speech discrimination with an 8-channel compression ...hearing aid can be very effective in some...
TRANSCRIPT
VeteransAdministration
Journal of Rehabilitation Researchand Development Vol . 24 No . 4Pages 161–180
Speech discrimination with an 8-channel compressionhearing aid and conventional aids in background ofspeech-band noise*E. WILLIAM YUND, HELEN J . SIMON,** and ROBERT EFRONVeterans Administration Medical Center, Neurophysiology-Biophysics Research Laboratory,Martinez, California 94553
Abstract—The design for a multichannel compressionhearing aid was developed from previous experimentaland theoretical work in our laboratory concerning pitchperception in normal-hearing subjects . The new hearingaid, implemented with off-line digital signal processing,was tested on twenty subjects with sensorineural hearingloss using speech sounds in a background of speech-spectrum noise. Five signal-to-noise ratios (+ 15 to – 5dB)were used at two noise levels (60 and 70 dB SPL).Hearing-loss subjects listened to these stimuli under threedifferent conditions : a) processed by the new multichan-nel compression hearing aid ; b) processed by a conven-tional hearing aid; and c) unprocessed. The performanceof normal-hearing subjects with the unprocessed stimuliprovided another condition against which the perform-ance in the two hearing aid conditions could be evaluated.Both aided conditions provided improved performanceover the unprocessed condition and the multichannelcompression aid produced better performance than theconventional aid . In the case of 4 of the 20 subjects, withless severe gradually sloping hearing losses, the newmultichannel compression aid produced near-normal per-formance even at low signal-to-noise ratios . Some aspectsof the results also suggested that learning to use the aidwas more important in the case of the multichannelcompression aid than in the case of the conventional aid.These results indicate that a multichannel compressionhearing aid can be very effective in some individuals withsensorineural hearing loss and is superior to a conven-tional hearing aid in most subjects.
*This work was supported by the VA Rehabilitation Research andDevelopment Service . A preliminary report of this work was given atthe 12th International Congress on Acoustics (26).
**Presently with the Smith-Kettlewell Eye Research Institute, SanFrancisco, CA .
INTRODUCTION
A multichannel compression hearing aid is one inwhich the amplification in each frequency band, orchannel, decreases as the amplitude of the signal inthat channel increases. The dependency of amplifi-cation upon amplitude also is usually different forthe various channels . Using such an aid, an individ-ual with sensorineural hearing loss (SNHL) couldexhibit both normal thresholds and normal discom-fort levels for narrow band stimuli and, in that verylimited sense, may be said to exhibit "normalhearing ." It is this obviously oversimplified view ofmultichannel compression that originally may havemade it seem so promising and perhaps that stillmakes us continue to try to demonstrate its pre-sumed superiority over simple linear amplification.
In one sense, however, such a view of multichan-nel compression cannot be dismissed as completelynaive. Consider the pattern of neural signals, forsome particular stimulus (either simple or complexto any degree), traveling up the auditory nerve in anormal-hearing subject . If it were possible to modifythis stimulus in such a way that the modified stimulusproduced the normal pattern of neural signals in theauditory nerve of a patient with cochlear hearingloss, then the patient should exhibit "normal hear-ing" for that stimulus, however complex it mightbe. A device that could "normalize" the auditorynerve signals for all possible stimuli for a particularSNHL patient, would be the "perfect" hearing aidfor that patient . Although it may never be possibleto make this "perfect" hearing aid, our current
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understanding of SNHL indicates that mulitchannelcompression would be an important componentprocess in such a device. In SNHL, low intensitystimuli generate reduced responses or no responsewhile high intensity stimuli generate essentially nor-mal-magnitude responses and the threshold as wellas the rate of response growth may vary withfrequency . This phenomenon, called recruitment inthe context of loudness perception, seems to beexactly the problem for which multichannelcompression is the solution.
Demonstrating the efficacy of multichannelcompression has proven much more difficult thanrecognizing its potential . In their review, Braida etal. (3) conclude that previous studies have notunambiguously shown an advantage for mulitchan-nel compression over a well-fit linear aid . Braida etal . attribute the lack of positive results primarily tothe technical and methodological complexity of theproblem rather than to an essential flaw in the ideaof multichannel compression . Among the specificdifficulties discussed by Braida et al. are : a) lack ofsubject training with compressed signals, b) use ofspeech stimuli that do not cover a wide enoughrange of intensities, c) lack of testing in noise,especially at lower signal-to-noise ratios (S/N), d)lack of a suitably chosen linear aid control conditionagainst which the multichannel aid was compared,e) improper subject selection, f) distortion producedin the compression system that degrades speechrecognition, and g) failure of the compression systemto compress the speech signals into the intendedintensity range.
Villchur (21) agrees that the advantage of multi-channel compression has not been thoroughly dem-onstrated, although his emphasis differs somewhatfrom that of Braida et al . On one point, the needfor a larger intensity range in the speech test ma-terial, Villchur and Braida et al . seem to be incomplete agreement . Villchur stresses the need fora well-fit linear control condition, but also a realisticone that would be acceptable to the user in therange of conditions encountered outside the labo-ratory . In terms of subject selection, Villchur (againin agreement with Braida et al .) suggests concen-trating efforts upon those subjects with severelyreduced residual dynamic ranges.
Three studies published more recently have in-cluded at least some multichannel compression:Laurence, Moore, and Glasberg (11) ; Nabelek (16) ;
and Walker, Byrne, and Dillon (22) . Walker, Byrne,and Dillon, using a mixed (compression/expansion)system, reported no general advantage of that sys-tem over a linear control, although they did noteimprovement in one subject for stimuli loud enoughto be predominantly in the compression phase ofthe device . Nabelek reported an advantage for asingle channel (wideband) compression system butmuch less, if any, for multichannel compression.Laurence, Moore, and Glasberg reported betterperformance for their two-channel compression aidthan for their linear system, both in quiet and innoise . However, their compression aid actuallycontains two separate compression stages, a singlechannel stage (not unlike Nabelek's wide bandsystem) followed by a two-channel stage . SinceLaurence, Moore, and Glasberg did not test eithercompression stage separately, it remains to be de-termined whether the success of their aid resultedfrom the action of the initial single channel compres-sion or the later two-channel stage . Thus, includingthese recent studies, there remains no convincingevidence for the advantage of multichannel compres-sion in hearing aid design.
The purpose of the current study was to test aparticular multichannel compression hearing aid de-veloped from previous experimental and theoreticalwork in our laboratory in normal-hearing subjects.The critical parts of the previous work as it relatesto our mulitchannel compression system are a) thetheoretical model formulated to explain certain di-chotic pitch effects (25) and b) the development ofa method to measure one parameter of the model—the intensity-response (I-R) transduction functionof the cochlea at any frequency (6) . From oneperspective, the current study tests how effectivelythe model can represent an individual subject'sSNHL, when its I-R parameters are measured 0
that subject. The individualized "SNHL" model isused to determine the type of signal processing (i .e .,the hearing aid) that must be applied to its input inorder to make its output, as much as possible, likethe output of the model with a "normal-hearing"I-R mechanism . (For further details on the modelsee Yund and Efron (25), and on the I-R measure-ments see Efron, Yund, and Divenyi (6) . Sometypical I-R typical functions for one SNHL subjectand how to derive amplification parameters fromthese are illustrated in the Method Section below .)
163Section III . Speech Processing Hearing Aids : Yund et al.
METHOD
SubjectsTwenty-nine volunteers were recruited to partic-
ipate in this study from local speech and hearingcenters and from the community. Seventeen menand three women (S2, S7, S11) ages 45—77 years(mean 64 years), with varying degrees of cochlearpathology, completed the study. All volunteers wereselected on the basis of peripheral hearing functionusing pure-tone air conduction, speech-receptionthreshold testing, tympanometry, reflexometry, andperformance-intensity functions for phoneticallybalanced words (9) using recorded CID W-22 wordlists (8) . Several audiometric examinations wereadministered to each over the course of the research;where minor threshold changes occurred, only themost recent values will be presented.
Audiometric test results for all subjects wereconsistent with cochlear pathology . Etiologies wereunknown for some of the subjects ; however, othersubjects had familial histories of hearing loss withaging and 16 reported exposure to excessive noiselevels . Eleven of the subjects were hearing aid usersat the time of these experiments (S4, S6, 510, 511,S12, S13, S15, S16, S18, S19, S20) . Of these, sevenowned and wore the hearing aid that was used asthe control (conventional) aid (S6, S 11, S12, S13,S15, S16, S18) . In one of these, S15, the subjectbegan using the aid in the month before his speechtesting started: he decided to purchase the recom-mended aid and began using it immediately.
All subjects were native speakers of English andnone had a history of middle ear pathology, neu-rological problems, or alcohol/drug abuse . All sub-jects were paid for their participation.
Test MaterialsThe test materials consisted of the closed-set
Nonsense Syllable Test (NST) developed by Resnicket al . (17) . In the original version of this test, thesubject was required to select the consonant heardfrom among a limited number of response foils ineach of eleven subsets . These subsets differed inconsonant voicing (voiced or unvoiced), position ofconsonants (initial or final), and vowel context (/a/,/i/ or /u/) . The syllables had been originally recordedby a male speaker within the carrier phrase "Youwill mark please ." However, for our experi-ments, the syllables were presented in isolationrather than in this carrier phrase .
The NST was chosen because of its high reliabilitywhen administered to hearing-impaired listeners withmild-to-moderate losses (5), its sensitivity to changesin performance with alteration of hearing aid param-eters and its ability to provide information aboutthe consonant error patterns made by the listener(13) . Walker, Byrne, and Dillon, (23) demonstratedlearning effects with their version of the NST (usinga female Australian speaker) . Such learning effectsshould not confound the results in the critical com-parisons in this study because of the multiple,randomized presentations of the test conditions andthe practice sessions that were not included in thedata analysis . In the methods section of a laterpaper, Walker, Byrne, and Dillon (22) present anexcellent analysis of the reasons for using the NSTin experiments like ours and we need not repeatthat analysis here.
The stimuli were digitized at a rate of 20 kHzwith 12-bit precision from the prerecorded tape andall further processing was done by digital means.After any of the remaining carrier phrase was trimmedaway, the stimuli were filtered into eight bands usingthe ILS (Interactive Laboratory Systems, SignalTechnology, Inc.) signal processing package . Weused six half-octave bands from 750 to 4000 Hz inaddition to one above 4000 Hz and one below 750Hz. With the exception of the low-pass band, thedigital Butterworth filters had slopes outside theirbandpass of 50 to 70 dB per octave ; the slope ofthe low-pass band was 20 to 40 dB per octave . Thebands crossed at – 3 dB at 595, 841, 1189, 1682,2378, 3364, and 4757 Hz . For the unprocessedcondition, these bands were recombined after filter-ing while for the processed conditions the bandswere selectively amplified before recombination.Amplification within bands could be a simple linearconstant—for a conventional aid—or a function ofthe signal level determined individually for thesubject—for our model aid . Peak clipping orcompression (based on input or output levels) wasdone after recombination to complete the conven-tional aid . Twelve of the conventional aids werelinear with peak clipping; two (S9, S16) had outputcompression ; six (S4, S7, S8, 510, S19, S20) hadinput compression . Speech spectrum noise (SSN),shaped to the long-term average speech spectrum(7) was added to the syllables prior to the processingthat corresponded to the aids . The duration of thenoise was 1 second .
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ProcedureSpeech stimuli were generated from the computer
through a 16-bit digital-to-analog converter system,low-pass filtered at 8 kHz (Frequency Devices model901F), attenuated (Hewlett-Packard model 350D),and delivered to the subject either through a Dan-avox SM-W wide band receiver coupled to thesubject's earmold or, in five cases (S1, S3, S4, S11,S I7), through a TDH-39 earphone and circumauralcushion . Earphones were routinely calibrated witha Bruel and Kjar sound level meter (Model 2203)with an octave filter set (Model 1613) . The receiverswere calibrated to the earphones using a loudnessbalance procedure with normal hearing listeners.All test items were presently monaurally to the earjudged to be the best ear for amplification.
Testing was done in a single-walled IAC booth in2-hour sessions with rest periods between each runof the 91 syllables . Within each run, the order ofpresentation of the 91 syllables of the NST wasindependently randomized . Each trial of a run con-sisted of the following parts : a) A numbered list ofthe syllables in one subset of the NST was displayedon a computer terminal in front of the subject ; b)One of the stimuli from that subset was presented;and c) The subject entered the number of the syllable(s)he had heard . This response entry was the com-puter's signal to begin the next trial . Subjects weretold that a response was required to go on to thenext trial and that they should just guess if they hadno idea what sound they had heard . They also weretold that they should use the sound they heard toreduce the size of the set from which they had toguess, whenever possible. Prior to testing withstimuli in noise, each of the first 10 subjects (S1 toS10) ran at least 20 2-hour sessions without noise,including unprocessed, conventional aid and modelaid stimuli . These quiet sessions served primarilyas training since all aids tested worked well in quiet.For this reason, we reduced the number of 2-hoursessions without noise to at least 10 for S11 to S20.
There were three experimental aid conditions ateach S/N : a) "unprocessed," b) "conventional"aid, and c) "model" aid . Two noise levels (60 and70 dB SPL) each including the five S/N values (+ 15,+ 10, +5, 0, and — 5 dB) means that speech signalswere presented at intensity levels corresponding tosoft (55 dB SPL) and to loud speech (85 dB SPL)and were centered around the typical input level fora hearing aid wearer of 70 dB SPL according toWalden, Schuchman, and Sedge (24) . Eight of the
subjects (Sl, S2, S4, S5, S6, S7, S8, S11) ran the60 dB noise condition before the 70 dB noisecondition. Within noise conditions, the five S/Nwere always run in descending order, beginning with+ 15 and ending at -5 or earlier if the subject wasunable to perform above chance . Within each S/Ncondition, runs for each aid condition were done inrandom order ; in this way learning effects, if any,were distributed equally across aid conditions.
As indicated above, all stimulus processing wasdone off-line in a digital computer . In the unpro-cessed aid condition, the stimuli were filtered intothe eight bands and then the bands were merelysummed back together again . We were unable tohear any differences between these unprocessedstimuli and the original digitized stimuli deliveredthrough the same experimental system ; normal sub-jects also performed the task equally well with eitherset of stimuli . In the unprocessed noise conditions,the noise was similarly filtered into eight bands andrecombined.
In the conventional aid condition, each of theeight bands of the speech sound or speech soundplus noise was amplified according to the frequencyand gain characteristics of the subject's own orindependently recommended hearing aid . For thosesubjects using their own aids for the conventionalaid condition, we did not always know the rationaleby which the aid was prescribed. For the others,the independent hearing aid recommendations weremade by a practicing audiologist in the communityusing the Carhart procedure (24) . We chose theaudiologist by professional recommendations and apersonal interview and then placed essentially nolimitations on her normal hearing aid evaluationprocedure. In this way, we should have obtained aconventional aid control condition that represents agood fit for the subject in terms of frequency-gaincharacteristics and also a realistic hearing aid to usein the variety of conditions encountered outside thelaboratory . The measured gain (Phonic Ear HC 2000Hearing Aid Acoustic Computer) for each aid usedis included in the data figures (Figures 4-.7) . In termsof the experiment, these gain curves represent the"real ear gain" of the conventional aid conditionover the unprocessed condition ; since both sets ofstimuli were delivered to the subjects in exactly thesame way, the gain curves in the figures illustratethe entire difference between unprocessed and theconventional aid stimuli . The curves were measuredwith the volume setting for most comfortable loud-
165Section 111 . Speech Processing Hearing Aids : Yund et al.
ness (MCL) by the subject with a 65 dB SPL input.The model aid is an 8-channel compression aid
where the amplification characteristic in each chan-nel is determined with the model of Yund and Efron(25) using the method of Efron, Yund, and Divenyi(6) to measure the I-R functions . Since the modeland the methods have been described elsewhere,only their application to the present problem willbe presented here . I-R functions for a young normal-hearing subject at the frequencies 500, 750, 1000,1500, 2000, 3000 and 4000 Hz are illustrated inFigure 1 . Each panel has the right and left ear I-Rfunction for one frequency . On the ordinate in eachpanel is the response of the model's intensity-response transduction stage ; intensity is plotted on
the abscissa. Note that the normal-hearing subject'sI-R functions are essentially straight lines, exceptfor 500 and 750 Hz . Figure 2 illustrates I-R functionsmeasured on our SNHL subject S4 . This subject'sI-R functions are typical of our SNHL population:not only do they begin at higher intensities (repre-senting his elevated thresholds) but also the func-tions show much more curvature than those of thenormal-hearing subject at the same frequency . (A.more detailed examination of I-R functions in SNHLsubjects is beyond the scope of this report and willbe prepared for a future publication .)
Determining the amplification characteristic at afrequency using the normal and the SNHL I-Rfunction at that frequency is quite simple . For
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Figure 1.I-R functions for normal-hearing subject, DG whose thresholds were 10 dB HL [ANSI (1)] or better between 250 Hz and 8 kHzin both ears . Each panel illustrates left and right ear I-R functions at the frequency given in the upper left of the panel . Theresponse in arbitrary units is plotted as a function of intensity in dB SPL . The straight dashed line in the 500 and 750 Hz panelsindicates the approximate position of the I-R functions for the higher frequencies for purposes of comparison.
166Journal of Rehabilitation Research and Development Vol . 24 No . 4 Fall 1987
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Figure 2.I-R functions for SNHL subject S4 . Each panel illustrates left and right ear I-R functions at the frequency given in the upperleft of the panel . The response in arbitrary units is plotted as a function of intensity in dB SPL . The straight dashed line is inthe same position as in the 500 and 750 Hz panel of the normal-hearing subject of Figure 1 and thus at 1000, 1500 and 2000Hz indicates the position of the normal-hearing I-R functions.
example, if we find that the normal curve yields aresponse of 20 for a 30 dB SPL input, then we lookat the I-R curve of the SNHL subject to determinewhat input would be required to generate a 20response—let it be 65 dB SPL . That would meanthat the aid should amplify a 30 dB input at thatfrequency by 35 dB and thereby cause the impairedear to generate the "proper" response, i .e ., a
response of 20 . Figure 3 illustrates such amplificationvalues, plotted as a function of frequency andintensity, derived from the curves shown in Figures1 and 2 . Figure 3 is thus a graphical representationof the amplification characteristics of our multi-channel compression hearing aid for subject S4.Scaling assumptions in the I-R function calculationscause the amplification to fall to 0 dB at or below
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Section III . Speech Processing Hearing Aids : Yund et al.
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100 dB SPL in all frequency bands and also preventthe model aid from ever producing any output levelover 100 dB SPL . It should be emphasized that,although we are doing an intensity correction toeffectively normalize the I-R function in each band,the methods used to measure the I-R functions allinvolve only pitch judgements . No loudness judge-ments are required for these corrections.
Several additional aspects of the processing sys-tem must be determined before the amplificationcharacteristic of Figure 3 could be applied to astimulus . The parameters of the filters and thefrequency bands used have been described above.We used the compression method suggested byRobinson and Huntington (18) with a 10 ms sym-metrical averaging window ; this yields an attack andrelease time, defined in the usual way, of approxi-mately 4 ms . We have no reason to believe thatthese or other parameters are optimal since we madeno attempt to optimize them .
RESULTS
Individual data for each of the 20 subjects arepresented in Figures 4-7 . The three panels in onerow of each figure are for one subject . The left andmiddle panels in each row illustrate the data fromthe 60 dB and the 70 dB SPL SSN conditions,respectively . In these panels, percent correct syl-lable identification is plotted as a function of thesignal-to-noise ratio (SIN) . The three curves in eachleft and middle panel represent the subject's per-formance with unprocessed, or unaided, stimuli(filled circles and dotted line), with the individuallyrecommended conventional aid (X symbols anddashed line), and with the 8-channel compressionmodel aid (open circles and solid line) . The uncon-nected diamonds represent the performance of nor-mal-hearing subjects with the unprocessed stimuli.Each data point is a mean of five repeats of the 91stimulus NST set . The panel on the right in each
168Journal of Rehabilitation Research and Development Vol . 24 No. 4 Fall 1987
row illustrates the subject's pure tone audiogram(filled circles and solid line) and the conventionalaid gain (X symbol and dashed line) . In this panel,dB HL or dB gain, respectively, is plotted as afunction of frequency in Hz.
The subjects fell reasonably well into three groups:a) The ten subjects of Figures 4 and 5 have the moregradually sloping audiograms . b) The five subjectsof Figure 6 have the more steeply sloping audi-ograms . c) The five subjects of Figure 7 have lossesthat are different and/or more complex than thoseof the subjects in Figures 4, 5 and 6 . First we willpoint out some of the interesting aspects of the datain these figures and then we will present tables tosummarize the statistical analyses that were doneon each subject's data . In one sense, the statisticaltables are almost unnecessary . On the scale at whichthe figures are drawn, points that are not touchingeach other always represent values that are signifi-cantly different from each other.
Consider, first, the data of the subjects with themore gradually sloping losses presented in Figures4 and 5 . Perhaps the most obvious aspect of theseresults is that the absolute and relative positions ofthe curves representing the unprocessed, conven-tional aid and model aid conditions varies consid-erably across subjects (different rows) and some-what less across noise levels within subjects (leftversus middle panel in the same row) . Comparingthe unprocessed results with the normal-hearingresults indicates that all ten of these subjects havea clear deficit in recognizing speech sounds in noise.Of these subjects, S14 is least impaired with theunprocessed sounds while S19 is most impaired.Indeed, only S14 was able to perform much abovethe chance level of 12 percent at -5 dB S/N, acondition at which normal-hearing performance (dia-mond symbol) is about 50 percent.
Both conventional and model aids produce sig-nificant improvement over unprocessed perform-ance for all but a few points and these all occurredat higher S/N where the unprocessed performanceis at its maximum. For S14 there were even twoconditions (+ 15 and + 10 dB S/N, 70 dB SPL SSN)at which the aids produced a performance decre-ment. In four of these ten subjects, S2 and S4 inFigure 4 and S15 and S17 in Figure 5, the model aidresults are quite close to those of the normal-hearingsubjects . It was S4 who produced the most consist-ently high scores with the model aid across allconditions and S17 who produced the only score
equal to normal-hearing under any condition (0 dBS/N, 70 dB SPL SSN) . In further comparing themodel and conventional aid conditions, we see thatsix of these ten subjects (S2, S3, S4, S15, S17, S18)were able to obtain consistently higher scores withthe model aid at 60 dB SPL SSN than with theconventional aid, while in no case in these subjectsdid the conventional aid outperform the model aid.At 70 dB SPL SSN, only S2 and S4 maintain theentire model aid curve above the conventional aidcurve, although for S3, S17 and S18 the model aidcurve is still predominantly above that of the con-ventional aid . For S19, however, the conventionalaid performance was consistently superior to themodel aid performance at 70 dB SPL SSN.
The difference in the relative performance of theaids between the 60 and 70 dB SPL SSN conditions,may represent an effect of learning to use the modelaid rather than a true effect of noise level . Earliersubjects (S1-S10) ran a longer set of stimulus in-tensities without noise, a set which was reduced forthe later subjects when it became clear that onlythe noise experiments were able to differentiatebetween aids . The later subjects in this group allran the 70 dB SPL SSN condition before the 60 dBSPL SSN condition. Since the clear differencebetween noise conditions (model aid relatively betterat 60 dB SPL SSN) occurred only in the latersubjects (Figure 5), it seems more likely to be dueto learning to use the model aid than to the differencein noise levels.
Figure 6 shows the results for subjects with steeplysloping audiograms . Although, as a group, thesesubjects benefited less from either aid, they dem-onstrated a very similar pattern of relative perform-ances with respect to the model and conventionalaids : The model aid performance was more oftensuperior to that of the conventional aid . The modelaid curve is consistently below the conventional aidcurve at 70 dB SPL SSN for only one subject, S20(possibly due to a learning effect, see above).
Figure 4 . (opposite page)The data for subjects in the first half of the gradually slopingaudiogram group . Each row contains data from one subject.The left and middle panels illustrate results of the NST for the60 and 70 dB noise conditions, respectively . Percent correct isplotted as a function of S/N for three conditions for the SNHLsubject (three lines, see key above panels) and for normal-hearing subjects (unconnected diamond symbols) . The rightpanel illustrates the SNHL subject's pure-tone audiogram withdB HL [ANSI (I)] plotted against frequency and the conven-tional aid gain with dB gain plotted against frequency .
169Section HI Speech Processing Hearing Aids : Yund et al.
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170Journal of Rehabilitation Research and Development Vol . 24 No. 4 Fall 1987
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Figure 5.The data for subjects in the second half of the gradually sloping uudiogrum group . Otherwise the same as Figure 4 .
171
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®
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Figure 6.The data for subjects in the steeply sloping audiogram group . Otherwise the same as Figure 4 .
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Journal of Rehabilitation Research and Development Vol . 24 No . 4 Fall 1987
Unprocessed
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173.20
Section III . Speech Processing Hearing Aids : \f und et al.
The results of the subjects in the miscellaneouscategory are illustrated in Figure 7 . Of these, subjectS7, with a relatively mild, flat hearing loss, producedhigher scores with no aid (unprocessed) under sixof the ten signal and noise combinations . Only atlow S/N and at the lower noise level did the modelaid prove to be of noticeable benefit, but it wasalways superior to the conventional aid for thissubject . S8 and S11 both have reverse slope losses,but the conventional aid gave better performancethan the model aid for S8 at most conditions andthe reverse is true for S11 . S12 almost belongs inone of the sloping-loss-subject groups, both in termsof his hearing loss configuration and his performanceon the NST. S16 performed better with the conven-tional aid, but his results on the various pitch testsinvolved in the 1-R function phase of the experimentsindicated that he has a much more complex hearingloss than any of the other 19 subjects in the exper-iment. Consideration of the complexities of thishearing loss will be deferred until the discussion.
Now that the presentation of the data for all 20subjects is completed, we will present the summarytables of the statistical analyses . An analysis ofvariance was done, individually for each subject andnoise level, on the raw data used to compute thepercent scores of Figures 4-7 . These analyses aresummarized in Table 1 where the significance levels
Table LSummary of Analyses of Variance.
60 dB SPL SSN
for the main effects of aids (unprocessed, modelaid, conventional aid), and the interactions of aidsand S/N, are given . The main effects of S/N are notincluded in the table because they were significantat the p<0 .001 for all subjects at both noise levels.As expected, and as was seen in Figures 4-7, everysubject performed better at higher S/N ratios . Themain effects of aids were significant at p<0 .001 forall subjects at 60 dB SPL SSN and also at 70 dBSPL, SSN except for S8 (n .s .) and S14 (p<0 .05).The interaction of aids and S/N was significant for19 subjects at 60 dB SPL SSN (excluding S17) andfor 17 subjects at 70 dB SPL, SSN (excluding S5,S10 and S20).
Based upon these analyses of variance, TukeyWSD (10) post hoc analyses were performed forboth the main effect of aids and the aids x S/Ninteraction . Tables 2, 3, and 4 illustrate, respectively,the comparisons of the model aid and the unpro-cessed condition (M vs U), the conventional aid andthe unprocessed condition (C vs U), and the con-ventional and the model aid (C vs M) . Within thebody of these tables, both the superior conditionand the significance level are given : a) The superiorcondition is identified by the letter in the table entry,U for unprocessed, C for conventional aid, and Mfor model aid . A hyphen (-) appears if neither wassuperior . b) The significance level is indicated by
70 dB SPA, SSN
Subject Aids Aids X S/N Aids Aids X S/N
SI <0.001 <0.001 <0 .001 <0.01S2 <0.001 <0.001 <0 .001 <0.001S3 <0.001 <0.05 <0 .001 <0.001S4 <0.001 <0.001 <0 .001 <0.001S5 <0.001 <0.001 <0 .001 n .s.S6 <0.001 <0 .001 . <0 .001 <0 .001S7 <0.001 <0.005 <0 .001 <0.005S8 <0.001 <0.001 n .s . <0 .001S9 <0.001 <0.001 <0 .001 <0.001S10 <0.001 <0.001 <0.001 n . s.Sib <0.001 <0.001 <0 .001 <0.001S12 <0.001 <0.001 <0 .001 <0 .001S13 <0.001 <0.001 <0 .001 <0 .05S14 <0.001 <0.001 <0 .05 <0 .001S15 <0.001 <0.001 <0.001 <0 .01S16 <0.001 <0.001 <0.001 <0 .001S17 <0.001 n .s . <0 .001 <0 .001S18 <0.001 <0.001 <0 .001 <0 .05S19 <0.001 <0.001. <0 .001 <0 .001S20 <0.001 <0 .01 <0 .001 n .s .
174
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the number of letters, one for <0 .05 and two for<0.01 . To make these summary tables slightly easierto read, upper case letters are used for the maineffect columns and lower case letters for the inter-action columns . Tables 2—4 indicate with statisticalsignificance essentially what we have already seenin the Figures 4-7 . Either aid is almost always betterthan no aid and the model aid is more often betterthan the conventional aid . Looking specifically atthe comparison between the model and conventionalaids in Table 4, we see the following: a) Only in S8at 60 dB SPL SSN was the conventional aid superiorin the main effect . b) Only S16 never performedbetter with the model aid than the conventional aid,i .e ., there is no "M" or "m" in the S16 row ofTable 4. c) Five subjects produced a significant maineffect in favor of the model aid (S1, S2, S4, S12,S18) at 60 dB SPL SSN and one, S4, maintainedthat at 70 dB SPL SSN . d) Six subjects neverperformed better with the conventional aid than themodel aid (S2, S4, S7, S11, S17, S18).
DISCUSSION
Before beginning the discussion of these particularresults and their relationship to other work in thearea of multichannel compression hearing aids, two
general points of major significance must be consid-ered . The first is the general problem that anymultichannel compression hearing aid is a verycomplex device with a large number of parameterswhose values must be determined prior to its im-plementation . All of the frequency, intensity, andtime parameters involved in such a device must bedefined. While it is difficult to specify how manyparameters such a device might have, it is clear thatlittle is known about determining optimal values forany of them . We do not know which parametersmight be critical or even how to set most of themso that their values will not have a negative impacton hearing in SNHL subjects . If, in addition, dif-ferent parameters have different effects in differentSNHL subjects, then the difficulty of developing amultichannel compression hearing aid increases evenfurther.
In the face of such complexity, investigators stillwishing to study multichannel compression must atthe onset of a research program make many deci-sions based upon general knowledge and a prioritheoretical assumptions concerning the nature ofnormal hearing and the nature of the functionaldeficit(s) in individuals with SNHL . To the extentthat the general knowledge and theory is correctlyapplied to the problem and, even more importantly,to the extent that the assumptions concerning the
Table 2.Summary of Tukey WSD Analyses (Model vs Unprocessed).
60 dB SPL SSN
70 dB SPL SSN
Subject ALL 15 10 5 0 — 5 ALL 15 10 5 0 — 5
S I. MM mm mm mm mm mm MM mm mm mm mm mmS2 MM mm mm mm mm mm MM mm mm mm mm mm
S3 MM mm mm mm mm mm MM mm mm mm mm mmS4 MM mm mm mm mm mm MM mm mm mm mm mmS5 M — mm mm mm mm — mm mm mm mm mmS6 MM mm mm mm mm mm MM mm mm mm mm mmS7 — 1111 uu mm mm mm — uu uu uu uu mS8 — uu . mm mm mm — — uu uu uu mm mmS9 MM mm mm mm mm mm MM mm mm mm mm mmS 10 MM mm mm mm mm — M mm mm mm mm —S11 MM mm mm mm mm mm MM mm mm mm mm mmS12 MM mm mm mm mm mm — uu mm mm mm mmS13 MM mm mm mm mm mm MM mm mm mm mm mmS14 M — mm mm mm mm — uu uu — mm mm
S 15 MM mm mm mm mm mm MM mm mm mm mm mm
S16 MM mm mm mm mm mm M uu mm mm mm mm
S17 MM mm mm mm mm mm MM mm mm mm mm mm
S18 MM mm mm mm mm mm — mm mm mm mm mm
S19 MM mm mm mm mm mm M mm mm mm mm mm
S20 M mm mm mm mm — M mm mm mm mm —
175
Section III . Speech Processing Hearing Aids : Yund et al.
functional deficits in SNHL are correct, investiga-tors studying multichannel compression should ex-pect to find a significant degree of success, at leastin some subjects . Given the complexity of theproblem, a significant degree of failure should beexpected in some subjects as well . In interpretingthe results in such a context, a high degree ofsuccess in a small number of subjects offers criticallyimportant support for the basic idea . When suchhigh performance is obtained in some subjects,relatively poor performance in other subjects canbe viewed more as an opportunity to learn aboutthe optimal parameters than as a general failure ofthe basic idea. Furthermore, statistical tests for anentire group of subjects should not be applied inthis context and thus, there must be enough dataover a large enough range of conditions within onesubject to evaluate the performance of an aid oneach subject individually.
A second general problem that has plagued re-search in multichannel compression hearing aids hasbeen the question of how to evaluate any perform-ance benefit obtained with such a device . Theobvious answer to this question, "Use the appro-priate other hearing aid, i .e ., the best one not in themulitchannel compression category," only furthercomplicates the problem, since the optimal choiceof parameters for such "other" hearing aids is not
much better understood than it is for multichannelcompression aids . But how can a multichannelhearing aid be evaluated without comparing itsperformance to that of the "other" aid? One criticalpoint of comparison is the performance of thehearing-loss subject with no aid at all ; clearly, any"good" hearing aid must provide a significant im-provement over no aid at all . Another critical pointof comparison is the performance of normal hearingsubjects ; clearly, any hearing aid that producednormal performance for a subject with SNHL, wouldbe a very good hearing aid for that subject . Ofcourse, any hearing aid (multichannel compressionor other) would be expected to fall somewherebetween these two extremes, but its position in thatrange is surely a critical piece of information aboutits performance . We would also argue, however,that some other hearing aid must be included in amultichannel compression study . The other aid isnecessary if only to establish that the particularmethods chosen do not produce virtually the samelevel of performance with any reasonable hearingaid. In this sense, the other aid becomes as much areference condition to evaluate the experimentalprocedures as it is a control condition against whichto evaluate the multichannel compression aid.
In our present experiments we chose as the otheraid condition a commercially available hearing aid
Table 3.Summary of Tukey WSD Analyses (Conventional vs Unprocessed).
60 dB SPL SSN
70 dB SPL SSN
Subject ALL 15 10 5 0 -5 ALL 15 10 5 0 -5S1 — cc cc cc — — CC cc cc cc cc ccS2 CC cc cc cc cc cc C — cc cc cc ccS3 CC cc cc cc cc cc CC cc cc cc cc ccS4 CC cc cc cc cc cc CC cc cc cc cc ccS5 — cc c cc — — — — cc cc cc —S6 CC cc cc cc cc cc CC cc cc cc cc ccS7 — uu uu uu — cc — uu uu uu uu uuS8 CC — cc cc cc cc — uu uu — cc ccS9 CC cc cc cc cc cc CC cc cc cc cc ccS10 CC cc cc cc cc — C cc cc cc cc —S11 CC cc cc cc cc cc CC cc cc cc cc ccS12 C cc cc cc cc cc — uu cc cc cc ccS13 CC cc cc cc cc cc CC cc cc cc cc ccS14 C — cc cc cc cc — uu uu c cc ccS15 CC cc cc cc cc cc CC cc cc cc cc ccS16 CC cc cc cc cc cc CC uu cc cc cc ccS17 CC cc cc cc cc cc CC cc cc cc cc ccS18 CC cc cc cc cc cc — cc — cc cc ccS19 CC cc cc cc cc cc CC cc cc cc cc ccS20 C cc cc cc cc — CC cc cc cc cc —
176
Journal of Rehabilitation Research and Development Vol . 24 No . 4 Fall 1987
fit by an independent practicing audiologist becausethat method at least provides a realistic control aidthat could be used outside the laboratory . It can beargued that this is not the best possible other aid,but the actual frequency-gain characteristics of theconventional aids used (Figures 4-7, right panels)puts them well within the range of accepted values:our conventional aid frequency-gain slopes generallyhave more high frequency emphasis than the LDLcharacteristics (12) and less than the most extremeL4 curve used by Lippmann, Braida, and Durlach(14) . Furthermore, our conventional aid was indi-vidually fit to the subject and it has the same fidelityand low distortion as the multichannel compressionand unaided conditions because, except for thedigital processing that represents the action of theother aid, all other parts of the system are identicalThe above considerations suggest that only veryminor differences (in an unpredictable direction)would exist between our conventional aid conditionand linear control conditions used in other studies.
The results presented here (Figures 4--7) demon-strate that the model aid can be very effective forsome subjects and is usually more effective than awell-fit conventional aid . In terms of the generalevaluation criterion presented above, the most im-portant results occur in 4 of the 20 subjects (S2, S4,S15, S17) where the performance with the model
aid approaches that of the normal-hearing subjectson this same task . The two earlier subjects, S2 andS4 in Figure 4, showed this high degree of perform-ance on both noise levels while the other two, S15and S17 in Figure 5, showed such consistently highperformance only on the 60 dB noise condition.These differences may be a result of a practice effectsince both S15 and S17 ran the 70 dB noise conditionbefore the 60 dB noise condition and the latersubjects (S11 to S20) had only half the number of"quiet" sessions prior to "noise" sessions as didthe earlier subjects (SI to S10) . Laurence, Moore,and Glasberg (11) report a practice effect with boththeir compression aid and their high fidelity linearaid and Braida et al . (3) discuss practice effects inexperiments with a multichannel compression hear-ing aid.
Whether or not the results for S15 and S17 alsoinclude a practice effect, it is clear that in the caseof 4 of the 20 subjects, the model aid was able toproduce near-normal performance on this difficulttask of speech recognition in noise . It should alsobe stressed that the near-normal performance oc-curred not only under the relatively easy, high S/Nconditions, but also under the most difficult, low S/N conditions. This low S/N performance is espe-cially important because it represents those condi-tions outside the laboratory where almost every
Table 4.Summary of Tukey WSD Analyses (Model vs Conventional).
60 dB SPL SSN
70 dB SPL SSN
Subject ALL 15 10 5 0 -5 ALL 15 10 5 0 --5
S1 M mm mm mm mm mm — cc — — mm mmS2 M mm mm mm mm mm — mm mm mm mm mmS3 — — mm mm mm mm — cc m mm — mmS4 MM mm mm mm mm mm MM mm mm mm mm mmS5 — cc mm mm mm mm — mm mm mm mm mmS6 — — — — — — — mm mm cc — ccS7 — mm mm mm mm mm — mm mm mm mm mmS8 CC cc c cc cc cc — mm — — cc ccS9 — cc — mm cc cc — cc — — — —S10 — mm mm — cc — — mm mm — — —S 11 — mm — mm mm mm — mm mm mm — mmS 12 MM mm — mm mm mm — cc cc mm mm mmS 13 — mm — — cc c — mm mm mm mm —S14 — — — mm cc cc — mm — — cc ccS 15 — mm mm mm mm mm — cc cc mm — —S16 — cc — cc cc cc — — c cc cc ccS 17 — — — mm mm mm — mm — — mmS 18 MM mm mm mm mm mm — mm mm — — —S 19 — cc cc — mm mm — cc cc cc cc ccS20 — mm mm cc mm — — cc cc cc — —
177Section III . Speech Processing Hearing Aids : Yund et al.
normally available speech cue would be critical formaintaining communication.
It is interesting that all four of the subjects whoachieved near-normal performance come from thegroup of ten subjects (Figures 4 and 5) characterizedas having more gradually sloping audiograms . Fur-thermore, these four high performance subjects alsohave relatively mild hearing losses compared to theother subjects in that group. Mean thresholds foreach of the ten subjects for 500-4000 Hz (see Figures4 and 5) are, in order of increasing threshold : S2(34 .3 dB HL), S17 (35 .0), S4 (44.3), S14 (44 .3), S15(45 .0), S9 (47 .1), S3 (48.6), S6 (52 .1), S18 (52 .8),S19 (55.0) . Thus, according to mean threshold value,the four high performance subjects have the first,second, third, and fifth least severe hearing lossesin this group of the subjects . Whether the severityof the hearing loss places some direct limitation onthe individual's ability to extract information fromthe compressed signal or whether the effect is a lessdirect one (e .g., where learning to use the systemis merely more difficult) cannot be determined atthis time . Perhaps even subjects who are not se-verely impaired need training to effectively use theinformation in the compressed signals . It also wouldbe valuable in future research to determine howwell normal-hearing subjects perform these discrim-inations with compressed signals of the type usedhere.
None of the subjects in the group characterizedas having more steeply sloping audiograms (Figure6) achieved near-normal performance with the modelaid. However, the two subjects in the steep-slopegroup who achieved the highest level of performancewith the model aid, Si and S13, were also the twowho have the least severe losses . Si and S13 arethe only subjects in this group with thresholds below80 dB HL for 2, 3, and 4 kHz . Overall, subjects inthis group demonstrate a less consistent advantagefor the model aid over the conventional aid, espe-cially in the more severe cases (510, S20) . Perhapsthere are special problems in implementing acompression mechanism in a frequency region wherethe thresholds, and thus also the predicted amplifi-cation functions, are changing very rapidly . Anotherpossibility (surely true, at least, for S10 above 2kHz) is that the residual sensitivity at higher fre-quencies is insufficient to be useful in extractinginformation from the signals in the higher frequencychannels . A third possiblity, already suggested above,is that more learning is required for the subject to
extract this information in bands where the hearingloss is greater.
One of the subjects in the miscellaneous group,S7 of Figure 7, showed a high level of performancewith the model aid, especially at -5 to +5 dB S/Nwith 60 dB noise . However, her unaided scoreswere so high under all conditions that, outside thelaboratory, she would probably not find any hearingaid worthwhile at this time . Subject S8 has a reverseslope hearing loss between 500 Hz and 2 kHz withvery little threshold elevation at 2 and 3 kHz andan increasing loss beyond 3 kHz. He performed thespeech task better with the conventional aid thanwith the model aid, perhaps because the model aidamplified signal components in the lower frequenciesmore, and those in the 2 to 3 kHz range less, thanthe conventional aid . Lutman and Clark (15) re-ported that 2 kHz was a critical frequency in deter-mining speech performance in their experiments.Clearly, the model aid does not work well in thecase of a hearing loss like that of S8 . S11 has a lessextreme reverse-slope audiogram and performedbetter with the model aid than the conventional aid.She also showed much more of an unaided deficitthan did S8 . Future studies on S8 and S11 as wellas other such subjects might yield a better under-standing of such subjects' abilities to use multi-channel compression systems.
S16 is a subject who demonstrated several uniqueaspects in his hearing loss : a) The sloping portionof his audiogram is all in the region around 500 Hz.b) From 1 to 8 kHz, his audiogram is virtually flat.c) A 90 dB SPL speech signal with conventional aidprocessing was too loud (so uncomfortable that helistened to only a few such stimuli, although hisconventional aid did include output compression).d) Loudness discomfort levels were less than 100dB SPL above 750 Hz . e) Above 1 kHz, he displayedan abnormally large amount of binaural diplacusiswith the right ear needing a frequency about 10percent higher than the left to yield a matching pitchup to 3 kHz but then approaching 4 kHz the matchwas about 7 percent in the opposite direction.[According to van den Brink (20), normal differencesare less than 4 percent ; also see Shambaugh (19) fora discussion of binaural diplacusis in patients withhearing loss .] In S16, the conventional aid wasgenerally superior to the model aid and it is temptingto speculate that the success of the conventionalaid is due to the fact that it amplifies less in theregion from 750 Hz to 3 kHz than one might want
178Journal of Rehabilitation Research and Development Vol . 24 No . 4 Fall 1987
in looking at the audiogram shape . Further study ofthis subject could be very interesting from both aclinical and a theoretical perspective.
To the extent that there is any pattern in themodel aid results across subjects, the model aid wasleast effective in comparison to the unaided condi-tion at the high S/N and that effect was moreapparent at the 70 dB noise condition than at 60 dBnoise. In this case, we cannot attribute the 70 dBversus 60 dB noise difference to learning to use themodel aid because S7 and S8, who most clearlyshow the effect, are among those subjects who ranthe 60 dB noise condition prior to the 70 dB noisecondition. Nevertheless, the overall effect as afunction of S/N is not very surprising, since at thehigher S/N the signal components are at higherintensities than noise components . Under thesecircumstances, the model aid tends to amplify signalcomponents less than noise components and, ineffect, reduce the S/N . The difference in effects at60 dB and 70 dB noise may be a result of a greatereffective S/N reduction because of where stimulusand noise components fall on the amplificationfunctions at the different intensities or a result ofincreased masking at the higher intensity . At low S/N the effective reduction of S/N no longer occurs(since signal and noise are at almost the same level)and other, beneficial effects may occur : a) Lowlevel signal components in channels where relativelylittle noise occurs may be amplified—this is essen-tially the explanation that accounts for the positiveeffect of the "frequency equalized" linear aid atlow S/N . b) Signal components in channels wherethere is a relatively high average noise level mayoccur at times when the noise is, by chance, at alow level . If the system can respond quickly enoughto the chance reduction of the noise and increaseits amplification to bring that signal component to alevel that the subject can use, it could produce aneffective increase in the overall S/N relative to eitherthe unprocessed or the conventional aid condition.This explanation for the compression aid's successat low SIN is related to what presumably is happen-ing in the normal-hearing subjects without any ex-ternal amplification . Any signal components suchas those described above would be helpful to thenormal-hearing subject because they are suprathresh-old and thus can be used whenever a negativefluctuation in the noise permits them to be detected.It should be noted that point (b) above would not
apply to all or even most multichannel compressionsystems, but only to those with narrow channelsand short release times and where at least the noisecomponents were in the compression phase of thechannel involved . The narrow channels and shortrelease times give the system an opportunity toapply its compressive function to enhance the lowerlevel signal relative to the noise.
Points (a) and (b) in the previous paragraph mayexplain why Walker, Byrne, and Dillon (22), usingthe same type of stimuli, found primarily negativeresults for a multichannel compression/expansionsystem . Low level components either in a differentchannel or in the same channel at a different timeare relatively reduced in the expansion range of thedevice. They found positive results only at the highintensity condition where stimuli were mainly in thecompression range of the device . Indeed, accordingto the rationale of the model aid, restricting thecompression phase of such a device to higher in-tensities (with either linearity or expansion at lowerlevels) would increase the difference between theneural information coming from the cochlea in theSNHL subject as compared to the normal-hearingsubject.
Barfod (2) used a similar rationale and equalloudness contours to determine "hearing loss" asa function of frequency and intensity . Those hearingloss functions (his Figure 3) are not dissimilar to theamplification function plotted in our Figure 3. edemonstrated that a 4-channel compression systembased on the hearing loss functions was as good asthe optimized linear control for speech at 65 dBSPL with noise at various levels (S/N from + 15 dBto -5 dB) . It is probably true, as Barfod asserts,that the 4-channel compression system would workbetter than the linear control at other speech in-tensities, but that should be demonstrated . Barfod'sdata would be much easier to interpret if normal-hearing results and these subjects' unaided resultshad been included . To apply the second generalpoint from earlier in this discussion, consider howdifferently one might interpret Barfod's data if theperformance of the 4-channel and the linear systemswere near-normal as opposed to near-unaided . Con-sider also, how easy it would be to compare Barfod'sdata to the data presented here if it were possibleto normalize the performance of any subject ineither study to a numerical scale where 0 representedunaided and 100 represented normal .
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Section Ill . Speech Processing Hearing Aids : Yund et al.
In summary, the 8-channel compression hearingaid studied here was designed to precisely compen-sate for deficits in the subject's cochlear I-R trans-duction mechanism, deficits that vary with fre-quency and intensity . The particular I-R functionsused here were developed from a model formulatedto explain dichotic pitch interactions . The model aidis a compression device because of the shape of theI-R functions and a multichannel compression devicebecause that shape varies with frequency . Althoughno other specifications of the aid are defined by themodel, many narrow channels with fast acting, non-distorting compression mechanisms seem desirable.Using a broad range of test conditions (55 to 85 dBSPL speech sounds and 60 and 70 dB SPL speechspectrum noise), we have shown that a particular8-channel compression system developed aroundmodel I-R functions can be a very effective hearingaid. The aid is more effective for more graduallysloping audiograms and for less severe hearinglosses. Four subjects with less severe, gradually
sloping losses produced near-normal performancein the model-aid condition . Some aspects of thepresent results suggest that learning to use the modelaid may be important . Future work will includedetailed analyses of the error patterns in the speechresults presented here as well as other psychophys-ical studies on these subjects in an attempt tounderstand the differences among subjects and theirdifferential success with our 8-channel compressionsystem.
ACKNOWLEDGMENTS
We are very grateful to all the subjects who have listenedwith such good nature to so many tones and nonsense syllablesthroughout the course of these experiments . We also thank allof those who have provided technical and administrative supportto this project—they were always willing to make that extraeffort when it was needed.
REFERENCES
1. American National Standards Institute : American NationalStandard Specifications for Audiometers . ANSI S3 .6-1969.New York : American National Standards Institute, 1969.
2. Barfod J : Multichannel compression hearing aids : Exper-iments and considerations on clinical applicability . InSensorineural Hearing Impairment and Hearing Aids, C.Ludvigsen and J . Barfod, Eds . Scand Audio/ Suppl 6 :315-340, 1978.
3. Braida LD, Durlach NI, De Gennaro SV, Peterson PM, andBustamante DK : Review of recent research on multibandamplitude compression for the hearing impaired . In TheVanderbilt Hearing-Aid Report, State of the Art ResearchNeeds, G . A . Studebaker and F . H . Bess, Eds . UpperDarby, Pennsylvania : Monographs in Contemporary Au-diology, 1982.
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