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Journal of Rehabilitation Researchand Development, Vol . 23 No . 1 1986pages 17-24

Multichannel Syllabic Compression afor Severely Impaired Listeners

STEVEN DE GENNARO, Sc . D.

LOWS D. BRAIDA, Ph . D.

NATHANIEL I . DURLACH, M.A.

Research Laboratory of ElectronicsMassachusetts Institute of TechnologyCambridge, Massachusetts 02139

aThis research was supported by a grantfrom the National Institute of Neurolog-ical and Communicative Disorders andStroke, Grant No . R01-NS12846.

b Dr . De Gennaro is currently with the IBMThomas J . Watson Research Center,P.O. Box 218, Yorktown Heights, NewYork 10598

NOTE : Please address correspondenceto : L. D. Braida, 36-747 M .I .T., Cambridge,Massachusetts 02139 .

Abstract — Two listeners with congenital hearing losses charac-terized by flat audiograms and dynamic ranges of 18-33 dB weretested with three compression systems and one (reference) linearamplification system . The compression systems placed progres-sively larger amounts of speech energy within the listener's residualdynamic range, by raising to audibility and compressing 25, 50, and90 percent of the short-term input amplitude distribution in each of16 frequency bands . The comparison linear system was defined byadjusting six octave-wide bands of speech to comfortable levels.System performance was evaluated with nonsence CVC syllablespresented at a constant input level and spoken by two talkers.Extensive training was provided to ensure stable performance . Theresults were notably speaker-dependent, with compression con-sistently providing better performance for one speaker, linearamplification for the other . Averaged over speakers, however, therewas no net advantage for any of the compression systems for anylistener. The use of high compression ratios and large input rangestended to degrade perception of initial consonants and vowels.Under some conditions, however, final consonant scores werehigher with compression than with linear amplification . Compres-sion generally enhanced the distinction between stops and frica-tives, but degraded spectral-concentration and relative-intensitycues required to identify place of articulation.

INTRODUCTION

Amplitude compression has often been suggested as a meansof overcoming the reduced dynamic range and the recruitmentcharacteristic of many types of sensorineural hearing loss : e .g .,Braida et al ., 1979 (1) . However, syllabic compression has notgenerally improved speech reception for moderately impairedlisteners when compared with carefully chosen linear amplificationcoupled with some form of overall level normalization : e .g . (2, 3, 4,5) . This study, further details of which are available in De Gennaro,1982 (6), was concerned with assessing the utility of multibandsyllabic compression for listeners with severe sensorineural hear-ing loss, since these listeners represent the population believedmost likely to benefit from some form of amplitude processing:see Vilchur, 1973 (7), and Peterson, 1980 (8) . There was, however,no clear method of specifying an "optimum" compression strategyfor a particular listener, due to both the lack of prior analytic studiesand the large number of free parameters required to specify amultiband compression system .

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DE GEN NARO et al . : Multichannel Syllabic Compression

The purpose of this research was to conductexperiments to define appropriate compressioncharacteristics (given properties of the hearing lossand the speech materials) and also to evaluate theusefulness of compression for severely impairedlisteners relative to carefully chosen linear ampli-f

METHODS

ication .

System Design

Intuitively, it seems that there is a trade-off in-volved in the use of compression to increase thespeech energy presented to an impaired listener.While intelligibility should be enhanced by raisinga wider range of speech sounds to audibility, thecorresponding alterations of normal intensity cuescan result in perceptual degradations . Therefore,compression of the entire speech range into theauditory area of a listener with a severe hearingloss is not likely to be optimal, since the high com-pression ratios required degrade resolution forboth overall intensity and details of spectral shape.On the other hand, if only peak speech levels weremade audible, intelligibility would also be low be-cause many speech elements would remain in-audible.

To determine the "optimal" range of speechlevels to present to an impaired listener, three com-pression systems which differed parametrically inthe amount of speech energy presented above thelistener's elevated detection threshold were inves-tigated. These systems placed 25, 50, or 90 percentof the short-term amplitude distributions in each of16 frequency bands within the listener's residualauditory area.

The static characteristics for these systems werelinear below the compression thresholds in eachband, and employed a single compression ratiowithin the compression region . In each band, thecompression threshold was set at the desiredcumulative level (25, 50 or 90 percent), and the gainreflected the amplification required to raise the cumu-lative level to the listener's detection threshold.The degree of compression applied to each bandwas determined through an empirical fitting processin which listeners chose among systems with iden-tical compression thresholds and band gains, butdifferent static compression ratios.

To provide a reasonable match between thespeech spectrum and the listener's narrow residualdynamic range, the fitting procedure was conductedin six separate frequency bands . Four of the fre-quency bands, centered at 500, 1000, 2000, and

4000 Hz, were each one octave wide . The lowestfrequency band, centered at 200 Hz ., was one andtwo-thirds octaves wide, and the highest frequency

band, centered at 7100 Hz., was two-thirds of anoctave wide.

A 16-band compression system, as described byColn in 1979 (9), with bandwidths roughly equal toorNnel bandwidths, true rms detectors, and inde-pendent compression applied to each band, wasused to process speech signals . An independentrms detector was used to determine the short-termsignal level for each band . The time constant of thedetector was inversely proportional to the width ofthe corresponding input fiber, so that the fasterlevel-variations in the wider bands could be followed.Detector time constants were determined by single-pole lowpass filters and varied from 18 ms in thelowest frequency band to 0 .2 ms in the highestfrequency band: see de GennarVet al ., 1981 (10) . Inthe final system implementations, the compressionratio selected for each of the 16 bands was derivedby linearly smoothing the compression ratios chosenby the listeners across frequency.

Linear System Design

The comparison linear amplification system wasa variant of the OMCL (Octaves at Most ComfortableLevel) system defined by Lippmann et al . in 1981 (5).It was chosen because it had generally providedhigh levels of performance when compared to otherlinear and compression systems in several previousstudies . The frequency gain characteristic for thelinear amplification system was determined empir-ically by having the listeners adjust isolated bandsof speech to levels that were consistent with maxi-mum intelligibility and long-term comfort.

The same six frequency bands used to determinethe static compression ratios for the compressionsystems were used in this fitting process.

Subjects

Two !ieteners, both of them 29-year-old womenwith severe bi\cdenal oeneorineural hearing loss ofoongenital origin, participated in this study . Eachhad relatively constant dynamic range over thefrequency range of 125 to 8000 Hz. Both ears weretested for one listener, who is identified as "SubjectTTR" for the right ear results and as "Subject TTL"for the left ear results . This listener used a hearingaid for her right ear only.

One ear was tested for the second listener, "Sub-ject PBR". This listener also used a hearing aid inher right ear only.

Pure tone detection thresholds were determined

4-

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Journal of Rehabilitation Research and Development Vol . 23 No . 1 1986

through an adaptive forced-choice paradigm whilediscomfort thresholds were measured with pulsedtones using an adaptive tracking algorithm. Detec-tion and discomfort thresholds are presented inFigure 1.

Subject PBR had the most severely restrictedresidual dynamic range, with a mean dynamic rangeof only 18 dB between 125 Hz and 8000 Hz . SubjectTTL had an average range of 21 dB, while subjectTTR had the largest residual range with an averageof 33 dB .

Speech Materials

The speech materials used in the identificationexperiments were nonsense CVC syllables spokenin an la!-CVC context by two talkers, one male(Speaker BH), and one female (Speaker CL) . TheCVC syllables were formed randomly from a set of16 initial consonants, 6 vowels, and 16 final con-sonants. The consonant set consisted of set

is, a, 1,

I, u, UI.

140

120

100

DISCOMFORT'0

DETECTION

SUBJECT TTR \®®®°~®®®®`~

®\

FIGURE 1Tone detection and discomfortthresholds of the subjects tested.

80

60

140

120

100

80

60

125

250

500

1000

2000

4000

8000

FREQUENCY (Hz)

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DE GENNARO et al . : Multichannel Syllabic Compression

TAoms1Average Static compression ratios*

Speaker BH

System

Subject 25% -C 50%-C 90%-C

TTR 1 .1 1 .8 1 .8TTL 2 .1 3 .7 4 .2PS R 1 .8 3 .6 3 .9

Speaker CL

System

Subject 25%-C 50%-C 90%-C

TTR 1 .3 1 .6 1 .9TTL 1]] 3 .1 4 .0PBR 2 .1 2 .9 3 .3

*Results have been averaged over the 16 frequency-dependentcompression ratios used for each subject and each system.

FIGURE 2Cumulative amplitude level distri-butions for the speech materialsused in the intelligibility tests .

The 800 CVC syllables for each speaker werestored digitally (with 12-bit resolution and 10-kHzaudio bandwith) to allow random presentationsequences, in order to reduce order effects andlearning of stimulus artifacts . In addition, theoverall rms level of each utterance was normalized.The cumulative amplitude level distributions derived(10) from measurements made on 800 CVC syllablesspoken by each of the talkers are presented inFigure 2. These distributions, along with the detec-tion thresholds for each subject, were used todefine the compression thresholds and band gainsfor the 25, 50, and 90 percent compression systems.

Amplitude Properties of Processed Speech Materials

In general, the listeners were quite consistent inthe selection of static compression ratios for the25, 50, and 90 percent systems, and in the selectionof the frequency gain characteristic for the linearOMCL system . The compression ratios chosen arepresented in Table 1 . The subjects generally re-sponded to increases in the range of audible speech,from 25 to 50 to 80 percent, with higher effective

500

1000

2000

FREQUENCY wz

40

20

80

60

04000125 250

80

60

40

20

0

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Journal of Rehabilitation Research and Development Vol . 23 No . 1 1986

FIGURE 3Relation of cumulative level distri-butions for processed speechmaterials (speaker CL) to the de-tection and discomfort levels forsubject PBR.

compression ratios. Also, smaller compressionratios were chosen by the subject with the largestdynamic range.

After fitting, amplitude distributions of the short-term output levels for each of the systems investi-gated were measured . For example, the cumulative!evul distributions for the three compression sys-tems and the comparison linear system for subjectPBR and speaker CL are shown in Figure 3 . Meas-urements such as these suggest that the subjectsselected compression characteristics that presentedthe peak 1 percent speech levels significantly

below the discomfort thresholds measured withpulsed tones.

The frequency gain characteristics chosen bythe subjects also tended to flatten the cumulative!evel distributions across frequency to provide abetter match to the detection thresholds . For theless severely impaired subject, TTR, approximately50 percent of the speech range was presentedabove threshold with the OMCL linear system . Forsubjects TTL and PBR, roughly 10-to-25 percent ofthe speech range was audible with linear amplifica-tion . Again, the subjects selected presentation

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DE GENNARO et al . : Multichannel Syllabic Compression

levels that held the 1 percent peak levels signifi-cantly below the measured discomfort levels inmost frequency bands.

Identification Experiments

Given the severity of the impairments and thenumber of experimental conditions, extensive train-ing was provided to ensure a stable basis for systemcomparisons . In a study of compression publishedby Peterson in 1980 (8), large variations in perform-ance were observed during initial presentations ofboth compressed and linearly processed materials.Consistent performance was achieved only after8,000 to 10,000 trials for a severely impaired subject;even then, additional learning appeared to takeplace, but at a fairly slow rate.

The experiments in this study were structured toseparate training effects from the comparativeanalysis of system performance. Identificationexperiments were conducted over 16 sessions foreach subject . In each session, all four systemswere presented with a practice list of 50 CVC items,followed by a test list of 150 to 200 CVC items . Theresults from the first 8000 trials were considered"training" and were not included in the final systemcomparisons. For the smallest unit of analysis (1system, 1 speaker, after training), each subjectcompleted 1000 identification trials . Discarding the200 practice trials, this provided roughly 50 pres-entations of each initial consonant, 133 presenta-tions of each vowel, and 50 presentations of eachfinal consonant.

The primary measures of system performance inthis study were the average percent correct scoresfor the initial consonant, vowel, and final consonantcomponents of the CVC test materials . Overall sys-tem performance was characterized by the phonemeand item percent correct scores.

RESULTS

Phoneme and item scores obtained by each sub-ject with each system are presented in Table 2 . Ingeneral, the three compression systems tested inthis study were, at best, equivalent to the comparisonlinear system in terms of overall performance levels.There was no net improvement with compression,whether measured by phoneme or item scores, forany of the subjects . Compression was better thanlinear amplification only in the final consonantposition for the two more severely impaired sub-jects, and these improvements with compressionwere small — between 4 and 7 percentage points.

For the least severely impaired subject, TTR, the

50 percent and 90 percent compression systemsand the OMCL linear system were statisticallyequivalent, and were better than the 25 percentcompression system by roughly 5 percentage points.In general, however, subject TTR achieved quitehigh levels of performance as demonstrated byphoneme scores of 75-to-81 percent across thelinear and compression systems tested.

For the two more severely impaired subjects, TTLand PBR, the 25 percent and 50 percent compressionsystems and the OMCL linear system were generallyequivalent in terms of overall performance, andwere better than the 90 percent system by 6-to-11percentage points . However, the performance scoresfor these subjects were rather low, ranging between52 percent and 69 percent of phonemes correct.

The results of the identification experiments, how-ever, were highly speaker-dependent for the two moreseverely impaired subjects . Compression con-sistently improved performance with one speaker(CL) while degrading performance with the other(RH). In many instances, the difference in perform-ance across speakers was comparable to or largerthan the corresponding differences across systems.For example, for the 90 percent system, scores forspeaker CL were 14 percentage points higher thanfor speaker BH — while for the OMCL system, theywere 8 points lower . Although the causes of thisspeaker dependence are not well understood, itmay be due to differences in articulation and differ-ences in masking effects associated with greaterconcentration of low-frequency energy with speakerBH. There may also have been more significant in-

TABLE 2Identification scores*Phoneme and item scores

Phoneme identification scores

System

Subject

OMCL-Lin 25%-C 50%-C 90%-C

TTR

80 .1

75 .4

81 .3

80 .4TTL

67 .9

69 .0

67 .5

57 .4PBR

57 .5

59.1

56 .0

51 .6

Item identification scores

System

Subject OMCL-Lin 25%-C 50%-C 90%-C

TTR 50.5 40 .9 52 .3 50 .6TTL 29 .4 32 .8 30 .0 21 .1PBR 14 .9 17 .1 14 .8 12 .9

*Scores have been averaged over the two speakers.

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Journal of Rehabilitation Re-search and Development Vol . 23 No . 1 1986

teractions between the dynamic properties of thecompressor and the lower fundamental frequencyof speaker BH.

The systems produced different error patterns -Even when comparable levels of performance werefound across the systems tested, analysis of theresponse error patterns indicated that phoneticfeature reception was significantly different undercompression and linear amplification . While con-sonant feature reception was usually influenced byspeaker and by syllable poaition, aeveral broadtrends were noted, particularly for the two moreseverely impaired subjects . Compression generallyimproved the subject's ability to distinguish betweenstops and fricatives . On the other hand, compressiondegraded the spectral-concentration and relative-intensity cues required to identify the particularstop or fricative that had been presented . Andunder compression, place of articulation and dura-tion were significantly degraded . The observedconfusions under compression are consistent withthe spectral flattening introduced by the independentaction of the 16 bands in the speech processingayatern.

With compression, 10, [, sl were often labeled as/f/ ' and /y, 3, zl were often confused with /v/ . Confu-sions were also common between 13/ and /zl, andbetween if/ and /s/ with compression . With linearamplification, confusions between stops and frica-tives (forexannp!e, between 101 and /y/ and /d/,and between /b, d, gI and Iv/) were more common.

With the 90 percent compression system, vowelperception was also degraded for the two moreseverely impaired subjects, by as much as 8 to 20percentage points . These subjects often incorrectlyresponded with Id to presentations of the other fivevowels in the test set . This result is also consistentwith the spectral flattening caused by independentcompression in the different bands of the compres-sion system, since /El was the most neutral of thevowels tested, with the most uniformly spacedformants and the flattest spectrum.

CONCLUSIONS

The compression systems tested in this studywere at best equivalent to the comparison linearsystem when performance was measured by eitherphoneme or item scores for isolated CVC syllables.Compression did improve final-consonant scoresfor the two more severely impaired subjects, but itoften significantly degraded initial-consonant andvovvol scores . The results of the identification mx'

periments were highly speaker-dependent . Com-pression consistently improved performance withone speaker while degrading performance with theother. Averaged over speakers, however, there wasno net advantage for compression.

This study was also concerned with determiningappropriate compression system characteristicsgiven properties of the subject's hearing loss.Usually, the best performance was achieved withsystems employing moderate compression ratiosand compression regions spanning the top 25-to-50percent of the short-term input speech range . Com-pression of 90 percent of the input range into theresidual auditory area often resulted in substantiallydegraded speech reception.

High levels of performance were generallyachieved with linear amplification for the lessseverely impaired subject, for speech materialsthat were presented at a constant input level . It hasbeen suggested that the use of speech materialswith fixed overall presentation levels provides afalse advantage for comparison linear systems,since compression is capable of equalizing pres-entation level differences. While it is clear that theintroduction of significant item-to-item !evel varia-tion will degrade performance with linear amplifi-cation, the results of these experiments indicatethat performance will also be significantly degradedwith compression . If syllabic compression is usedto compensate for overall presentation !evel varia-tions, then compression ratios and input rangesmuch larger than those used in the current studywould be required. Given the degradations observedwith the 90 percent system, it is likely that com-pressing wider input ranges into the subject'snp eidual auditory range, with high compressionratios and a large number of independently proc-essed bands, would result in severely degradedperformance . Similar degradations have been ob-served by Bustamante fornnu!tiband compression-limiters (11 ) .

The results of this study suggest that well-chosenlinear amplification, coupled with some form ofautomatic volume control to reduce long-term !evelvariations, may be more beneficial than syllabiccompression for subjects with flat losses anddynamic ranges of 30 dB or more . While normalhearing was not restored by the systems tested inthis study, the less severely impaired subjectachieved phoneme scores of roughly 80 percent onnonsense CVC syllables under linear amplificationwith materials presented at a fixed overall level.

The performance scores for the subjects with moreseverely restricted dynamic ranges were generally

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DE GENNARO et al . : Multichannel Syllabic Compression

REFERENCES1 . Braida LD, Durlach NI, Lippmann RP,

Hicks BL, Rabinowitz WM, Reed CM:Hearing Aids : A Review of Past Re-search on Linear Amplification, Am-plitude Compression, and FrequencyLowering . ASHA Monograph Number19, 1979.

2 . Barfod J : Multichannel CompressionHearing Aids : Effects of Recruitmenton Speech Intelligibility . Report No.11, The Acoustics Laboratory, Tech-nical University of Denmark, 1976.

3 . Abramovitz R: The Effects of Multi-channel Compression, Amplification,and Frequency Shaping on SpeechIntelligibility for Hearing ImpairedSubjects . Unpublished Ph . D . Thesis,The City University of New York,1979.

4. Braida LD, Durlach NI, De GennaroSV, Peterson PM, Bustamante DK:Review of Recent Research on Muli-band Amplitude Compression forthe Hearing Impaired . In TheVander-bilt Hearing-Aid Report, State of theArt Research Needs, G . A . Stude-baker and F . H . Bess, Eds. UpperDarby, Pennsylvania : Monographsin Contemporary Audiology, 1982.

5 . Lippmann RP, Braida LD, Durlach NI:Study of multichannel amplitudecompression and linear amplificationfor persons with sensorineural hear-ing loss . J Acoust Soc Am 69(2):524-534, 1981.

6 . De Gennaro SV : An Analytic Studyof Syllabic Compression for SeverlyImpaired Listeners . Unpublished Ph.D . Thesis, Department of ElectricalEngineering and Computer Science,M .I .T ., Cambridge, Mass ., 1982.

8 . Peterson PM : Further Studies of thePerception of Amplitude CompressedSpeech by Impaired Listeners . Un-published S .M . Thesis, Departmentof Electrical Engineering and Com-puter Science, M .I .T ., Cambridge,Mass ., 1980.

7 . Villchur E : Signal processing to im-prove speech intelligibility in per-ceptive deafness . J Acoust Soc Am53 :1646-1657, 1973.

9. Coln MCW : A Computer ControlledMultiband Amplitude Compressor.Unpublished S .M. Thesis, Depart-ment of Electrical Engineering andComputer Science, M .I .T ., Cambridge,Mass . 1979.

10. De Gennaro SV, Krieg KR, Braida LD,Durlach NI : Third-Octave Analysis ofMultichannel Amplitude-CompressedSpeech . Proceedings of IEEE ICASSP,1981.

11. Bustamante DK : The Effect of Com-pression Limiting on Speech Intel-ligibility for the Sensorineural Hear-ing Impaired . Unpublished S .M . The-sis, Department of Electrical Engi-neering and Computer Science,M .I .T ., Cambridge, Mass ., 1981.

quite low even under the best presentation condi-tions . For those subjects, it is clear that a simplemapping of the input speech range into the residualauditory area is inadequate, particularly if the map-ping is carried out independently in a large numberof frequency bands . While it is certainly necessaryto overcome the lack of audibility, the mappingmust also preserve important acoustic differencesbetween speech sounds . A more appropriate com-pression strategy would reduce overall level varia-tions while maintaining, or perhaps emphasizing,relevant acoustic cues. For example, the relativelygood performance achieved with 25 percent of thespeech range above threshold suggests that pres-ervation of spectral peaks is more important tointelligibility than the audibility of lower amplitudespeech levels . Future work should concentrate onsystems that employ interband control algorithmsto preserve relevant short-term features while re-ducing overall level variations