Main Components of the Hearing Mechanism: Outer Ear Middle Ear Inner Ear Central Auditory Nervous System Structures of the Outer Ear Auricle (Pinna) Collects sound Localization Amplifies sound (approx. 5-6 dB) External Auditory Canal: Approx. 1 inch in length S shaped Lined with cerumen glands Outer 1/3 surrounded by cartilage Inner 2/3s surrounded by mastoid bone Tympanic Membrane: Thin membrane Forms boundary between outer and middle ear Vibrates in response to sound Changes acoustical energy into mechanical energy The Ossicles: A: Malleus B: Incus C: Stapes Smallest bones in the body Acts as a lever system Footplate of stapes enters oval window of the cochlea Stapedius Muscle Connects stapes to wall of middle ear Contracts in response to loud sounds (called the Acoustic Reflex) Eustachian Tube (AKA: The Equalizer) Lined with mucous membrane Connects middle ear to nasopharynx Equalizes air pressure Structures of the Inner Ear Cochlea Snail shaped organ with a series of fluid-filled tunnels Converts mechanical energy to electrical energy Organ Of Corti: The end organ of hearing Contains stereocilia and hair cells. Hair Cells: Frequency specific High pitches= base of cochlea Low pitches= apex of cochlea Vestibular System Consists of three semi-circular canals Shares fluid with the cochlea Controls balance Central Auditory System VIIIth Cranial nerve or Auditory Nerve Carries signals from cochlea to brain Auditory Cortex Temporal lobe of the brain where sound is perceived and analyzed How Sound Travels Through The Ear Acoustic energy, in the form of sound waves, is channeled into the ear canal by the pinna 2. Sound waves hit the tympanic membrane and cause it to vibrate, like a drum, changing it into mechanical energy 3. The malleus, which is attached to the tympanic membrane, starts the ossicles into motion 4. The stapes moves in and out of the oval window of the cochlea creating a fluid motion 5. The fluid movement causes membranes in the Organ of Corti to shear against the hair cells 6. This creates an electrical signal which is sent up the Auditory Nerve to the brain The brain interprets it as sound! Audiometer An Audiometer is a machine, which is used to determine the hearing loss in an individual. Pure Tone Audiometer works on the principle of presenting specific pure tone signals to the subject and determining the intensity at which they can barely hear these signals. They are calibrated in terms of frequency and output. Classification Audiometers Pure tone audiometer Speech audiometer Air conduction Bone conduction Speech thresholds AIR & BONE CONDUCTION: Air conduction is the transmission of sound through the external & middle ear to the internal ear. Bone conduction refers to the transmission of sound to the internal ear mediated by mechanical vibration of the cranial bones & soft tissues. Threshold of Hearing The threshold pressure level of a sound is the lowest level at which an observer can discriminate between the desired sound and the noise background always present in the auditory sytem. Audiometer Generally employed transducers in audiometer are the following: Earphone Microphone Electret microphone MEMS based micrphone Bone-vibrator Loud speakers Earphones: Earphones are usually of the moving coil type and gives reasonably flat frequency response upto 6 KHz after which their sensitivity decreases rapidly. They are not specially designed for audiometric applications but for communication purposes. Used in hearing aids in their miniature form. Microphones: These are used to translate wave motion in air into electrical signal. The first one which are carbon button type which changes resistance with the air pressure. The second one is the electrodynamic type in which the voltage is induced in a coil by its motion relative to a magnet Third type-condenser where capacitance of a condenser is varied by the vibration of one of the condenser plates. Electret Microphone Electret Condenser Microphone, as the name suggests is a parallel plate capacitor and works on the principle of a variable capacitance. It consists of two plates, one fixed (called the back plate) and the other moveable (called Diaphragm) with a small gap between them. An electric potential charges the plate. When sound strikes the diaphragm it starts moving, thereby changing the capacitance between the plates which in turn results in a variable electric current to flow. MEMS based microphone The ADMP801 is a high quality, ultralow power, analog output, bottom-ported, omnidirectional MEMS microphone designed specifically for hearing aid applications. It is fully pick-and-place and reflow compatible, offering an option to save on cost using a mechanized assembly process as compared to ECMs that require manual assembly processes. The device offers excellent environmental and temporal stability, and multiple ADMP801 MEMS microphones can be configured in an array to form a directional response, facilitating sound of voice localization. Bone vibrators Bone vibrators is a vibration device which is supposed to be pressed against a reasonable hard part of the human head, which could be the forehead or more common the mastoid, a bone right behind the ear and therefore close to the hearing organs inside the head The vibrations will be transmitted through the bones to the inner ear where it is detected. The bone vibrator should be held firmly into place on the head by a headband, by glasses or build into a hat/helmet of some sort. They are of the hearing aid type in which the transduction mechanism changes the alternating current into a vibratory force through a diaphragm. The diaphragm and its basic mechanical parameters like mass, compliance and resistance are important in establishing its response chs. Though convenient it is very ineffecient means of transduction and has arather limited and peaky frequency response. The plane circular contact area of a bone vibrator is recommended to be mm2 It is heald in position by a headband. Loudspeakers They are used to deliver auditory stimuli when it is not possible to have close coupling of the transducer to the ear. Audiometers A typical Audiometer: Identification of hearing loss. Screening audiometers : Used to separate two groups of people. Simple audiometer An audiometer will essentially have an oscillator driving a pair of head phones and is calibrated in terms of frequency and acoustic output. Pure tone audiometers and speech audiometers are two main groups of audiometers and are grouped according to the basis of the stimulus they provide to evoke audio response. The intensity range of most audiometers starts from approximately 15 dB above normal to 95 below normal over a frequency range from approximately 500 to 4000 Hz. Pure tone audiometer A pure tone is the simplest type of auditory stimulus. Generate test tones in octave steps from 125 to 80000Hz, the signal intensity ranging from 10 dB to dB. Frequency range of Hz. Changes in threshold sensitivity associated with various middle ear surgical procedures can be monitored more accurately with pure tone than speech tests. Speech audiometer To carry out tests with spoken voices. These tests are particularly important before prescribing hearing-aids. CONSTRUCTION: A double band tape recorder is preferred to interface the two channel audiometer units. Masking noise is supplied by the noise generator. The two channels supply the two head-phones or the two loud speakers of 25 W each. Noise White noise: White noise is a noise containing all frequencies in the audible spectrum at approximately equal intensities. Saw tooth noise: Saw tooth noise is a noise in which the basic repetition rate is usually that of the mains voltage & contains only those frequencies that are multiplies of the fundamental. Bekesy Audiometer System OPERATION: The instrument generates a pure-tone signal which is presented to him through an air-conduction earphone. The subject is asked to press a switch when the tone is heard and to release the switch when it is not heard. A pen connected to the attenuator traces a continuous record of the patients intensity adjustments on an audiogram chart, producing a graphic representation. Bekesy Audiometer System ELECTRICAL SECTION Oscillator circuit Modulator circuit. Automatic attenuator. Control circuits Master clock generator MECHANICAL SECTION Carriage device Writing system Electrical section Oscillator circuits: This oscillator generates test signals with frequencies of 125, 250, 500, 1000, 1500, 2000, 3000, 4000, 6000, and 8000Hz. This sequence is first presented to the left ear automatically, each tone for 30s, and then to the right ear, the shift between the frequencies being noiseless. After both ears have been tested, a 1 kHz tone is presented to the right ear to provide a useful indication of the test reliability. Electrical section Modulators: The models of modulators are available Pulse or Cont. In the Pulse mode the test signal is modulated giving a signal which is easily recognized by the patient. In the Cont mode no modulation is applied, giving a signal suitable for use, while calibrating the audiometer. Electrical section Attenuator: The attenuation range is 100dB, thereby covering the range of hearing levels from 10 to + 90 dB. When the test is initiated, the attenuator starts at its top position of dB and then increases the level with a rate of 5 dB/s. The pen drive is controlled by means of the hand switch operated by the patient. Pressing the switch decreases the output from the potentiometer and thereby the level in the ear phones. Earphones The earphones are a matched pair with distortion which is typically less than 1%. Mechanical section: Mechanical carriage with the writing system is driven by a stepping motor via a toothed belt. Earphones The speed and direction of rotation of the motor are automatically controlled via the logic control system. When the test is initiated and the patient indicates that he hears the signal by pressing hand switch, the carriage moves along the X-axis (Frequency axis) of the audiogram in tune with the frequency of the test signal. When the complete test is finished the carriage and writing system returns to the start position. Writing System Operation: operated by the pen drive, which is driven by a stepping motor. The pen drive moves the pen, and with it the wiper of the automatic attenuator, along the Y-axis (hearing level axis) with a constant speed corresponding to the change in attenuation of 5dB/s. The direction of movement of the pen is determined by the position of the hands witch operated by the patient. Limit switches are also included with the pen drive. Audiogram Chart The audiogram is printed in standard 5 format (148*210mm). MASKING IN AUDIOMETRY Need for masking: In case of monaural & asymmetrical binaural hearing losses, there is a serious difficulty in obtaining accurate measures of hearing for the poorer ear. This problem can be overcome by eliminating responses from the better ear by masking in order, to shift the threshold to high level, permitting greater intensities to be presented to the poorer ear without any danger of cross-over. MASKING IN AUDIOMETRY Efficiency: Masking efficiency depends upon the nature of masking sound as well as intensity. A pure tone can be used to mask other pure tones. Evoked response audiometer Five major subsystems The tone generator EEG amplifier The programmer Signal averaging computer Chart recorder