forces, but the sound wave cycle and the movement cycle of the structure that the sound wave influences are out-of-step with one another in mass and stiffness dominated systems. As such, their relationship lends itself to depiction in a phasor diagram where stiffness reactance and mass reactance are 180o out of phase with one another and the terms are labeled: RA = Acoustic Resistance XA = Mass Reactance -XA = Stiffness Reactance To observe how the impedance elements are diagrammed, click Immittance. From the submenu displayed, click Impedance and select Example 1. Now, click the Next in sequence to observe the impedance elements diagrammed. This example assumed that we measured in arbitrary units the following values for reactance and resistance: XA = 1.0 -XA = -5.0 RA = 4.0 The net reactance of -4.0 is the arithmetic sum of the positive mass reactance (1.0) and the negative stiffness reactance (-5.0). The


impedance value is the vector quantity derived by forming the hypotenuse of the right triangle, and ZA = Acoustic Impedance is 5.7. Note that the phasor points to the "stiffness dominated" quadrant (i.e., lower half of screen). This does not mean, however, that the ear did not have any mass reactance. Rather, this ear had more stiffness than mass. From geometry, recall the similarity in finding the impedance value to that of solving for the hypotenuse of a right triangle when the length of the two sides is known. The relationship between XA and -XA (both XA terms are summed to a single value) and RA can thus be expressed as:

That is, the absolute impedance equals the square root of the sums of resistance and reactance squared. Impedance is expressed in acoustic ohms, a unit of measurement that conveys the degree of opposition to the flow of sound. We use the notation |ZA| rather than ZA to imply that the above formula specifies the magnitude of impedance only. For, without also specifying the fraction of time between sound energy impacting the middle ear and its resultant motion, any number of combinations of resistance and reactance could yield the same absolute impedance. Now click Immittance, Impedance, and Example 2. Observe that the resulting impedance value is the same as that in Example 1 even though the individual values of the components were different. Note that both systems have the same absolute impedance, yet Example 1 is a system dominated by stiffness; whereas Example 2 is a system dominated by mass. In summary, impedance is the vector quantity of resistance and reactance that is described completely when phase information is provided. See Lilly (1972) for a thorough explanation. The terminology used to specify completely an ear's impedance includes the frequency of the sound used to stimulate the ear as well as the phase angle. In so-called "polar" notation:

ZTM = 1530 \ -74.1°\ 250 Hz is read, "impedance at the TM (tympanic membrane) equals 1530 acoustic ohms at negative 74.1 degrees for 250 Hz. Clearly, this impedance value is considerably larger than the abnormally small values used to illustrate the principles of calculating impedance in the computer simulation above.


You may experiment with different values of resistance and reactance by doing the following: Click Immittance, Impedance, and Custom values. Then follow the instructions to enter your own values.

Acoustic Admittance Measuring the impedance of the middle ear is not without some technical difficulties. It turns out that it is easier to measure the flow of acoustic energy through a system than to measure its impedance. See Van Camp and Creten (1976) and Newman and Fanger (1973) for a complete discussion. Unlike impedance, admittance is an expression of the ease of energy transfer through the middle ear. Whereas the symbol for acoustic impedance is ZA, the symbol for acoustic admittance is YA. Impedance and admittance are reciprocals such that:

The impedance components of reactance and resistance have counterparts in admittance termed susceptance and conductance. Their relationship is seen below.

ACOUSTIC IMPEDANCE |ZA| ACOUSTIC ADMITTANCE |YA| •Resistance (RA) •Conductance (GA) •Mass Reactance (XA) •Negative Susceptance (-BA) •Stiffness Reactance (-XA) •Positive Susceptance (BA) |Z |= ( R ) +( X )A A

2A

2 | |A A2

A2Y = ( G ) +( B )

Since impedance and admittance are reciprocals, the ear with otitis media, for instance, will exhibit a high impedance to the transmission of sound energy and a low admittance to the transmission of the same sound energy. To review, the impedance of the middle ear governs its capacity to move in response to the sound waves impinging on the tympanic membrane. Since measuring the impedance of the middle ear poses technical difficulties, it is more feasible to measure the admittance of the ear. These two measures, impedance and admittance share a reciprocal relationship. Thus, an ear with otitis media would have a higher than normal acoustic impedance (ZA) while the same ear would have a reduced acoustic admittance (YA).


Static Admittance The admittance of the ear is recorded in one of two measurement states: 1) while air pressure against the eardrum is systematically changed (tympanometry) and 2) while the eardrum is at rest (static admittance). While it is confusing to navigate through the impedance and admittance nomenclature, it is important to grasp how the two concepts are related as admittance concepts are used more frequently in the audiology literature. With low frequency stimulation of the ear the mass (-BA) and resistive (GA) elements of admittance contribute little to the overall admittance of the middle ear system. Rather, the stiffness component, positive susceptance (BA), derived from the tympanic, oval and round window membranes, predominates when stimulated by low frequency sounds. Note that this is true whether one is measuring in admittance or impedance units. Stated another way, at low frequency stimulation the ear is dominated by the stiffness reactance component which in admittance terminology is the positive susceptance (BA). Because the stiffness component predominates at low frequencies it is said to be "stiffness dominated." Describing the stiffness of the ear is made simple by equating it to the compliance of a volume of air that is just as compliant. The logic of equating the ear's compliance to an equivalent volume of air is straightforward. An enclosed cavity of air has a certain compliance or springiness to it. For instance, when attempting to inflate an air mattress, one notices it is easier to inflate when there is little air in the mattress than when nearing the end of the process and the mattress is close to full inflation. This is because the enclosed volume of air has a springiness or compliance that is dependent upon its volume. An unfilled air space having a large volume (such as a very large, deflated balloon) is highly compliant when inflation begins. Near full inflation there is very little space left for air to fill, and the compliance is reduced considerably. Since cavity size determines compliance, we may equate a structure's compliance to that of an equivalent volume of air. Whether the immittance manufacturer describes their admittance-measuring instrument as an impedance bridge, a middle ear screener, or an otoadmittance meter, the admittance of the ear is usually equated to the compliance of an equivalent volume of air, and expressed in cubic centimeters (cm3) or milliliters (ml). Aural acoustic immittance is the term used to denote the impedance and/or admittance as it is measured by commercially available


instruments. Refer to the ANSI standard (ANSI S3.39-1987) for a description of the specifications for aural acoustic immittance instruments. Observe the following patient data: Right: Left: YTM = 0.85 ml YTM = 0.40 ml @ 226 Hz @ 226 Hz We may conclude: 1.The admittance (YTM) at the right ear TM is 0.85 ml when measured with a 226 Hz tone. 2.The admittance (YTM) at the left TM is 0.40 ml as measured with the same frequency. 3.The right ear is more compliant than the left (because its admittance value is greater) or said another way, the left ear has more stiffness. 4.The right middle ear has the same overall admittance as an enclosed cavity of air of 0.85 ml. But, the external canal is not 0.85 ml in volume, nor are any of the cavities or structures 0.85 ml. The right middle ear system does, however, have the same overall admittance that a cavity of air having a volume of 0.85 ml has. Recall that acoustic admittance as expressed in equivalent volume is appropriate only when the ear is stimulated with a low frequency tone. This is because at low frequencies the mass and resistive components are negligible in normal middle ears and the stiffness component (BA) is very close to the overall admittance (YTM). To illustrate, the left ear of one of the authors was measured in sequence first for YTM @ 226 Hz (the overall admittance being expressed in equivalent ml) then only for acoustic susceptance (BA) @ 226 Hz (expressed in mmho) with the following results: YTM=1.1 ml, BA=1.0 mmho. Thus, YTM @ BA for low frequency sounds. The mmho (pronounced milli-mow) is the unit of admittance measurement and for practical purposes is used when the admittance components cannot be expressed in equivalent milliliters. This occurs at higher frequencies where the mass and resistive components play a greater role in the overall middle ear response. At middle ear resonance, approximately 800-2000 Hz (Hunter and Margolis, 1992), the mass and stiffness components are equivalent. Above the resonance frequency the system's overall admittance is controlled more by the mass component. Because of the additional electronics required to measure the other admittance components and


the high degree of clinical sensitivity provided alone by the 226 Hz probe tone, some immittance manufacturers do not include the instrumentation to measure the mass and resistive components separately. However, some middle ear pathologies clearly are better detected with higher frequency probe tones. When the higher frequency probe tones are used, the mass and resistive components of the total admittance are measured individually. For higher frequency probe tones, such as 678 Hz or 1000 Hz, the mmho is the unit of measurement. One may see notation such as: YA(678 Hz) = 5.62 mmho; that is, the admittance measured with a 678 Hz probe tone is 5.62 mmho (see the table below). Commercially available software has allowed for the measurement of immittance components from 250 to 2000 Hz as well as determining the ear's resonant frequency. While beyond the scope of this program, multi-frequency tympanometry will be a valuable clinical procedure in further explaining the dynamics of middle ear dysfunction. Hunter and Margolis (1992) provide an excellent tutorial on multi-frequency tympanometry.

The Relationship between Test Frequency and Measurement Unit Low Frequencies (220, 226 Hz) High Frequencies (678, 1000 Hz)

• Overall admittance (YA or YTM) expressed in equivalent volume (ml)

• Overall admittance (YA) in mmho • Susceptance (BA) in mmho

• Susceptance (BA) in mmho • Conductance (GA) in mmho

Summary This chapter has introduced the physical concepts essential to understanding the measurement units in the electro-acoustic assessment of the middle ear structures. In the next chapter you will begin using these concepts in the procedure known as tympanometry.

Instruction Manual Audiology Clinic V2 Chapter Eight 77

Chapter Eight

Tympanometry The basic immittance test battery consists of two tests along with the subtests associated with them. The two tests are: • Tympanometry • Acoustic Reflex Tympanometry is the measurement of the change in the admittance of the middle ear while a positive to negative air pressure sweep is introduced into the sealed ear canal. The admittance of the middle ear is deduced by measuring the ear's effect on a "probe tone" which is simultaneously presented into the canal.

Tympanometry Procedure The patient is seated while examination of the pinna and external canal structures is performed. Otoscopy of the canal and TM is carried out with the intention not only of assisting in the interpretation of the immittance findings but more importantly, to determine if a condition exists that would preclude performing the immittance tests. Such conditions include but are not limited to

foreign objects in the canal

drainage or open sores in the canal

whenever probe tip insertion is painful

recent TM or middle ear surgery Assuming the immittance instrument is in the tympanometry mode and the probe tone frequency has been selected, one may next insert the probe. A common technique is to insert the probe tip into the ear canal with one hand while the other hand gently pulls the pinna laterally and back with a motion similar to that used in otoscopic examination. This procedure is an attempt to slightly straighten the cartilagenous portion

78 Chapter Eight Instruction Manual Audiology Clinic V2

of the canal to assist in probe tip insertion. Upon probe tip insertion the air pressure sweep is commenced, usually by pressing a button, and the tympanogram is plotted. To observe the simulation of a tympanogram being plotted, click Case and Select Case. Choose Case 2. Now click Test, then Immittance. Click the Plot button on the lavender-shaded immittance instrument and observe the tympanogram being plotted from positive to negative pressure. You will have plotted the Right Tympanogram. To close the demonstration, click Case on the main menu and Close Case from the drop- down menu. Since it is the delivered air pressure changes in the sealed ear canal that produce the dramatic effects on sound transmission through the middle ear and generate these fascinating tympanograms, the measurement of air pressure is of substantial interest to us. Air pressure measurement in tympanometry is notably different than other expressions of air pressure. That air pressure is expressed in mmH2O is an extension of the convention whereby we express atmospheric pressure with a mercurial barometer by observing the height of a column of mercury that any given air pressure will support. In tympanometry, however, we use the term relative air pressure implying that the air pressure measured in the ear canal is the same as atmospheric pressure in the locale where the measurement is taking place. A +100 mmH2O ear canal pressure means the pressure is 100 mm greater than the ambient pressure. An ear canal pressure of -150 mmH2O means the pressure is 150 mm less than atmospheric pressure. The decaPascal (daPa) has replaced mmH2O as the unit of measure for air pressure. You may recall from studying the decibel that the Pascal (Pa) is the MKS (meter/kilogram/second) unit of pressure. Since 1.02 mmH2O is equivalent to 10 Pa or 1 daPa, we often interchange the air pressure units expressed in mmH2O with decaPascals. Thus, 50 mmH2O @ 50 daPa. Take note of the X (horizontal) and Y (vertical) axes on the screen. The X axis represents relative air pressure changes expressed in mmH20 or decaPascals (daPa). The air pressure began at +200 daPa, swept negatively through 0 daPa (atmospheric pressure), and ended at -400 daPa. The shape of the tympanogram may vary slightly as the rate and/or direction of the air pressure change is altered (Shanks and Wilson, 1986). The pressure extremes (+200 and -400 daPa) are used to place the tympanic membrane (TM) into an immobile position. Consider how tight and stiff the skin of a balloon is when it is fully inflated. Having the TM rigid effectively decouples (for measurement purposes) the middle ear from the ear canal which has its own impedance characteristics. When the TM is made maximally stiff and the admittance is measured with a low frequency probe tone, the admittance of the volume of air in the ear canal alone is estimated.


This is necessary since we ultimately want to know the admittance of the middle ear beginning at the TM, not at the end of the probe tip assembly, which would encompass a measurement of the column of air in the ear canal as well. Modern immittance meters perform the calculations that separate the admittance effects of the ear canal from the intended measurement of the middle ear. The Y axis represents the acoustic admittance expressed in milliliters (ml) of equivalent volume. Note that as the air pressure is reduced, admittance increases, then is maximized at 0 daPa, and again declines as negative air pressure stiffens the TM. The static admittance (YTM) is calculated by subtracting the peak admittance from the admittance observed at the ±200 daPa points. In this example the admittance of the ear is 1.1 ml. Note that the resulting trace, the tympanogram, is a measure of admittance as a function of changing air pressure. Within the probe tip assembly are three essential components used in the measurement of middle ear immittance; a loudspeaker, a microphone, and an air pump with a pressure transducer. To see an animated demonstration of the probe tip mechanism, click Immittance on the Main menu. Click Probe tip and observe the probe mechanism diagram. Click the Normal and Pathological buttons to observe the influence of eardrum stiffness on the measured sound pressure in the sealed ear canal during tympanometry. The point of the demonstration is to compare the size of the sound waves traversing the eardrum into the middle ear cavity vs. the size of the sound waves being reflected off the eardrum and back to the microphone at ambient pressure. It is instructive to stop and analyze what is actually taking place within the sealed ear canal as the tympanogram is being generated. First, it is essential that there be an air-tight (hermetic) seal of the probe tip within the confines of the external auditory canal. The sealing of the canal insures that the air pressure extremes targeted by the pneumatic pump may be realized. Once the +200 daPa air pressure is achieved, either manually by the clinician or automatically by the instrument, the loudspeaker transduces a 226 Hz pure tone into the ear canal. With the introduction of +200 daPa air pressure into the sealed ear canal, the eardrum becomes rigidly fixed in place. The rigid eardrum absorbs little (i.e., reflects most) of the probe tone, and for measurement purposes, essentially separates the canal from the middle ear. The microphone in the probe assembly measures the amount of sound pressure transmitted by the loudspeaker that is reflected off the eardrum. Furthermore, this sound pressure changes when the air pressure is varied from positive to negative pressure. As the air pressure is gradually reduced and the eardrum becomes more mobile, the sound pressure in the canal is


reduced as well, since a less rigid eardrum will transfer more energy through the middle ear than when rigid. As the air pressure approaches atmospheric pressure (0 daPa) the eardrum's mobility is maximized. This is because when the air pressure is equal on both sides of the eardrum (the normal situation); maximum sound transfer occurs creating a "peak" on the tympanogram. As air pressure varies from positive 200 daPa through 0 daPa, the intensity of the reflected probe tone decreases as TM rigidity is reduced. As air pressure moves through 0 daPa toward -400 daPa, the intensity of the reflected probe tone increases within the sealed canal as the TM is once again made rigid. So, changes in the reflected sound pressure of the probe tone indicate the amount of sound energy being transferred from the ear canal into the middle ear. The immittance instrument converts these probe tone sound level changes to a measure of one of several "immittance quantities" that may include susceptance (BA) expressed in acoustic millimhos, or compliance or admittance expressed as an equivalent volume, or even in arbitrary units. To review: 1.Air pressure is varied in the sealed ear canal. 2.The intensity of the reflected probe tone in the sealed canal changes as the air pressure influences eardrum mobility. 3.When the air pressure in the external canal matches that of the middle ear cavity, maximum sound energy transfer occurs, creating a tympanometric peak. 4.The air pressure where the peak occurs is an approximation of the air pressure in the middle ear cavity. 5.The amount of eardrum mobility observed at the height of the peak is expressed as the ear's admittance.

Tympanometric Norms The following norms were suggested by Margolis and Heller (1987) and presented in the ASHA guidelines for the screening of middle ear disorders (ASHA, 1990). The norms represent a 90% range of observed immittance values. The normative values for children are from 3-5 years. 1. EAR CANAL VOLUME: Children 0.4 - 1.0 cm3, mean 0.7 cm3; Adults 0.6 - 1.5 cm3, mean 1.1 cm3. The norms are expressed in equivalent cubic centimeters, (cm3 ) ,and denoted by Vec. Ear canal volume is a significant measure in that it can help eliminate the possibility of measurement artifact. For instance, an ear with a perforation of the TM may show a tympanogram of the same shape as


an ear having otitis media with effusion. One of the ways these pathological conditions are distinguished is with the canal volume measurement. When the TM is not intact, the probe tone circulates throughout the middle ear cavity as well as in the external canal. This will yield a volume measurement much greater than the 1.5 cm3 upper limit. The actual volume will depend upon the state of the middle ear mucosa. The canal volume measure can also aid in the detection of measurement artifact, especially in young children where a probe tip improperly positioned against the canal wall (easily done with young children) may produce a spurious tympanogram. The manner in which the canal volume measure assists in discriminating among several artifactual and pathological states will be presented in the Case Studies that follow. 2. PRESSURE PEAK: -150 to +50 daPa: The middle ear cavity may be thought of as a gas chamber capable of a range of internal static pressures. That the normal range of pressures includes both positive and negative values is recognition that the Eustachian tube is not perfect in its role of sustaining atmospheric pressure within the middle ear cavity, nor is middle ear pressure solely determined by Eustachian tube dynamics (Sadé and Luntz, 1990). Recall that the middle ear air pressure is denoted on the tympanogram by the place of the tympanometric peak on the X axis. Pressure values outside the normal range may signal the onset of conditions likely to result in otitis media. The clinical significance of pressure values outside the normal range is dependent upon other tympanometric factors such as the static admittance measure and the acoustic reflex test battery. It is a common mistake to over-interpret the tympanometric pressure peak since it may be transient, and its significance depends largely on the case history, otoscopic inspection, and other test data. The range of suggested normative values (-150 to +50 daPa) used in this program does not imply a cut-off for normal vs. pathological states. Rather, it is a guide based on clinical experience of the range of peak pressure values typically observed in healthy middle ears, and will be used to classify tympanograms to aid in their discussion. 3. STATIC ADMITTANCE: Children 0.2 to 0.9 ml, mean 0.5 ml; Adults 0.3 to 1.4 ml, mean 0.8 ml. Static admittance is the quantification of middle ear admittance, i.e., the height of the tympanogram (YTM). With experience, many clinicians find the shape of the tympanogram more useful than the admittance quantity per se. Recall that static admittance when measured with a 226 Hz probe tone may be expressed in equivalent volume, and as such equates the immittance of the ear to that of an equivalent volume of air. 4. GRADIENT AND TYMPANOMETRIC WIDTH: The visual inspection of the tympanogram yields much information. One of the finer points of tympanogram interpretation has to do with its shape


near the pressure peak, which the gradient measure quantifies (Brooks, 1969). The point of interest is the region + 50 daPa on either side of the pressure peak. The calculation of the gradient yields the ratio of the admittance around the tympanometric peak to the overall admittance of the ear. Typically, tympanograms characterized as rounded or shallow will yield gradients less than or equal to 0.2. A gradient greater than 0.2 is considered normal. Tympanometric width (TW) is likewise a measure conveying the “roundedness” of the tympanogram. TW is the amount of air pressure distance that a defined region near the peak of the tympanogram encompasses along the abscissa of the tympanogram. For instance, a rounded tympanogram may have a TW of 200 daPa, while a normal tympanogram may have a TW of 60 daPa. Normative values for TW relate more to referral criterion for screening of otitis media with effusion and as such will be discussed in the Immittance Screening section found at the end of this chapter. Tympanograms displaying comparative tympanometric widths are seen in Figs 8-1a and 8-1b. Both the gradient and the TW are tympanometric measures calculated by the immittance instrument. Both of these measures of tympanometric width has been shown to be associated with identification of middle ear effusion (Roush et al, 1995).

Figure 8-1a. Tympanometric width of 130 daPa


Figure 8-1b. Tympanometric width of 60 daPa

Tympanogram Classification The following tympanograms exemplify the classification system that audiologists have adopted for common tympanometric forms as conceived by Liden (1969) and Jerger (1970). Remember, the classification of a tympanogram is not used as the definitive means of contributing to the diagnosis of a middle ear condition. 1. TYPE A TYMPANOGRAM (see Fig 8-2) The type A tympanogram is the normal tympanogram and is characterized by: 1. Normal static admittance (as described by the height on the Y axis), and 2. Normal peak pressure (as described by the position on the X axis). Remember that the tympanogram reflects the status of the middle ear only. A patient with profound sensorineural loss will present with a type A tympanogram if the middle ear is normal. The type A tympanogram verifies normal TM mobility and normal ventilatory capacity of the Eustachian tube. Some cases of stapes fixation present the type A tympanogram.

Fig 8-2 Type A tympanogram 2. TYPE B TYMPANOGRAM (see Fig 8-3) The type B tympanogram has either a flat shape or a slightly rounded appearance. This results from a maximally stiffened TM usually due to middle ear effusion or its residual effects, and/or negligible air space in the middle ear cavity.


Fig 8-3 Type B tympanogram 3. TYPE C TYMPANOGRAM (see Fig 8-4) The type C tympanogram is characterized by excessive negative middle ear pressure as demonstrated by the peak of the tympanogram occurring at a value more negative than –

Fig 8-4 Type C tympanogram 150 daPa. The static admittance, YTM,(reflected by the height of the tympanogram) may not necessarily be affected by the negative pressure. 4. TYPE AS TYMPANOGRAM (see Fig 8-5) The AS tympanogram is characterized by normal middle ear pressure (remember that normal may include a range of pressure values from -150 to +50 daPa) but abnormally reduced admittance, YTM, (height). This tympanogram is observed in an ear with normal ventilatory capacity but an overall stiffened vibratory mechanism. Ossicular fixation and thickened TMs from chronic (but currently resolved) otitis media yield the AS tympanogram.


Fig 8-5 Type AS tympanogram 5. TYPE AD TYMPANOGRAM (see Fig 8-6) The AD tympanogram is characterized by: 1) normal middle ear pressure, and 2) excessive static admittance. Observe that the Y axis has been rescaled. Accordingly, this tympanogram extends to nearly 6 ml. Scarred tympanic membranes as well as middle ears with ossicular discontinuity will yield this tympanogram form.

Fig 8-6 Type AD tympanogram

Tympanometric Screening For Middle Ear Disorders While there is not a routine in this program to simulate tympanometric screening for middle ear disorders, the subject is of considerable interest to those who work with school-aged children. Screening for middle ear disorders (ASHA 1979, ASHA 1990) has evolved to the current state (ASHA 1997) in part, due to the recognition of troublesome medical over-referral rates of children using the previous screening guidelines. Because tympanometric peak pressure is quite variable, and negative middle ear pressure alone is a poor predictor of the onset of middle ear effusion, the present guidelines (ASHA 1997), focus on the tympanogram to determine the necessity for medical referral. The recommended referral criteria are: Ytm<0.3mmho or TW > 200 daPa. The 1997 guidelines also benefit from previously lacking specificity and sensitivity data provided by studies such as Nozza,

86 Chapter Nine Instruction Manual Audiology Clinic V2

1995; and Roush et al., 1995. You are encouraged to refer to the entire document (ASHA 1997) prior to undertaking screening for middle ear disorders.

Summary This chapter has described the fundamentals of single probe-tone tympanometry and introduced you to the basics of tympanogram interpretation. Acoustic reflex data adds a significant refinement to the diagnostic process. In the following chapter you will have a chance to integrate audiometric and immittance information in a case-by-case format.

Chapter Nine

Acoustic Reflex The reflexive contraction of the stapedius muscle upon presentation of a sufficiently intense sound can provide diagnostic information about the state of the middle ear, the cochlea, the VIIIth (auditory) cranial nerve, auditory brainstem pathways, and the VIIth (facial) cranial nerve. That the sound can be presented only to one ear yet produce a contraction in both ears has led to confusion regarding the reporting of

Instruction Manual Audiology Clinic V2 Chapter Nine 87

reflex thresholds. That is, a physiologic reaction to sound presented to, say, the left ear can be monitored in the right ear as well as in the left ear! Which ear then is being tested? It is conventional practice to specify acoustic reflex thresholds for the ear that is stimulated. A review of the neural pathway responsible for the reflex will help to clarify this issue and facilitate an understanding of the clinical significance of the test.

Ipsilateral Stapedial Reflex It is instructive to observe the reflex's ipsilateral pathway, the neural pathway responsible for a reflex contraction occurring in the same ear that received the reflex eliciting sound. This can be accomplished in the clinical setting because the reflex-eliciting sound stimulus can be presented to each ear individually, whereas in everyday life it is more likely that both ears would be in the sound field that elicits the reflex. To observe an animation of the ipsilateral reflex arc click Immittance on the menu. Next click Ipsilateral reflex arc and proceed as indicated. Upon sound stimulation, the sensory route begins with neural discharges from the cochlea, in this case the right cochlea, through the auditory nerve to the first complex in the central auditory nervous system, the cochlear nucleus. Describing the neural action in its simplest form, when the neural discharges received at the right cochlear nucleus reach a threshold value, a pathway connecting the cochlear nucleus to the right motor nucleus of the facial nerve is activated, resulting in a contraction of the right stapedius muscle. Note, that not animated is the fact that when right cochlear nucleus is activated, the superior olivary complex on the opposite, left side of the brainstem is stimulated, initiating a reflex contraction of the left stapedius muscle as well. Thus, the ipsilateral stapedial reflex does not truly exist in the natural state because both left and right stapedial muscles contract. The term does however have significant clinical relevance as will be discussed below. Whether the tensor tympani muscle is activated in concert with the stapedius muscle remains unresolved. Borg (1973, 1977) claims that in humans the tensor tympani does not contract in response to acoustic stimulation. Not shown in the animations are the other neural pathways that demonstrate that the acoustic "reflex" is not entirely reflexive in that higher auditory centers may influence the occurrence and threshold of a contraction. Influences such as immediately precedent stimulation and anticipation of a loud sound are examples of such mediating factors. To observe an ipsilateral acoustic reflex, the probe tip is hermetically sealed in the ear to be tested, and the immittance meter is changed from the tympanometric mode to the reflex testing mode. The


results from tympanometry just obtained in the normal testing sequence are used in the reflex test in that the observance of the reflex is enhanced by presenting sound to the ear at an external canal pressure equal to middle ear pressure. Thus, if middle ear air pressure is -110 daPa, for reflex testing the external canal air pressure should be set at -110 daPa. An impedance mismatch occurs when there exists an air pressure differential between the external and middle ear air spaces which can inhibit the vibratory motion of the TM and ossicular structures. When testing reflexes, we want to maximize both the transmission of sound through the ear as well as our ability to detect minute changes to the ear's admittance caused by the resulting contraction of the stapedius muscle. Some electroacoustic immittance meters automatically set the reflex test mode's external canal air pressure to match that of the middle ear pressure just obtained from tympanometry. Note: In this computer simulation the appropriate air pressure for reflex testing is set automatically. Physiologically, sounds of sufficient intensity can elicit the acoustic reflex. Whether the reflex is governed by sound intensity or its loudness is open to debate (Margolis and Popelka, 1975). Whatever its triggering mechanism, the subsequent contraction of the stapedius muscle reduces the admittance of the middle ear thereby inhibiting the motion of the TM. The immittance instrument continually monitors the sound pressure of the probe tone in the external canal, so that any change in middle ear admittance will change the probe tone's transmission through the ear and hence its measured sound pressure in the sealed ear canal. To review the ipsilateral acoustic reflex: 1.The probe tip ear is the “test ear”. 2.A momentary (1-2 sec.) reflex eliciting sound is presented into the probe tip ear. 3.An acoustic reflex stiffens the ossicular chain and decreases acoustic admittance. 4.The probe tip microphone that has been monitoring the sound pressure of the probe tone in the ear canal records a sudden increase in sound pressure coincident with the decreased admittance of the middle ear. (Proportionally more of the sound wave is reflected off the TM and picked up by the microphone). 5.The nearly simultaneous reflex sound presentation and subsequent change in acoustic admittance is noted on a graphic display. To perform a threshold search for an ipsilateral stapedial reflex follow this procedure. (Note: If the simulation is not set up to test Immittance on Case 2 as on p. 58, click Case, Select Case, and choose Case 2.


Now click Test, then Immittance. Replot the tympanogram by clicking Plot.) Notice that under Stimulus Ear, Ipsilateral is selected and that under Probe Ear Right is selected. Under Mode click Reflex thresh., then click Plot and observe the trace for a right ipsilateral reflex at 500 Hz. The initial intensity is 80 dB. This produces a deflection of 0.05 ml, which is above threshold. Next reduce the intensity to 75 dB and click Plot; this results in a deflection of 0.02 ml which is the criterion we will adopt for threshold. Finally, reduce the intensity to 70 dB and click Plot; there is no deflection, so this intensity is below threshold. Accordingly, the ipsilateral threshold in the right ear at 500 Hz is 75 dB. Repeat this process by changing the frequency to 1000 Hz, 2000 Hz and finally to BBN (broad-band noise).

Contralateral Stapedial Reflex While the presentation of a loud sound to only one ear may elicit a reflex contraction, the reflex occurs in both ears. The bilateral nature of the stapedial reflex is best understood by observing its bilateral neural pathways. To see an animated demonstration of the contralateral reflex auditory pathway click Immittance. From the drop-down menu click Contralateral reflex arc and proceed as instructed. For contralateral reflex testing, the ear under test, in this case the left ear, receives the reflex eliciting sound, typically a pure tone, and the probe tip in the right ear monitors the occurrence of a reflex contraction. In the left ear, if the structures are intact and a tone is sufficiently intense, the neural output from the cochlear nucleus is sent to the ipsilateral and contralateral superior olivary complexes. From the left and right superior olives the respective motor neurons of the facial nerve are activated, resulting in a bilateral reflex contraction. As with the ipsilateral reflex animation, only one stapedial muscle is shown to contract. It is important to note that the right stapedius muscle does not contract because the left stapedius contracted. Rather, the neural signal for stapedial contraction was sent to both ears regardless of which ear received the sound. Left stapedial contraction was not a prerequisite for right stapedial activation. To perform a threshold search for a contralateral stapedial reflex follow this procedure. (Note: If the simulation is not set up to test Immittance on Case 2 as on p. 58, click Case, Select Case, and choose


Case 2. Now click Test, then Immittance. Replot the tympanogram by clicking Plot.) Notice that under Probe Ear, Right is selected. Under Stimulus Ear, click Contralateral. Under Mode click Reflex thresh., then click Plot and observe the trace for a left contralateral reflex threshold at 500 Hz at 80 dB. The deflection is 0.02 ml, which signifies a threshold. To confirm the validity of this response, repeat with an intensity of 75 dB (which yields no deflection—below threshold) and an intensity of 85 dB (which yields a deflection of 0.05 ml—above threshold). Note that the left ear was presented with the reflex-eliciting tone, while the presence of a reflex response was monitored in the right ear. Repeat the process at 1000 and 2000 Hz and for BBN. Finally click Tympanogram followed by Left ear and repeat the entire process for that ear. Given that we can measure both the ipsilateral and contralateral reflexes separately and given that we have a rudimentary knowledge of the neural architecture controlling the reflex, we can make some rather bold conclusions regarding the structural integrity of the peripheral and brainstem auditory pathways. Before we consider how auditory abnormalities influence the stapedial reflex, we should review the signal parameters under which the reflex functions normally.

Normative Stapedial Reflex Behavior 1. PURE TONES. For normal hearing subjects the acoustic reflex (ipsilateral and contralateral) is elicited by pure tones between 70 to 95 dB SL (Peterson & Liden, 1972). For the normal hearing patient this is 70 to 95 dB HL as well. The SL-HL distinction is important as we shall learn in discussing reflex thresholds for auditory disorders. 2. NOISE. The acoustic reflex is elicited by broadband signals, such as noises, at lower sound intensities than by pure tones (Peterson & Liden, 1972). The acoustic reflex threshold for white noise occurs about 10-15 dB below that observed for pure tones, and is related to the cochlea's summation of loudness for broadband signals. 3. TIME. The acoustic reflex follows the temporal (on-off) characteristics of a signal rather well so that an intermittent sound would produce a well-matched intermittent acoustic reflex. And as steady state sounds experience adaptation so, too, does the reflex. For reflex eliciting sounds above 1000 Hz, the reflex response declines in magnitude prior to 10 seconds of signal duration. A more rapid decay of the response, especially for sounds at and below 1000 Hz, is associated with acoustic neuromas. The test for this pathologic


condition is termed "acoustic reflex decay." (Consult Stach and Jerger, 1991, for a literature review). 4. THRESHOLD TECHNIQUE. Reflex thresholds may be obtained in an ascending or descending manner, usually in 2 or 5 dB increments. While the threshold searching method may vary, one should employ strict adherence to a threshold criterion. A 0.02 ml decrease in admittance has been recommended by several immittance manufacturers as the minimal reflex response upon which to base the presence of an acoustic reflex.

Clinical Patterns

Ipsilateral Acoustic Reflex in Sensorineural Hearing Loss For the purposes of the present discussion it is assumed that all the structures in the anatomical and neurological chain of the ipsilateral reflex arc are normal except for cochlear hearing loss. When the reflex eliciting signal of sufficient intensity is presented to the ear with the probe tip, an ipsilateral reflex is observed. Remember, the probe tip provides a means of both presenting a reflex eliciting signal and monitoring any admittance change that may result from the reflex. The reflex will occur in both ears, but for ipsilateral testing we record reflex thresholds only for the ear with the probe tip. Recall that in the normal hearing ear the acoustic reflex threshold occurs between 70-95 dB SL. In sensorineural ears however, the sensation level at which the reflex occurs is diminished as hearing loss increases. The audiograms on the next page illustrate this phenomenon.

Note that the hearing levels at which reflex thresholds were obtained were approximately the same for normal and sensorineural ears (85


dB). Therefore, the sensation level for reflex threshold decreases as a function of increasing hearing loss. The audiometric and reflex threshold data from the right audiogram are shown in the table below.

500 Hz 1000 Hz 2000 Hz 4000 Hz Air conduction HL 0 35 60 60 Reflex HL 85 85 85 85 Reflex SL 85 50 25 25

The rule is that reflex sensation level decreases with greater sensorineural hearing loss. This is the distinguishing reflex characteristic of sensorineural hearing loss, that in spite of the hearing loss an acoustic reflex may be observed for even severe hearing loss and may be elicited as low as 25 dB above the pure tone threshold (25 dB SL). What this means in practical terms is that if the output limit of the reflex eliciting tone is 110 dB HL, the maximum loss likely to produce a reflex is roughly 85 dB HL (110 minus the lowest likely SL at which the reflex is observed). In clinical experience, one seldom tests beyond 110 dB HL, so the upper hearing loss where a reflex may still be elicited is approximately 85 dB HL. It is important to remember that when testing the ipsilateral reflex, the state of the contralateral ear has no consequence on the outcome of the test ear, and no attempt is made to infer anything other than from the ear under test. As we shall see later with the contralateral reflex test, the state of the non-test ear is of paramount importance in interpreting information obtained from the test ear. It is often the case that the immittance test battery is the first and primary set of tests administered to a patient. The results then guide our subsequent diagnostic regimen. If this is the case, as one records reflex thresholds one will not be able to assess the reflex sensation level since the pure tone air conduction thresholds have yet to be obtained. This is not a drawback, however, as we ultimately rely on diagnostic patterns, the sum of which unfold as we progress through a battery of tests. When testing reflexes, caution should be exercised not to exceed a patient's comfort level for loudness or to present sound intensities capable of sustaining inner ear damage. A 500 Hz tone at 115 dB HL produces approximately 126 dB SPL, a potentially injurious level of sound even at short durations. There are occasions when one is only assessing the capacity of the auditory system to produce an acoustic reflex rather than to specify the reflex threshold per se. In such instances a broad band noise stimulus will elicit the reflex at considerably reduced sound levels, eliminating the necessity of excessive sound level presentation. Furthermore, an uncomfortably


loud sound may cause the activation of the facial nerve (and ultimately the stapedial branch of the facial nerve) through a "wince," that while appearing to be an acoustic reflex is instead a pure artifact of facial nerve activation. The following is a summary of sensorineural hearing loss and the acoustic reflex: 1.Reflex thresholds are approximately the same for the sensorineural and the normal ear. 2.Reflex sensation level (SL) is the air conduction (AC) threshold (in dB HL) minus the reflex threshold (also in dB HL). For example: Reflex Threshold=85 dB HL AC Threshold=10 dB HL Reflex SL=75 dB 3.Since reflex thresholds occur at the same Hearing Level (that is, the same place on the audiogram) for both normal and sensorineural ears, the reflex occurs at a reduced Sensation Level in the sensorineural ear. For example: Normal: Sensorineural: Reflex Threshold = 85 dB HL Reflex Threshold = 85 dB HL AC Threshold = 5 dB HL AC Threshold = 30 dB HL Reflex SL = 80 dB SL Reflex SL = 80 dB SL 4.An ipsilateral reflex testing the probe tip delivers both the probe tone and the reflex eliciting sound. The ear with the probe tip is the ear for which reflex thresholds are recorded.

Contralateral Acoustic Reflex In Sensorineural Hearing Loss In contralateral reflex testing the ear that is stimulated by the reflex- eliciting signal is the "test ear," and the contralateral ear has in it the probe tip which monitors the presence of a stapedial reflex contraction. Note, that the ear that is stimulated to have a reflex elicited is not the ear where immittance changes will be observed, but by eliciting the reflex in the right ear and monitoring its presence in the left, we can make inferences about the integrity of the entire contralateral reflex arc. Considering hearing loss of cochlear origin only, with normal contralateral neural pathways, what clinical patterns might be expected with sensory loss in the test ear? The first consideration is that the cochlear status of the probe ear is of no consequence. Refer again to the animated contralateral reflex arc diagram to verify that if the reflex is elicited in the left, whether the right cochlea is functional at all has


no bearing on the right facial nerve producing a reflex contraction of the right stapedial muscle. Because of this, the relationship between sensorineural hearing loss (SNHL) and reflex sensation level is the same for ipsilateral and contralateral testing. If the right test ear has a cochlear loss we can expect that the reflex will be elicited at reduced sensation levels re the right ear. After all, it is the right cochlea that is responding to sound. You may wonder of what use is the contralateral reflex test if the reflex thresholds are the same for ipsilateral and contralateral stimulation? Since ipsilateral and contralateral reflexes share the same sensation level relationship to pure tone thresholds, the power of comparing ipsilateral and contralateral reflex thresholds does not relate to any sensation level differences between the two modes of reflex stimulation. Rather, the power of the contralateral test is that many other structures have to be normal for the reflex to be observed. Moreover, by comparing the presence of the reflex as elicited ipsilaterally and contralaterally and by right and left ears we can deduce information about the integrity of important auditory and brainstem structures that cannot even be acquired by today's most sophisticated imaging technology. A 2x2 matrix (Figure 9-1) is useful in helping to see the significance of clinical reflex response patterns (see diagram below). Note very carefully what is implied by the terminology of the 2x2. "Right Ipsi" means that the reflex-eliciting stimulus is delivered to the right ear and that the probe is also monitoring the response in the right ear. In contrast, "Right Contra" means that the reflex-eliciting stimulus is delivered to the right ear, while the probe is monitoring the response in the left ear. Also realize the implications for measurement both in the clinic and with this simulation: the probe is physically placed in one ear at a time. If the probe is in the right ear, then right ipsilateral and left contralateral reflex thresholds can be obtained, before moving the probe to the left ear. Thus, referring to the 2x2 figure above, the shaded conditions would be obtained with the probe placed in the right ear, while the unshaded conditions would be obtained with the probe placed in the left ear. Assuming all other auditory and neural structures are normal, consider the clinical findings shown in the 2x2 matrix. Note that the decibel values in all the 2x2 matrices in the text


Figure 9-1. 2x2 matrix presenting reflex thresholds and in the simulation program are expressed in Hearing Level (dB HL). We conclude from these reflex thresholds: 1.For both ears reflex thresholds occur at normal Hearing Levels. 2.Since reflex thresholds occur at the same Hearing Levels in both the normal and sensorineural, ear we cannot conclude "normal hearing" from these findings alone. 3.Each ear may have as much as a 60 dB sensorineural hearing loss (85-25) since the acoustic reflex can occur as little as 25 dB above threshold in the cochlear-impaired ear. Note that the 2x2 matrix does not specify the frequency at which the reflex thresholds were derived. While there is a frequency effect for acoustic reflex thresholds (some normal hearing individuals have an absent or elevated 4000 Hz reflex threshold), none of the following examples has a frequency effect that would compromise the interpretation. For all subsequent 2x2 reflex matrices it is assumed that the reflex thresholds were elicited by a 1000 Hz tone. Fourteen (14) examples will now be presented. To see the necessary data, click Immittance and then click Examples. Note that in each


frame of the 2x2 reflex matrix, when the mouse pointer is in any given frame, a probe ear and stimulus ear diagram is displayed. In each of the following cases inspect the audiogram and the reflex thresholds. Note that the air conduction thresholds are designated by a red line for the right ear and a blue line for the left ear.

Refer to Example 1 Right Ipsilateral: Reflexes occur at normal Hearing Levels as expected. Left Contralateral (stim-L, probe-R): The left ear is presented with the reflex activating signal while it is monitored in the right. Again, the cochlear loss in the left is too great to elicit a reflex. It is of no consequence that the right ear (where the reflex is being monitored by the probe tip) has normal hearing. Remember, the cochlear status of the non-test ear does not influence a contralateral acoustic reflex. Left Ipsilateral: The left ear has too great a cochlear loss to produce a reflex from a 110 dB HL tone. Right Contralateral (stim-R, probe-L): Right contralateral means the right ear is presented with the reflex eliciting sound and it is the right ear for which we record its reflex threshold. The presence of the reflex in this condition is monitored in the left ear. Thus, with the reflex stimulus in the right ear and the probe in the left ear we would expect to observe stapedial reflexes at normal Hearing Levels. The cochlear status of the non-test left ear has no bearing on the observation of a reflex. The stimulation of the right cochlea activates a neural chain of events that by-passes the left cochlea. Consider these findings for a sensorineural hearing loss:

Refer to Example 2 Right Ipsilateral: The reflex sensation level is 45 dB (dB reflex threshold minus 50 dB hearing threshold) and consistent with SNHL. Left Contralateral (stim-L, probe-R) and Left Ipsilateral: The same findings seen in the right ear will apply to the left in this symmetrical cochlear loss. Right Contralateral (stim-R, probe-L): The observed reflex threshold is consistent with hearing loss of cochlear origin in the right. Consider these sensorineural hearing loss findings:

Refer to Example 3


Right Ipsilateral: While we expect the reflex to occur within a range of reduced sensation levels in the sensorineural ear, the upper limit at which we are willing to test may not yet be sufficient to elicit a reflex. As with this ear, the reflex may have been obtained beyond the 110 dB HL ceiling we have set. Left Contralateral (stim-L, probe-R): Stimulating the left ear produces reflexes at reduced sensation levels. The right ear thresholds do not influence our ability to observe the effects of a stapedial contraction in the right. Left Ipsilateral: The reduced reflex sensation levels are consistent with SNHL. Right Contralateral (stim-R, probe-L): The same reasoning holds for right ipsilateral and contralateral testing since in each instance it is the right ear that elicits the reflex. Conductive Hearing Loss. While elicitation of the acoustic reflex is expected in all but the severest of sensorineural losses, for most conditions affecting the conductive structures of the middle ear, the reflex is absent or elevated in threshold. Two generalizations regarding conductive hearing loss and the acoustic reflex are: 1.When the probe or "monitor" ear has a conductive pathology, the acoustic reflex will be absent. Even with neural pathway integrity and the resulting activation of the stapedius muscle, if the conductive condition either stiffens the ossicular chain and TM or decouples the ossicular chain from the TM, the acoustic reflex will not be observed. In the first instance, the ear already stiffened by otitis media is beyond further stiffening from the effects of a reflex contraction. If the ossicular chain is decoupled from the TM as is the case with ossicular discontinuity, the change in compliance produced by the reflex contraction is not transferred to the TM where its influence would affect the probe tone. 2.When the reflex eliciting signal is presented to the ear with the conductive pathology, reflex thresholds will be elevated or absent. That is, when the ear being stimulated has a 30 dB conductive loss, it will take 30 dB more sound energy to reach the cochlea and initiate the neural events precedent to a reflex contraction (to be monitored in the other ear). If the conductive elevation in threshold is great enough, there may be insufficient reflex eliciting intensity to produce a contraction, resulting in "absent reflexes." Consider the following case: Unilateral otitis media with effusion:


Refer to Example 4 Right Ipsilateral: The reflex threshold of 80 dB HL is expected since there is no conductive pathology in the right ear. The state of the left ear is irrelevant for ipsilateral right stimulation. Left Contralateral (stim-L, probe-R): In this situation the reflex may be elevated or absent. Consider that when this transient middle ear condition is resolved and the ear has normal air and bone conduction, a reflex threshold of 85 dB is measured. To overcome the current 30 dB conductive "pad" while stimulating the left, a 115 dB tone (85 + 30) would have to be used. Note that it is important to record the highest sound level used that failed to produce a reflex, since this sound level may convey the probable reason for an absent reflex. Left Ipsilateral: The absent reflex in the left to ipsilateral stimulation is consistent with our first rule, that when the probe ear has a conductive pathology, the acoustic reflex will be absent. The conductive type audiogram and type B tympanogram are consistent with otitis media. Right Contralateral (stim-R, probe-L): The right contralateral reflex is absent. While the right ear's normal conductive and cochlear function likely generate sufficient neural energy to elicit a reflex, it is the left conductively-involved ear that is the probe ear, hence the "absent reflex." Here the fluid-filled left middle ear is already stiffened to a degree such that even a strong reflex contraction would not alter the middle ear admittance. Now consider the second case of unilateral otitis media:

Refer to Example 5 Right Ipsilateral: The reflex threshold of 80 dB HL (and SL) is consistent with normal peripheral auditory function. Left Contralateral (stim-L, probe-R): Stimulating the left ear at an intensity sufficient to overcome the 10 dB pad while monitoring in the normal right middle ear produces a reflex at 95 dB. If reflex threshold is usually 80 dB in the left ear, then a 95 dB HL tone would overcome the 10 dB conductive threshold elevation and elicit a reflex. Left Ipsilateral: The reflex is absent since excessive stiffness in the left obscures a reflex. The conductive loss itself does elevate the reflex threshold, but the increased stiffness is the overriding factor for reflex absence.


Right Contralateral (stim-R, probe-L): While stimulating the right ear likely produced a bilateral reflex contraction, the conductive condition in the left probe ear obscured it. Consider the following case of bilateral otitis media:

Refer to Example 6 For each of the reflex testing conditions in the 2x2 matrix the probe ear has a conductive condition producing extreme stiffness with resultant absent reflexes. For instance, the right-ipsilateral condition would produce an absent reflex because the immobility of the TM in the right would not yield to the effects of an acoustic reflex. In the right-contra situation, the left ear is the probe ear, and the same generalization holds true. Even if sufficient intensity could overcome the conductive loss in the right and produce a contraction, the effects of that bilateral contraction would not be observed because of the stiffness of the left TM. Consider the following case of unilateral negative middle ear pressure:

Refer to Example 7 Right Ipsilateral: The extreme negative pressure changes the middle ear dynamics enough to prevent observing a reflex. Note that the negative middle ear pressure has not produced a measurable air-bone gap on the audiogram, yet the acoustic reflex is absent. Left Contralateral (stim-L, probe-R): The probe ear's stiffness from negative pressure obscures observation of a reflex. It is interesting that negative pressure may or may not yield an air-bone gap. As in this case, even if no measurable conductive loss exists, the tympanometric abnormality is often sufficient to yield the "absent reflex" state. Left Ipsilateral: Reflex observed as expected. Right Contralateral (stim-R, probe-L): The reflex falls within the normal 70-95 dB HL range, but we might suspect that it is slightly elevated due to the added stiffness of the stimulated right ear. It is worthwhile to note that the vibratory dynamics of the middle ear are altered by even the slightest of abnormalities that in this case are manifest in the right tympanogram but not on the audiogram. Observe the following case of bilateral occluded tympanostomy tubes (referred to as PE tubes hereafter):


Refer to Example 8 Occasionally an otologist may request a determination of PE tube patency. If the tube is patent, we will observe immittance findings similar to those of a TM perforation; flat tympanograms and excessive canal volume, with varying degrees of confirmation from audiometric air and bone conduction. When the PE tube is plugged, we often observe a type B tympanogram and normal canal volume. The present case illustrates this diagnostic pattern. Absent reflexes occur bilaterally as would be the case for any middle ear condition producing the degree of stiffness of these tympanograms. Remember, for each condition in this 2x2 matrix the probe ear has a conductive loss. Consider the following case of unilateral PE tube patency:

Refer to Example 9 Right Ipsilateral: Reflex response observed as expected. Left Contralateral (stim-L, probe-R): Observance of an elevated reflex is consistent with the conductive disruption in the stimulated ear requiring a greater intensity to elicit the reflex response. If the conductive condition were of greater severity, a stimulating tone of 100 dB would have been an insufficient reflex stimulator. Left Ipsilateral: With the probe in the left ear, we would anticipate absent reflexes here for the reasons detailed above. Right Contralateral (stim-R, probe-L): The general rule with the probe in an ear with a patent PE tube is to find an absent reflex. Consider that the reason for the tube was to evacuate the middle ear cleft of exudate and to permit air exchange. As such, the middle ear cavity is likely quite stiff and therefore unresponsive to further stiffening via stapedial contraction. Consider also that since the PE tube is patent, the probe tone is circulating throughout the external and middle ear cavities. If the ossicular chain was mobile enough to respond to stapedial contraction, the resultant change in probe tone intensity would likely be insignificant and not be considered reflective of a reflex response. Consider the following case of bilateral cerumen accumulation:

Refer to Example 10


Prior to immittance testing, otoscopic observation may reveal a cerumen occlusion that obscures a view of the TM. Tympanometry will show whether the cerumen is fully or partially occluding the external canal. Reduced canal volume and a flat tympanogram are consistent with occluding cerumen. If only a pin hole of air space around the cerumen exists in the canal, the probe tone may circulate and monitor TM dynamics during tympanometry and reflex testing. In this case, the normal tympanograms and reflex thresholds dictate that the cerumen has not fully occluded the external canal. This determination is useful in that since we know the TMs are intact, the canals may be irrigated. Had we discovered a TM perforation, cerumen management would have taken a different approach. Note that the reflex sensation levels are consistent with a sensorineural loss. Observe the following case of bilateral otosclerosis:

Refer to Example 11 The diagnostic signature of otosclerosis is reduced static admittance, absent or elevated reflexes, and an audiometric conductive loss. Reflexes may be present in the early stages of stapedial fixation. Reflexes are absent bilaterally in this case since in all test conditions, the probe ear is in an ear having conductive loss. Thus, in each of the 2x2 reflex test conditions a stapedial contraction would have no further influence on an ossicular chain already significantly reduced in compliance by the ossification process. Observe the following case of unilateral otosclerosis with stapedectomy:

Refer to Example 12 During the stapedectomy procedure the stapedial tendon is severed, rendering the stapedial reflex nonfunctional. That is, a reflex contraction would have no means to influence the movement of the stapes with the attaching tendon's disconnection. Right Ipsilateral: The reflex is absent as this ear has been stapedectomized. The air-bone gap in the right ear may or may not be of significant improvement over the presurgical audiogram. Yet, the primary consideration in the reflex's absence here is the fact that the ear has undergone a stapedectomy, rendering the stapedial reflex inoperative. Even if the air-bone gap had been completely closed with no measurable conductive loss, the reflex would be absent.


Left Contralateral (stim-L, probe-R): The bilateral reflex elicited with stimulation to the left will not be observed in the right ear due to the severed right stapedial tendon. Left Ipsilateral: The slightly reduced reflex SL is consistent with mild SNHL. Right Contralateral (stim-R, probe-L): Once the 10 decibel conductive component of the hearing loss is overcome by the intensity of the reflex activating tone, the right cochlea generates sufficient neural activity to initiate the neuronal chain of events to produce a contraction in the left ear, where the right contralateral reflex is monitored. Observe this case of unilateral ossicular disruption: Most cases of ossicular discontinuity from skull trauma involve incudostapedial joint separation. Since the stapedial tendon inserts on the neck of the stapes (which is medial to the common site of ossicular disruption), stapedial contraction will have no effect on the rest of the ossicular chain. Therefore, a "reflex absent" state will be observed in the probe ear with ossicular discontinuity. The ossicular disruption at the incudostapedial joint effectively decouples the stapedial tendon's influence from the tympanic membrane. Thus, as with other conductive conditions, the "absent reflex" conclusion refers solely to the absence of an observed reflex via the immittance equipment. With ossicular disruption the stapedial muscle may well contract, but the effects on the ear's immittance are not transmitted to the TM and therefore not "observable" by the immittance meter.

Refer to Example 13 Right Ipsilateral: The reflex is observed at normal Hearing and Sensation Levels. Left Contralateral (stim-L, probe-R): The elevated reflex threshold reflects the requirement of overcoming the effects of the conductive loss in the stimulated ear. Once a sound level capable of eliciting a reflex is reached, it is monitored in the right ear with no complications. Left Ipsilateral: Again, the probe and conductive ear pairing precludes the observance of a reflex. Right Contralateral (stim-R, probe-L): With the probe ear as the conductive ear, no reflex will be observed. Retrocochlear Hearing Loss (VIIIth Nerve Pathology)


The acoustic reflex pattern associated with VIIIth nerve pathology is dependent upon the location and size of the lesion. Acoustic neuromas typically involve the VIIIth nerve but may spread to brainstem structures as well. Reflex decay is the reduction in reflex amplitude greater than 50% of the initial compliance change over 10 seconds to a 10 dB SL reflex eliciting tone of 500 or 1000 Hz. For example, in an ear with a reflex threshold of 85 dB HL at 1000 Hz, the reflex decay test would be administered at 95 dB HL to this ear. In VIIIth nerve pathology we expect the reflex to be absent to stimulation between 70-100 dB HL on the affected side in about 65% of patients. Approximately 20% may have normal reflexes, while in 15% the reflex is present yet displays abnormal reflex decay. Thus, approximately 80% of VIIIth nerve patients will display abnormal reflex test patterns manifest in either absent or elevated reflexes or significant reflex decay (Jerger and Jerger, 1974; Olsen et al, 1975). Recall that reflex sensation level is reduced in the cochlear involved ear. One identifying feature of an acoustic neuroma ear is that the reflex occurs at normal sensation levels. Thus, with a 30 dB loss the reflex might occur at 115 dB HL (30 + 85 dB SL). If one tests for reflexes only to 110 dB then an erroneous "absent reflex" would be recorded for this ear. This situation again highlights the need to specify the upper intensity at which the reflex was not observed. With this simulation reflex decay may be tested in the ipsilateral and contralateral modes. In the early days of immittance testing when ipsilateral reflex testing was not available and/or suspect because of limitations of the instrumentation, the only means of assessing reflex decay was in the contralateral mode. That is, if a right acoustic neuroma was suspected, the right ear would be stimulated with a reflex eliciting tone while the probe in the left ear monitored the reflex response. Realize that the entire afferent pathways of the right ear and the efferent reflex pathway in the left ear must be normal to measure the status of the right 8th cranial nerve. If the left ear had excessive negative middle ear pressure, or adhesions from childhood otitis media, or any number of other conditions, one could not test reflex decay of the right ear. Using the ipsilateral mode, however, one could test right reflex decay and monitor such decay from the same ear. Of course, the contralateral reflex testing mode always yields more information about the integrity of lower brainstem pathways (whether testing reflex threshold or reflex decay) than does the ipsilateral test, yet there may be clinical situations in which reflex decay data cannot be acquired without relying on the ipsilateral testing mode.


To perform a normal stapedial reflex decay test, follow this procedure. (Note: If the simulation is not set up to test Immittance on Case 2 as on p. 58, click Case, Select Case, and choose Case 2. Now click Test, then Immittance. Replot the tympanogram by clicking Plot.) Furthermore, the stimulus ear should be Ipsilateral and the Probe Ear should be Right. Click Reflex decay. Reflex decay is measured with an intensity that is 10 dB above the reflex threshold. When the ipsilateral reflex threshold was obtained at 1000 Hz on p. 69, it was 75 dB. Accordingly the proper intensity at which to measure the reflex decay is 10 dB above that threshold, or 85 dB. So adjust the intensity to 85 dB. Click Plot. Observe that the amplitude of the tracing did not decay by more than 50%, yielding a normal response. To perform an abnormal reflex decay test, follow this procedure. Again with Case 2 select the Left ear. The ipsilateral reflex threshold for the Left ear obtained at 1000 Hz on p. 69 was 75 dB. Replot the tympanogram for the Left ear, if necessary. Then click Reflex decay. Set the intensity to 10 dB above the reflex threshold, or 85 dB. Click Plot. Observe that the amplitude of the tracing decayed more than 50%. In fact it decayed almost 100% (i.e., nearly returned to the baseline.), thus yielding an abnormal response. In summary, reflex decay testing assesses the reflex adaptation of the ear stimulated by a 10 second tone. It is preferable to stimulate one ear and monitor any decay in the other ear (contralateral mode), but the status of the probe ear may preclude this mode of testing on occasion. In such situations, ipsilateral reflex decay testing is appropriate. Observe the following case of unilateral right acoustic neuroma:

Refer to Example 14 Right Ipsilateral: The right ipsilateral reflex is absent because the right-sided acoustic neuroma interrupts normal synchronous firing of the eighth nerve. An insufficient neural code is sent to the reflex centers above the eighth nerve. Left Contralateral (stim-L, probe-R): This reflex is normal since the left contralateral reflex arc essentially involves the left auditory nerve and right facial nerve. The right acoustic neuroma is not a factor when the left ear is stimulated and the reflex response monitored in the right. Left Ipsilateral: Normal reflex thresholds observed since no part of the dysfunctional right eighth nerve is stimulated.

Instruction Manual Audiology Clinic V2 Chapter Ten 105

Right Contralateral (stim-R, probe-L): The involved ear has poor neural conductivity, and therefore the higher reflex pathways are not stimulated with a threshold reflex input. While the facial nerve on the left is intact, the neural disruption on the right precludes its activation.

Summary You have completed a comprehensive simulation course in the essentials of aural acoustic immittance. Next we complete the basic audiological test battery with an introduction to speech audiometry.

Chapter Ten

Speech Audiometry Standard speech tests require a relatively minor amount of time to administer in terms of the complete audiological test battery, yet the results of speech tests reveal vital information about the listener’s capacity to communicate. Two types of tests are routinely given as part of the regular protocol, but there are many specialized tests as well. The two most common tests are the speech recognition threshold (SRT) and the word recognition test (WR).

Speech Recognition Threshold

106 Chapter Ten Instruction Manual Audiology Clinic V2

Just as thresholds can be found for the pure-tone frequencies, the threshold for speech can also be measured. The nature of the speech materials used and the manner in which they are presented are of paramount importance to the outcome of the test. For the most part disyllabic words spoken with spondaic stress are used. That is, the word list consists of two syllable words pronounced with equal emphasis on each syllable. The original set of words was eventually reduced to a set of thirty-six, know as the W1 word list. Measuring a threshold involves very low intensities, and two syllable words spoken with equal stress on each syllable (called spondees) constitute an easy-to-recognize stimulus. To further facilitate perception, the listener is informed of the words in the set of test stimuli before the test begins and asked to repeat them. One way this can be accomplished is to deliver the words to the listener through the earphones at a comfortably loud intensity. Alternately, the clinician may choose to recite the words to the listener in a face-to-face situation. In either case care should be taken that the listener is understanding the words based solely on auditory and not visual cues. Thus, covering the mouth may be necessary. During this familiarization phase, if the listener misses a word, it is repeated until the listener comprehends it. If the listener is ultimately unable to repeat a particular word, it should be eliminated from the set and not used. The specific steps involved in conducting the threshold test will be covered below. The SRT provides two valuable pieces of information. First, it serves to confirm the validity of the pure-tone thresholds (more on this below). Some individuals falsify the intensity at which they detect pure-tones; that is they do not respond at threshold but rather at suprathreshold levels; i.e., they fake a hearing loss. Speech thresholds, in contrast, are more difficult to misrepresent because speech is a much more complex acoustic signal, and it has an intrinsic meaning. Second, the SRT forms the baseline intensity upon which other speech tests can be conducted. For example, if a listener’s SRT is 25, and you want to perform another test at an intensity 40 dB greater than the threshold, or 40 dB sensation level (SL), then the intensity dial is set at 65 dB for the ensuing test.

Word recognition The more significant of the speech tests is word recognition, or the capacity of the listener to understand speech. The result of such a test is recorded as a percentage that is intended to represent the listener’s ability to perceive everyday speech. The type of stimuli to use in order to measure word recognition is much more controversial than for the SRT. Generally speaking, the


materials can be subdivided into two broad groups: individual words and multiple words (phrases or sentences). The objective is, of course, is to assess the degree to which the listener can understand speech in ordinary conversation. The means of converting that goal into a reasonably simple and economical clinical tool is challenging. In addition, everyday speech occurs in a variety of listening conditions varying from the quiet of the dinner table to the moderate noise of the urban street corner to the extreme noise of a heavy construction site. Thus, speech is often measured both in quiet, meaning that just the speech signal is presented to the listener, and again in noise, meaning that a competing noise is mixed with the speech signal and delivered simultaneously to the test ear. We emphasize that both the speech and the noise are delivered to the same ear. Unlike the SRT, the WR test is given at a suprathreshold level, ideally a level that will optimize the listener’s ability to comprehend the words. The most commonly used level is 40 dB above the speech threshold, or 40 dB SL, although other levels are used as well. In fact, WR may be measured at several sensation levels, both in quiet and in noise. The most commonly used materials used to determine word recognition consist of a set of 50 monosyllables. There are several word lists that have been used historically, such as the W-22 and the NU-6 lists. There are other lists too, including lists that are comprised of sentences instead of individual words. When a monosyllabic word list is used, it is generally presented with an accompanying carrier phrase, that is the test stimulus is preceded by a phrase such as “Say the word a-word”, where a-word is one of the words on the list. The point of the carrier phrase is create a more natural speech event; we converse in sentences, not in isolated words. Furthermore, the stimulus (the word at the end of the phrase) is not to be emphasized, but rather is to be spoken as the final word of any conversational sentence. Each word is presented only once, and a tally is kept of the number of words repeated correctly by the listener and the number missed. The resulting score is converted to a percentage of correct responses. The lists of 50 words were carefully constructed to contain a balance of the sounds that occur in everyday English. Moreover the construction of a given test, like the NU-6, consisted of the development of several lists of different words which are of equivalent difficulty. That is, there are multiple lists of words, and using any of the lists should result in the same score on the same individual. The need for multiple lists arises when doing several tests on a listener. If the exact same word list were used repeatedly, the person’s score would be expected to improve as the person become increasingly familiar with the words after repeated exposure to them.


Recorded vs. Live Voice Presentation To assure consistency among repeated tests on the same listener and across different listeners, speech materials are available in a recorded format. Originally called “taped speech”, the tapes have now been supplemented by CD formats as well as digital formats stored in the memory of some audiometers or audiometers interfaced with computers. Recorded speech offers the advantage of generating precisely the identical stimulus every time the test is administered. In contrast, the clinician may choose to say the stimulus words. This method is known as monitored live-voice speech audiometry. In ordinary conversation the intensity of our voices varies by a few decibels. Because it is imperative that the speech stimuli be at a constant intensity for a given intensity dial setting, the clinician uses a VU meter to monitor the level of her or his voice in order to maintain a correctly calibrated signal. Thus, the level of the clinician’s voice is monitored to assure a consistent intensity from word to word. One major advantage of a live-voice presentation is that it allows much greater flexibility than does the recorded format. Even with monitoring, the acoustical event that is generated by the clinician is likely to vary somewhat from presentation to presentation, and certainly from clinician to clinician. This fact creates the incentive for using recorded materials. A given clinic will usually adopt one method or the other as established clinic policy. The Audiology Clinic provides an SRT and a WR score for most cases. The WR score represents the results of a single test done in quiet. These scores can be seen by plotting the audiogram for the particular case. A subset of ten spondees is used for the SRT in order to conserve disk space.

Descending Threshold Protocol The method of obtaining the SRT that is recommended by the guidelines issued by the American Speech-Language-Hearing Association (ASHA, 1988) makes use of a descending threshold technique. The signal level can be varied in 5 dB or 2 dB steps. The audiometer simulated by The Audiology Clinic only permits 5 dB changes, so instructions for utilizing that increment will be specified. The guidelines should be consulted to obtain the details of the 2 dB method, which is similar. There are two stages to the determination of the threshold. First is the preliminary phase. 1.Set the intensity to a level that is 30-40 dB higher than the estimated threshold and present one word.


2.If the word is missed, increase the intensity by 20 dB and again present one word. Repeat this step until the word is repeated correctly. 3.Decrease the intensity by 10 dB and present one word. If the word is missed, present a second word. 4.Continue as in Step 3 until both words are missed. 5.Increase the intensity by 10 dB and record this intensity as the starting level. At this point the test phase begins. 1.Present five words at this level and note the correct or incorrect response to each word. 2.Decreased the intensity by 5 dB and present five more words. 3.If the first six (6) words are not repeated correctly, then increase the intensity by 10 dB and record this value as the new starting level. 4.Continue presenting five words at each level scoring the responses until all five words are missed. 5.Calculate the threshold as follows: SRT = starting level - number of words correct + 2 It should be pointed out that the guidelines recommend this procedure to assure consistency among clinics. However, the modified Hughson-Westlake procedure that was described in Chapter 2 has also been used to measure the SRT. The procedure for establishing the SRT using that method is exactly like that used to get a pure-tone threshold. Experiment with both methods or use the formula required by your clinic or as directed by your instructor or supervisor.

Calibration of the Speech Signal As previously mentioned, before conducting speech tests it is mandatory to calibrate the speech circuit regardless of whether live-voice or recorded speech is performed. We will use recorded speech stimuli. If your computer has the proper sound card, both the test stimulus and the listener’s response can be heard. If there is not a sound card, then the word spoken by the clinician and repeated by the listener will appear in a bubble on the screen. Further, if you have audio capability but wish to defeat it, on the Options menu observe whether there is a check mark next to Sound on. If so, click Sound on to remove the check mark, and the sound will be muted and the words will appear in bubbles as described above. It is necessary to calibrate the speech circuit before doing a speech test. This is because different clinicians speak with different intensities. Even different recorded materials produce varying output levels. In order to maintain the calibration of the intensity of the


speech signal, the small VU meter on the clinical audiometer must be monitored to confirm that when the word is spoken, the indicator peaks at 0 dB (as shown in Figure 10-1 below). This fact assures that an intensity dial setting of, let’s say, 45 dB results in a word with an intensity of 45 dB being delivered to the listener.

Figure 10-1. VU meter. To calibrate the speech signal the audiometer must be in Speech Mode, so click on the Speech button in the Mode window of the audiometer. The frequency window will display the letters “Spch”. Now click Options, then Calibrate speech. The calibration window will open. It contains a large scale on the left side. The scale has a pointer, which will be randomly positioned. Click Play words. Either you will hear the words being spoken or see them in a bubble. Your task is to monitor the pointer of the VU meter on the audiometer as the words are produced. It needs to peak at the red “0” on the dial. If the pointer peaks below the “0”, move the pointer on the scale slowly upward until the needle on the VU meter peaks at “0”. On the other hand, if the pointer on the VU meter peaks above the “0”, drag the pointer on the scale downward. (Click Instructions if you forget what to do.) When the pointer of the VU meter on the audiometer peaks at “0”, the speech circuit is calibrated, and the actual test may begin. In the calibration window click Stop words, then close the window (click on the X in the upper right corner). You will observe that all menus become inactive during a speech event and are reactivated after the event concludes. This is because the production of words involves a complex event that cannot be interrupted.

Masking Speech Thresholds Just as pure-tone thresholds must occasionally be masked, so must speech thresholds be masked. The SRT is measured through the earphones, consequently it is an air conduction event. Accordingly, the masking rules for air conduction (Rule No. 1 and Rule No. 3) apply. With regard to Rule. No 1 the decision is clear: mask (retest) when one SRT is 40 dB or more poorer than the other for circumaural earphones (60 dB or more for insert earphones). Nevertheless the situation with regard to Rule No. 3 is more complex because you are comparing the SRT in one ear to the bone conduction threshold in the other ear. But


speech is a multi-frequency stimulus; the SRT cannot be compared to the bone conduction threshold at a single frequency. It has been shown, however, that there is a close correlation between the SRT and the average of the thresholds at 500, 1000, and 2000 Hz. This average is called the pure-tone average, and as a rule the SRT should be very close to this value. The exception to the case occurs when there is a substantial difference between the levels at which these three pure-tone thresholds are realized. An example would be the following: 500 Hz = 20 dB, 1000 Hz = 25 dB, and 2000 Hz = 75 dB. Clearly, there is a precipitous drop in sensitivity between 1000 and 2000 Hz. In such instances, a two frequency PTA is often calculated, using the two frequencies that have the more similar thresholds, namely 500 and 1000 Hz. Accordingly, the two frequency PTA would be 22.5 dB. As already mentioned above, the SRT serves to confirm the validity of the pure-tone thresholds. There should be a close association between the SRT and the PTA, barring the occasional configuration that has vast differences among the thresholds in the 500 - 2000 Hz range. An SRT and PTA that differ by more than 10 dB should alert the clinician to a problem. An attempt should be made to resolve the difference, which could be due to clinical error on the part of the clinician, a misunderstanding on the part of the listener, or an equipment malfunction. If the difference cannot be resolved, then the clinician may suspect that the listener is deliberately falsifying the results (usually the pure-tone thresholds). In addition, the results of other tests, such as immittance, and other clinical information, for instance, the case history, can be immensely helpful in reaching the correct clinical interpretation of the status of the listener’s hearing.

Practice It is possible to obtain SRTs on all of the 100 cases in this simulation (50 cases in the Lite edition). The value obtained should agree with the pure-tone average (PTA), which is usually calculated by averaging the thresholds at 500, 1000, and 2000 Hz. With steeply sloping losses, or losses that drop precipitously after 1000 Hz, a two frequency average is sometimes used (500 and 1000 Hz). Because the SRT is constructed to be an easy test, the low frequencies are of more importance than the high frequencies, thus the rationale for the two frequency average. A disagreement of 10 dB or more between the SRT and PTA, particularly when the speech threshold is better than the pure-tone thresholds, should alert you to the possibility of an error in testing, an equipment malfunction, or worst of all a non-cooperative listener. It is essential to explain the discrepancy. The situation of the malingering listener is discussed in many audiology texts (Martin, 1994).


Summary Speech testing is an important aspect of the complete audiological evaluation. Hopefully you have become proficient at measuring the speech threshold. The word recognition test is even simpler to administer and will be reserved for actual clinical situations.

Instruction Manual Audiology Clinic V2 Index 113

Index

A

Audiometer 4, 7, 10–14, 10–14, 18–19, 18–19, 21–24, 26–27, 26–27, 33, 36, 51, 108–10, 108, 110

C

Crossover 8, 28–33, 28–33, 31–33, 36, 39, 42–45, 42–45, 49, 53–54, 53–54, 56–59, 56–59, 62

E

Earphones 10, 13, 16–18, 16–18, 21, 24, 26, 29–30, 29, 30–37, 32–36, 39–42, 39, 42, 44–47, 44–47, 51, 56–57, 56, 59–63, 59–63, 106, 110

L

Listener 4, 10, 13–15, 13–15, 18–22, 19–24, 26, 28–29, 28–29, 32–33, 35–39, 35–39, 38, 43, 47, 49–51, 49–51, 53, 57–58, 57–58, 64–68, 64–69, 105–7, 105–11, 109–11

M

Masking 8, 13, 21, 28–30, 28–31, 34–63, 34–63, 65–68, 65–68, 110

O

Overmasking 35, 41–45, 41–45, 52, 58–60, 58–60

P

Practice 12, 14, 20, 24, 41, 42, 47, 53, 60, 63, 66, 68, 87, 111

R

Reactance 70–73, 70–74 Resistance 29, 70–72, 70–72

T

Technique 14–16, 14–16, 21, 23–24, 23–24, 34–35, 34, 35, 48, 53, 77, 91, 108

Transducer 10, 26, 51, 70, 79

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