Preferred frequency response for two- and three-channel ... · or three independent channels ....

M Department ofVeterans Affairs

Journal of Rehabilitation Researchand Development Vol . 30 No . 3, 1993Pages 305—317

Preferred frequency response for two- and three-channelamplification systems

Donald D . Dirks, PhD ; Jayne Ahlstrom, MS ; P. Douglas Noffsinger, PhDDepartment of Veterans Affairs Medical Center, West Los Angeles, CA 90073 ; Department of Head andNeck Surgery (Audiology), UCLA School of Medicine, Los Angeles, CA 90024

Abstract —The purpose of these investigations was tocompare the preferred frequency-gain responses obtainedfrom two- and three-channel amplification systems . Thecurrent experiments were limited to a linear system inwhich the crossover frequency dividing the channels wassystematically varied . The subjects for the experimentwere nine individuals with mild to moderately severesensorineural hearing loss with various audiometric con-figurations. The subjects listened to continuous discourse,in noise, via a computer-controlled digital master hearingaid containing two real-time data acquisition processors.Initially, a modified simplex procedure was used toobtain preferred frequency-gain responses using severaldifferent crossover frequencies . A round-robin procedurewas then conducted in which each preferred responsefrom the simplex was compared with every other pre-ferred response . The frequency-gain responses chosenmost often for the two- and three-channel systems werecompared. The results showed no significant differencesbetween the preferred frequency-gain response for thetwo- versus the three-channel system . In addition, thepreferred response chosen most often was not consistentlyobserved at the same crossover frequency for all subjects,with the exception of those with steeply sloping hearingloss who chose 1,120 Hz as the first or second preferencefor the two-channel system . The round-robin results wererank-ordered according to the number of times eachfrequency-gain response was chosen . In general, subjectschose several frequency-gain responses at various cross-over frequencies, which were not significantly differentfrom each other statistically . The results of a final

Address all correspondence and requests for reprints to : Donald D.Dirks, PhD, Department of Head and Neck Surgery, UCLA School ofMedicine, CHS 62-132, Los Angeles, CA 90024 .

experiment suggested that physical similarities in thepreferred responses chosen at the various crossover fre-quencies played a role in the rank-ordering of thepreference judgments obtained in the original investiga-tion.

Key words : amplification systems, digital hearing aids,hearing aids, preferred frequency-gain response, sensorin-eural hearing loss, two- and three-channel amplificationsystems.

INTRODUCTION

The goal of most hearing aid selection proce-dures is to provide adequate gain so that speech cuesimportant for intelligibility are audible. This generalgoal is not easily achieved for many patients withcochlear impairment who have elevated thresholdstogether with recruitment, which reduces the dy-namic range . There is evidence (1,2) that an appro-priate amplification system for such patients shouldrestore the speech cues to normal loudness levels ateach frequency and amplitude . For most hearing-impaired individuals, however, the loudness func-tion and dynamic range of hearing varies over thefrequency spectrum. Theoretically, the ideal amplifi-cation system would contain a sufficient number offrequency bands or channels, each acting indepen-dently, to accommodate the variations in dynamicrange that exist at frequencies within the speechspectrum. In practice, several investigators (3,4)have advocated the use of two-channel amplification

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systems, in which the channels operate indepen-dently, and often contain some form of compres-sion .

Currently, several digitally controlled, program-mable hearing aids are available incorporating twoor three independent channels . Often, the signals aresplit into a high- and low- frequency channel,allowing for relatively independent processing ofconsonant and vowel cues, respectively . A thirdchannel, which allows for increased bandwidthflexibility, is also available in at least one digitallyprogrammable hearing aid . This might be especiallyuseful for individuals with large variations in bothaudiometric configuration and dynamic range acrossthe frequency spectrum (e .g., patients with steeplysloping high-frequency loss) . There is, however, nostrong evidence that demonstrates the superiority ofhaving three versus two independent channels . Thispaper contains results of our initial efforts todevelop and test a digital master hearing aid withtwo and three independent, adjustable channels.Specifically, the digital master hearing aid was usedto compare the preferred frequency-gain responsesobtained from hearing-impaired individuals utilizingthe two-channel versus three-channel systems.

An important characteristic of two- and three-channel systems is the flexibility to adjust thecrossover frequency(ies) that divide the channelsaccording to the preferences or requirements of thelistener. No rules or recommendations for choosingthe desired bandwidth of each channel have, as yet,been studied systematically . It is possible that thechoice of bandwidths preferred by the individualhearing-impaired listener may interact with thepreferred frequency-gain responses . In fact, anindividual listener may potentially find more thanone frequency-gain response acceptable as the cross-over frequency is varied . Because of this potentialinteraction between the preferred frequency-gainresponse and the crossover frequency(ies), the cur-rent investigation incorporated two measurementtechniques (one dependent on the other) to deter-mine a final frequency-gain/crossover frequencycombination that was preferred for the two- andthree-channel systems. The first, a modified simplexadaptive strategy (5) was used to determine apreferred frequency-gain response for each of sev-eral predetermined crossover frequencies. This pro-cedure resulted in a set of preferred frequency-gainresponses, each obtained with a different crossover

frequency. These responses were stored in adatabase for subsequent comparison with eachother, using a second measurement technique, around-robin paired comparison strategy . Since inthat strategy, each preferred response was comparedwith each other response, it was possible to rank-order the number of times each frequency-gain/crossover frequency combination was pre-ferred, as well as to determine the ultimate"winner(s) ." From these results, comparisons couldthen be made between the preferred frequency-gain/crossover frequency combination for the two-and three-channel systems.

METHOD

InstrumentationFigure 1 contains a block diagram of the basic

components of the experimental system . The speech(continuous discourse) and noise signals used in thisexperiment were recorded on digital audio tape(DAT) recorders . These same recorders were thenused to deliver the stimuli to the digital masterhearing aid. The output of the recorders wereattenuated (fixed attenuator, FATT) to achieve thedesired signal-to-noise ratio (S/N), mixed, and thendelivered to a compression/limiter device (having anattack time of 2 msec and release time of 45 msec) toreduce the effects of any spurious peaks that mighthave occurred in the speech or noise stimuli . Thethreshold of the compression/limiter was set at 85dB sound pressure level (SPL) . Followingpreamplification, the stimuli were delivered to ananti-aliasing filter to prepare the stimuli for digitalprocessing . The stimuli were then converted fromanalog to digital form using 16 bit resolution with asampling rate of 19.84 kHz . The digital processingsystem incorporated two real-time data acquisitionprocessors (Ariel DSP-16 +) operating in parallelunder PC-AT control . The computer also controlleda programmable attenuator (PATT) to establish theoverall SPL. The signals from the two Ariel boardswere mixed and delivered to a reconstruction oranti-imaging filter to smooth the output of thedigital-to-analog conversion. Both the anti-aliasingand reconstruction filters were programmed tolow-pass filter the signals with a cutoff frequency at7.0 kHz . The attenuation was 51 .5 dB at the Nyquistfrequency of 9 .9 kHz.

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Amplification Systems

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Figure 1.Block diagram of the basic components of the digital master hearing aid system . DAT = digital audio taperecorder ; FATT = fixed attenuator ; LIM = limiter ; AAF = anti-aliasing filter ; AIF = anti-imaging filter;PATT = programmable attenuator ; I/O = input/output board.

The processing system was programmed toproduce four filter bands with the transition bandbetween

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impulseresponse (FIR) digital filter . The various experimen-tal channels (or bandwidths) were formed by com-bining the one-third octave bands, starting with theband having a nominal center frequency of 0 .315kHz. The selection of the bandwidths of the desiredchannels was limited by the transition bandwidth of500 Hz. Band SPL could be varied in 1 dBincrements. The overall level of the stimuli deliveredto the earphones was limited to an output level of118 dB SPL (in a 6 cc coupler) by a second comp-ression/limiter.

SubjectsFor this experiment, nine subjects with mild to

moderately severe sensorineural hearing loss partici-pated. Three subjects had gradually sloping audio-metric configurations, four subjects had steeply slop-ing audiometric configurations, and two subjectshad flat or uniform audiometric configurations.Speech reception thresholds ranged from approxi-mately 15 to 50 dB HL . Of the nine subjects, sixsubjects were binaural hearing aid users and threesubjects had no prior experience with amplification .

Test Stimuli and InstructionsThe principal test stimulus used in this experi-

ment was continuous discourse (primary message)recorded by a male talker embedded in a small partynoise (background message) containing a variety ofmale and female talkers . The continuous discoursewas recorded on a DAT recorder in 45-minutesegments. Each recording contained a completestory on various topics with vocabulary found infifth-grade readers . Figure 2 displays the long-termroot mean square (rms) level of the stimuli atone-third octave bands from 0 .125 to 6.3 kHz.During the experiment, the signals were presented atan S/N ratio of + 3 dB. All subjects were able tounderstand the continuous discourse, because theinformation in the meaningful sentences containedmany contextual cues.

Subjects in this experiment were instructed tobase their judgments of preference on several dimen-sions of speech quality and intelligibility accordingto their individual standards . The instructions formaking the preference judgments were, thus,broadly conceived, and permitted judgments basedon the individual's own criterion of comparativeimportance of intelligibility and quality attributes.In our opinion, the preference judgment for speechin real-life situations is based on a variety of

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Figure 2.Long-term rms spectrum of the continuous speech stimuli andthe background party noise in one-third octave bands levels.

intelligibility and quality attributes that are highlyrelated and not readily separated.

In addition to the speech and noise stimuli,threshold testing was administered to each subjectfor descriptive purposes using the modifiedHughson-Westlake procedure (6) conducted in 5-dBsteps . Loudness judgments were also obtained fromeach subject using the Loudness Growth in 1/2-Octave Bands (LGOB) method described by Allen,et al . (7) . This procedure has been incorporated aspart of the Resound Digital Hearing System (8) andwas used in the study to describe the comfortloudness levels . In the LGOB procedure, one-halfoctave bands of noise, centered at 500, 1,000, 2,000,and 4,000 Hz are used as stimuli . The noise bandsare presented to the subject (via an insert earphone),randomized over frequency and level, and thesubject rates the loudness on a seven-point scalefrom "cannot hear" to "too loud ." A rating of"3," reported in this study, is approximately equiv-alent to a judgment of most comfortable listeninglevel.

ProceduresIn two previous experiments (9), comparisons

between the two- and three-channel hearing aidsystems were made using a master hearing aid . Forthose investigations, the signal was presented via aloudspeaker and delivered to a microphone locatedin the shell of a behind-the-ear hearing aid . After

modification by the digital master hearing aid, thesignal was routed to a receiver, also located in theshell of the hearing aid, and delivered to the subjectin the conventional manner via tubing and anearmold. This method, while realistic, led to severalproblems that may have interfered with the experi-mental results . Two of the problems were especiallydifficult to control . First, the input signal to themicrophone was modified by the head diffractioneffects and the position of the microphone on thelistener, and the output signal varied with the fit andtype of earmold . These factors often resulted indiscrepancies between the frequency response in thecenter and surrounding cells of the simplex matrixrelative to the desired levels . Various modifications(i .e ., Libby horn, dampers, etc .) were needed forevery subject in order to match the levels measuredat the probe microphone to the desired levels.Second, in a significant number of the subjects,acoustic feedback problems were encountered, espe-cially in cells of the simplex matrix where the gain inthe high frequencies was increased by as much as 12dB over the National Acoustic Laboratory (NAL)predicted gain (10) within the center cell (startingestimate) . In order to eliminate these problems, itwas decided to conduct the current experimentsunder earphone conditions . We did, however, incor-porate modifications in the software so that theheadphone SPL measured in a 6 cc coupler (via themaster hearing aid system) simulated the SPL at theeardrum for a sound field hearing aid condition . Toobtain the desired coupler response, correctionvalues suggested by Bentler and Pavlovic (11) for the6 cc-to-eardrum and soundfield-to-eardrum averagetransfer functions were applied to the long-term rmsmeasured speech levels (Figure 2).

In order to obtain the preferred frequency-gainresponses for two- and three-channel amplificationsystems, the modified simplex strategy was used,similar to the procedure described by Neuman, et al.(5), with the exception that the frequency responsesin the matrix were not based on loudness . Thesimplex procedure consisted of a 5 x 5 matrix of cellsfor the two-channel system, and a 5 x 5 x 5 matrix ofcells for the three-channel system . Each cell con-tained a different frequency-gain response . Thecenter cell of the simplex procedure was pro-grammed to provide the frequency-gain characteris-tics predicted by the revised NAL prescriptionprocedure (plus the correction values for simulating

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a sound field hearing aid condition, as previouslyindicated) for each individual subject. The responsecharacteristics in the remaining cells varied systemat-ically, in 6 dB steps, relative to the response of thecenter cell (NAL) . Thus, for the two-channel sys-tem, with a crossover frequency of 1,200 Hz, the cellon the vertex of the matrix immediately above thecenter cell changed by 6 dB in the high-frequencychannel while the level in the low-frequency channelremained the same as in the center cell . For a cell onthe abscissa adjacent to the center cell, the responsewas raised by 6 dB in the low-frequency channel,while the high-frequency channel remained un-changed . A similar arrangement was used for thethree-channel system, with a 6 dB increase in one ofthe three channels for each cell in the matrix.

The simplex strategy was conducted in twotrials. In the first trial, an initial estimate of thepreferred frequency-gain response was obtained us-ing a 12 dB step size . The starting point was alwaysthe response in the center cell of the matrix . For thetwo-dimensional simplex (two-channel system),paired comparisons were obtained between theresponses in two cells on the vertical axis, and then acomparison was obtained between two cells on thehorizontal axis. Depending upon the outcome, anew set of comparisons was presented, with thesimplex moving back and forth between the cellsdepending upon the outcome of the previous com-parisons, until a predetermined number of reversalswas reached. A detailed description of the methodand a graphic display of a two-dimensional simplexprocedure can be found in Neuman, et al . (5) and ishighly recommended for the interested reader . Thecomparative frequency-gain responses were variedby a manual switch controlled by the subject . Thesubject could switch back and forth between eitherof the two cells under evaluation as many times ashe or she wished . Once the subject decided whichresponse was preferred, a vote was made and then anew set of responses was automatically presented forcomparison . Testing was terminated during theinitial trial after three reversals in direction for boththe horizontal and vertical axes.

After an estimate of the preferred frequency-gain response was made from the initial trial, thatwinning response then became the starting cell forthe final estimate . The strategy just described wasagain utilized ; however, a smaller step size (6 dB)was used, and testing was terminated after five

reversals . The efficiency of the adaptive strategymethod was improved by this use of a large step sizeto obtain an initial estimate of the response followedby the use of a small step size to obtain the finalestimate.

The same design was used to obtain thefrequency-gain response for the three-dimensional(three-channel) simplex, except that the addition ofthe third dimension required additional comparisonsbeyond those in the vertical and horizontal plane toaccommodate the three components (low-, mid-,and high-frequency) . The two-dimensional simplexprocedure, in general, required 10-14 minutes forcompletion, while the three-dimensional requiredapproximately 15-20 minutes.

Because of the potential interaction between thecrossover frequency and the preferred frequency-gain response, several simplex procedures wereconducted with various crossover frequencies . Forthe two-channel system, results were obtained withcrossover frequencies at 0 .562, 0.891, 1 .12, 1 .78,and 2 .82 kHz . These crossovers were chosen becausethey appeared to provide a representative sample offrequencies across the important audible spectrumof speech.

For the three-channel system, the number ofpotential combinations for frequency crossoverswere numerous ; thus, it was necessary for practicalpurposes to delimit the combinations . Combinationswere chosen that included one low-frequency cross-over paired systematically with a mid- to high-frequency crossover . Using this rationale, fourcombinations of crossover frequencies were used:0.562/1 .120, 0 .562/1 .78, 0.562/2 .24, and 0.562/2.82kHz. Three additional conditions were also tested;0.891/2.24, 0.891/2.82, and 1 .12/2 .82 kHz. Theseseven combinations, therefore, provided a reason-able sample of crossover frequency conditions thatmight be utilized by hearing-impaired listeners . Insummary, there were five frequency-gain responseconditions for the two-channel system and sevenfrequency-gain response conditions for the three-channel system.

In the second part of the experiment, a round-robin strategy, also incorporating a paired compari-son method, was then used to compare eachpreferred response (within the two- or three-channelcondition separately) obtained from the simplexprocedure with every other preferred response atotal of six times . This procedure produced a final

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"winner" from the preferred frequency-gain re-sponses . In addition, the total number of times eachcompeting response was chosen could be rank-ordered for more detailed analysis.

RESULTS

Overall ResultsSeveral descriptive results will be reported ini-

tially to provide a general overview of the outcomeof the round-robin procedure.

Figure 3 illustrates the average preferred fre-quency-gain response resulting from the round-robinprocedure for the two- and three-channel systems insteeply sloping loss and gradual/uniform loss . Ap-plication of the analysis of variance to the dataindicated that there were no significant differencesbetween the preferred responses for the two- andthree-channel systems . Included in the figures arethe average predicted NAL responses for the samesubjects at six major test frequencies . Similar toearlier data from this laboratory (9) and from datareported by French-St . George, et al . (12), each ofthe two groups of subjects in this study preferredslightly more gain in the low-frequency region thanthe NAL prediction . This result was somewhat moreevident for the steeply sloping hearing loss group.

In regard to the preferred frequency-gain/crossover frequency combination, no singlefrequency(ies) was consistently chosen by all sub-

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Figure 3.Average preferred frequency-gain responses resulting from theround-robin procedure for the two- and three-channel systems.The left panel shows the results for the steeply sloping hearingloss group . The right panel shows the results for the graduallysloping/uniform hearing loss group . Included also are theaverage predictions for both groups from the NAL method .

experiments wherepreferred frequency-gain responses are obtained toinstruct the subjects to make judgments on the basisof speech intelligibility and quality factors . Becausethis study was conducted with a linear system, boththe speech and noise changed proportionally assubjects varied the frequency-gain responses withinthe matrix available . As a consequence, the Articu-lation Index predictions of intelligibility remainedthe same for all frequency responses available for anindividual subject . It is assumed then that intelligi-bility was generally constant for the comparisonsmade by an individual subject ; thus, it was reasonedthat speech-quality factors may have played a largerrole than speech intelligibility in the preferencejudgment of the subject.

While it was not possible to determine whatquality factors were critical to each subject in his orher judgment process, it was possible to estimateone factor, the loudness level of each preferredfrequency-gain response, and determine whetherpreference was dependent on loudness level . (Exam-ples of the five preferred frequency-gain responsesfor the two-channel system and the seven preferredfrequency-gain responses for the three-channel sys-tem, from two individual subjects, can be seen inFigure 6 and Figure 7 .) The loudness level of eachpreferred response was estimated in the followingmanner. First, the preference judgments were rank-ordered in terms of the number of times eachpreferred frequency-gain response was chosen dur-ing the round-robin procedure. Second, the loudnesslevel of each preferred frequency-gain response wasestimated using the Zwicker loudness summationmodel incorporating software based on Zwicker'soriginal programs for calculating loudness in one-third octave bands (13) . Frequencies lower than 200Hz and higher than 6,300 Hz were eliminated fromthis calculation because of the restricted frequencyrange of the master hearing aid . The loudness level

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jects, with one exception. Among subjects withsteeply sloping hearing loss, a crossover frequencyof 1,120 Hz was chosen as either the first or secondpreference for the two-channel system. For thegradually sloping/uniform loss group, although nosingle crossover frequency was favored consistently,in general, the preferred frequency-gain responseincorporated one of the mid-frequencies rather thana crossover at the lowest (562 Hz) or highest (2,820Hz) frequencies available.

It is not uncommon in

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of each frequency response was calculated from theaided or amplified speech spectrum obtained frommeasurements of the long-term rms values of thespeech for each preferred frequency response . Theseloudness levels for each response were rank-orderedfrom the highest to lowest in terms of loudness foreach subject. The rank-ordered results from eachsubject were then averaged in contingency tables,and chi-square analysis was conducted separately forthe results from the two- and three-channel systems.The group results from both analyses were found tobe nonsignificant, indicating that the preferencejudgments made during the round-robin procedurewere independent of loudness level.

Rank-order correlations between loudness leveland preferred frequency-gain response from theround-robin procedure were also conducted for eachindividual subject . The relationships varied greatlyfrom an inverse correlation of – 0 .90 for onesubject to a + 0 .50 for another subject . As a group,no clear trend could be found relating loudness levelto the "winner" of the round-robin procedure.

Detailed ResultsAs indicated previously, the results of the

round-robin procedure were rank-ordered accordingto the number of times each competing preferredresponse (at the various crossover frequencies) waschosen . These results indicated that most subjectspreferred several frequency-gain responses at variouscrossover frequencies that statistically (chi-squareanalysis) were not significantly different from eachother. Since this general result was applicable toeach subject, results from two subjects will bedetailed as a representative demonstration of groupdata .

Figure 4 shows the descriptive results from asubject (BH) with a steeply sloping hearing loss . Thepanel on the left illustrates the audiogram describedin SPL values at the eardrum . Also included in theleft panel are the results of the LGOB test . Recallthat the loudness test incorporates loudness ratingsfrom "cannot hear" to "too loud" ; however, theresults shown on the graph are only those rated as"comfortable." The top right panel shows thepreferred aided speech spectrum response for thetwo- and three-channel systems (these results wereobtained by adding the preferred gain to thelong-term rms speech spectrum at one-third octaveintervals), while the relative gain is depicted in the

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Figure 4.Descriptive results from a subject (BH) with a steeply slopinghearing loss . The left panel includes the pure tone audiogramand comfort levels from the loudness test . The upper right panelshows the aided speech spectrum and the bottom right panelshows the relative gain for both the two- and three-channelsystems.

lower right panel . Similar results are shown inFigure 5 for a typical subject (LH) with graduallysloping hearing loss.

Despite the large differences in hearing loss foreach subject, especially in the low- and mid-frequency region, the preferred gain responses aresimilar ; therefore, the preferred aided speech spec-trum values for the two subjects are nearly equiva-lent. It has been shown that hearing-impairedsubjects require amplification such that the ampli-fied speech spectrum corresponds roughly to theloudness contour of comfort loudness levels ratherthan to audiometric configuration (10,14) . Interest-ingly, despite large differences in audiometricthresholds, the comfort loudness levels as well as thepreferred amplified speech spectrum for both sub-jects are similar. The preferred amplified speechspectrum values for both subjects, shown in theright upper panels of Figure 4 and Figure 5, areslightly lower than the comfort loudness levelsmeasured with the LGOB procedure . Assuming thatthere should be general correspondence between thecomfort loudness level and the preferred amplifiedspeech spectrum, the discrepancy between thesevalues for our subjects is of interest . Recall that theloudness levels in this study (using the LGOB) werebased on frequency-specific narrow-band signals,while the preferred aided-speech spectrum was basedon broad-band continuous discourse . No allowancehas been made for the loudness summation mosthearing-impaired subjects experience when listening

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Figure 5.Descriptive results from a subject (LH) with a gradually slopinghearing loss . The left panel includes the pure tone audiogramand comfort levels from the loudness test . The upper right panelshows the aided speech spectrum and the bottom right panelshows the relative gain for both the two- and three-channelsystems.

to broad-band stimuli as compared with narrow-band stimuli . This loudness summation effect varieswith input level and the individual hearing-impairedlistener but, in general, can be quite large (2), and isprobably responsible for the reduced levels of theaided speech spectrum relative to the frequency-specific comfort loudness levels.

Figure 6 shows the preferred frequency-gainresponses at the various crossover frequencies forthe subject (BH) with the steeply sloping hearingloss. The upper panel of the figure illustrates theresults from the two-channel system, while thebottom panel contains similar data from the three-channel system . Also shown in each panel is thenumber of times each preferred frequency-gainresponse was chosen during the round-robin proce-dure. Similar results for the subject (LH) withgradually sloping hearing loss are shown in Figure 7.

Two observations from the data in Figure 6 andFigure 7 are noteworthy . First, the results of theround-robin procedure do not lead to a clear-cut"winner" within the two- or three-channel systemsfor either subject . Chi-square analysis was appliedto the number of times each preferred response waschosen. In each of the four comparisons (two foreach subject), the chi-square was significant(<0 .01). This significant result was attributed to thelowest rank-ordered preferred response . (When thislatter preferred response was removed from theanalysis, the chi-square was not significant .) Thus,the results would suggest that for both the two- and

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Figure 6.Preferred frequency-gain responses, at various crossover fre-quencies for the two- and three-channel systems, for subjectBH. Also included are the number of times each response waschosen, at the various crossover frequencies, during the round-robin procedure.

three-channel system, several preferred frequency-gain responses, with different crossover frequencies,might be acceptable to the subjects . Second, it wasobserved that within each condition, the preferredfrequency-gain responses were often quite similar.The similarity between the physical characteristics ofmany of the preferred responses prompted us toquestion whether or not subjects could, in fact,appreciate the differences among the responses whencomparing the samples. Thus, an additional experi-ment was conducted to determine the degree towhich the subjects, who participated in the initialexperiment, could appreciate the differences betweenthe preferred responses using the continuous dis-course stimuli.

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Figure 7.Preferred frequency-gain responses, at various crossover fre-quencies for the two- and three-channel systems, for subjectLH . Also included are the number of times each response waschosen, at the various crossover frequencies, during the round-robin procedure.

Supplemental ExperimentFor this investigation, the round-robin proce-

dure was again utilized, but instead of making apreference judgment, the subjects were instructed toprovide a "same/different" judgment between thespeech samples heard for the various preferredfrequency-gain responses that had been chosen inthe first experiments . Each frequency-gain responsewas compared with every other response a total offour times.

A control condition was included in this experi-ment in order to verify that the subjects couldreadily perform the same/different task . Two foilfrequency-gain responses (two NAL responses) werepaired against each other in the round-robin proce-

dure. Within the experiment, these two foil re-sponses were the only ones that were identical inphysical characteristics . No subject had difficultyidentifying the two foil responses as the same.

Results . For data analysis, given that eachresponse was compared four times with every other,judgments were considered as "same" if a subjectrated three or four of the four comparisons betweenany two frequency-gain responses as identical . Judg-ments were considered "uncertain" if two of thecomparisons were rated the same and two different.A judgment of "different" was used if the subjectrated three or four of the comparisons as differentfrom each other . Subjects were instructed to ignorethe obvicus fact that each segment of the continuousdiscourse contained different sentences . Similar tothe earlier instructions for making preference judg-ments, each subject judged the comparative frequen-cy-gain responses as same or different depending onhis or her own internal criteria of variations in thequality or intelligibility of the speech.

The results of the same/different task for thenine subjects revealed two somewhat discrete pat-terns of judgments . One, represented by the resultsfor subject BH, indicated that a majority of thecomparisons were judged to be the "same ." Theresults of BH are shown in Figure 8 in terms of thepercent of occurrence of the various ratings for thetwo- and three-channel systems. Note that the"same" judgment was predominant among thepreferred responses for the two-channel system.

Figure 8.Percentage of occurrences of the various ratings from thesame/different task for subject BH.

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While the "same" judgment continued to be in themajority for comparisons made with the three-channel system, the difference in the ratings between"same" and "different" were somewhat moreevenly divided . This result seems reasonable if theearlier data (Figure 6), which illustrated the pre-ferred frequency-gain responses, are reviewed.Those data indicated relatively little difference in thephysical characteristics of the preferred frequency-gain responses chosen for the two-channel system.For the three-channel system, however, the spread inthe physical characteristics of the preferred frequen-cy-gain responses was relatively large in the high-frequency channel, and most likely accounted forthe more evenly divided same/different ratings forthat condition.

Five subjects in this experiment followed thegeneral pattern of BH, implying that the lack of adistinct preferred frequency-gain response (from theround-robin procedure) may be closely related to theinability of these subjects to differentiate perceptu-ally among many of the competing frequency-gainresponses . With one exception, the subjects in thisgroup were those with the steeply sloping hearingloss .

Figure 9 contains the pattern of same/differentjudgments for subject LH . As can be seen, amajority of the comparisons were judged to be"different," especially for the three-channel system.Inspection of the preferred frequency-gain responsesof LH (Figure 7) again shows relatively large

100w• 90c

80L• 70U

o• 60

4--

50ow 40

q• 30c

20Lw 10a.

0

Figure 9.Percentage of occurrences of the various ratings from thesame/different task for subject LH .

differences between the responses for the three-channel system, not only in the high-frequencyregion but also in the low- and mid-frequencyregions . For the two-channel system, variationsbetween the preferred responses in both the low- andhigh-frequency regions are observed; however, thedifferences are not as large as those found for thethree-channel system . Four subjects, with graduallysloping or uniform loss, followed the pattern ofLH. For those subjects it appeared that even thoughthey were able to perceptually differentiate betweenthe responses, a clear-cut preference still did notresult from the round-robin procedure . This resultmay suggest that a range of frequency-gain re-sponses would be acceptable for that subset ofsubjects.

As indicated previously, the simplex procedureresulted in five preferred frequency-gain responsesfor the two-channel system and seven for thethree-channel system . From the round-robin proce-dure a winner was identified, although inspection ofthe data showed that several frequency-gain re-sponses were chosen nearly as often as the winner . Itseems clear from the results of the supplementalexperiment that some subjects found it difficult todiscriminate between several of the frequency re-sponses that were physically similar . Therefore, forthose subjects, the round-robin results may havebeen significantly influenced by this factor.

An additional analysis was performed on theresults from the supplemental experiment . Of inter-est was the degree to which the frequency-gaincurves differed from each other before the curveswere judged to be dissimilar . To obtain this infor-mation, the percent of same or different judgmentswas analyzed as a function of the rms differencesbetween the preferred frequency-gain responses . Thedifferences were quantified in the following manner.First, the difference between each frequency-gaincurve was calculated at each one-third octave band.The rms difference was then calculated as the squareroot of the mean of the sum of the differencessquared . This calculation provided us with distribu-tions of rms differences for the two- and three-channel systems . The distribution of rms differencesranged from 1 dB to 11 dB . The percent of times a"same" (three or four out of four times) or"different" (three out of four times) judgmentoccurred was calculated for each rms difference . Arating of "uncertain" was given to comparisons

SomeUncertain (same=dif F .)

= Different

2 Ch .

3 Ch.

LH

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DIRKS et at .


where the "same" and "different" rating occurredequally.

Figure 10 (two-channel system) and Figure 11(three-channel system) illustrate the percent of timessubjects judged two comparative frequency-gainresponse curves as the "same," "uncertain," or"different" as a function of the distribution of rmsdifferences . As might be anticipated, for small rmsdifferences between curves (1-4 dB), a majority ofthe comparisons were judged the same . For 4 dBrms differences, the same/different judgments wereeither equal (three-channel system) or approachingequality (two-channel system). For larger differences(>4 dB), a majority of the comparisons were judgedto be different . Some irregularities occurred at thehighest rms difference principally because of thesmall number of samples on which to base theestimates. These results suggested that at leastone-half of the hearing-impaired subjects couldperceptually distinguish rms differences of 3-4 dB ofcontinuous speech in noise . Interestingly, for thesimplex procedure we chose a 6 dB step size, based apriori on preliminary observations . Given the resultsfrom the same/different task just reported, the 6 dBstep size appeared in retrospect to be a reasonablechoice. For many subjects, a smaller step size wouldhave led to indecisive behavior and thus test poortest efficiency . A larger step size, however, may nothave yielded sensitive enough data.

DISCUSSION

The results from this experiment demonstratedno significant differences between the preferredfrequency-gain responses for two- or three-channelamplification systems . This observation applied toboth a group of subjects with steeply sloping hearingloss and to a group with gradually sloping oruniform hearing loss . It should be stressed, how-ever, that these studies were conducted under linearamplification conditions, and it is quite possible thata different set of results will apply for systems withvarious types of compression amplification . Con-ducting investigations similar to the current study,but incorporating compression parameters, wouldbe especially timely because of advances in technol-ogy that have already made two- and three-channelhearing aids commercially available .

2 Channel System

Figure 10.Percentage of times subjects judged comparative frequency-gainresponse curves in the two-channel system as "same," "un-certain," or "different" as a function of the rms differencesbetween any two preferred frequency responses.

3 Channel System

Figure 11.Percentage of times subjects judged comparative frequency-gainresponse curves in the three-channel system as "same," "un-certain," or "different" as a function of the rms differencesbetween any two preferred frequency responses.

Although a representative sample of crossoverfrequencies was available to the subjects, no singlecrossover frequency was consistently chosen by allsubjects even when grouped by audiometric configu-ration. This result, however, does not imply that all

4 5 6 7 8 9 10 11RMS Differences (dB)

Same

MINUncertain

1!1l]Different

316


available crossover frequencies and the resultant

low-frequencyfrequency-gain responses were equally acceptable .

quality.That frequency-gain curves from several crossoverfrequencies were chosen approximately the samenumber of times may imply that the choice wasmore closely related to the suprathreshold loudnesscontour (which was observed at more equivalentSPL levels across the frequency range than atthreshold) rather than the more varied audiometricshape at threshold . While the round-robin procedureappears to be a very useful technique for establish-ing an acceptable crossover frequency, this adaptivemethod is not highly efficient for clinical purposes.In our opinion, no systematic clinical procedure bywhich to select an appropriate crossover frequencyfor two- or three-channel hearing aids is currentlyavailable. It would be especially useful, however, ifprinciples could be established possibly employingthe shape of the loudness contour at comfort level asthe means by which to establish an appropriatecrossover frequency in the individual case.

As a group, the subjects preferred an averagefrequency-gain response (Figure 3) that approxi-mated the NAL predictions, except in the lowfrequencies where more gain was routinely preferredthan predicted by the NAL. This finding is inagreement with results reported by French-St.George, et al . (12). There are several factors thatmay have contributed to this preference of morelow-frequency gain than predicted by NAL. First,the speech signal in the current study, and that ofFrench-St . George, et al ., were both presented atrelatively low input levels (55-62 dB SPL) . While itis implied that the NAL prediction is applicable atall input levels, there is evidence that the preferredfrequency-gain response may change with input level(15). Second, as reasoned previously in the currentpaper, it appeared that sound quality may haveplayed a larger role than intelligibility in the prefer-ence judgments. In the validation studies of theNAL (16), both perceived intelligibility and pleas-antness (quality) of the speech were considered to beof importance. Generally, the importance of themid- and high-frequencies has been associated withimprovement in intelligibility. However, evidence(17,18) is also available indicating that increasingthe amount of low-frequency gain resulted in judg-ments of better sound quality . Since the continuousdiscourse in the current study was generally under-stood by the subjects, perhaps they chose increased

gain in order to improve sound

ACKNOWLEDGMENTS

The authors would like to thank Theodore Bell,PhD, for his help in the interpretation of the statisticalanalyses; Donald Morgan, PhD, for his critical com-ments ; and Laurn Langhofer for programming assis-tance.

REFERENCES

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