Audibility and recognition of stop consonants in normal and hearing ...

Audibility

and recognition of stop consonants in normal

and hearing-impaired subjects ChristopherW. Turner and Michael P. Robb CommunicationSciencesand Disorders,SyracuseUniversity,80.5South CrouseAvenue,Syracuse, New York 13244-2280

(Received 13 November 1986;acceptedfor publication27 January 1987)

The purposeof this studywasto examinethe effectof spectral-cueaudibility on the recognitionof stopconsonants in normal-hearingand hearing-impairedadults.Subjects identifiedsix syntheticCV speechtokensin a closed-setresponsetask. Each syllablediffered only in the initial 40-ms consonantportion of the stimulus.In order to relate performanceto spectral-cueaudibility,the initial 40 ms of eachCV were analyzedvia FFT and the resulting spectralarray waspassedthrougha sliding-filtermodelof the humanauditorysystemto accountfor logarithmicrepresentation of frequencyand the summationof stimulusenergy within critical bands.This allowedthe spectraldata to be displayedin comparisonto a subject'ssensitivitythresholds.For normal-hearingsubjects,an orderly functionrelatingthe percentageof audiblestimulusto recognitionperformancewasfound,with perfect discriminationperformanceoccurringwhenthe bulk of the stimulusspectrumwaspresented at suprathreshold levels.For the hearing-impairedsubjects,however,it wasfoundin many instancesthat suprathresholdpresentationof stop-consonant spectralcuesdid not yield recognitionequivalentto that foundfor the normal-hearingsubjects.Theseresults demonstratethat while the audibilityof individualstopconsonants is an importantfactor influencingrecognitionperformancein hearing-impairedsubjects,it is not alwayssufficientto explainthe effectsof sensorineural hearingloss. PACS numbers: 43.66.Sr, 43.70.Jt, 43.71.Es, 43.71.Gv

INTRODUCTION

Although someresearchershave shownthat sensorineuralhearing-loss subjectsexhibitpoorerthan normalrecognitionof stopconsonants (e.g.,Owensetal., 1972),others have not found such differences(e.g., Van Tasell et al., 1982). Instancesin whichstop-consonant recognitionscores for hearing-losssubjectsare comparableto thoseof normalhearingsubjectsgenerallyoccurwhenthe subjects'hearing lossesare mild and the presentationlevelfor the speechmaterials is high, which suggeststhat the audibility of the speechstimuliis an importantfactor determiningsubjects' performance.Consider,for example,speechmaterialsin which stop consonantsare embeddedin a consonantplus vowel (CV) context.The presentationlevelsof the individual stopconsonantsare typically as much as 20-30 dB less than thoseof accompanyingvowels (Fletcher, 1953). Becausespeechpresentationlevelsare typically expressedin terms of the levelsof the intensitypeaks (usually the vowel sounds),it is likely that the lack of audibility of individual stop consonantsfor hearing-impairedsubjectsmay be an overlookedfactor in explainingtheir recognitionperformance.The presentstudyexaminesthe effectsof audibility of individual stop consonantson the recognitionperformanceof normal and hearing-impairedsubjects. Considerableresearcheffortshave beendirectedat sepPortionsofthispaperwerepresented at the 111th MeetingoftheAcoustical Societyof Americain May 1986 [J. Acoust.Soc.Am. Suppl.1 79, S24 (1986)].

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aratingaudibilityfactorsin speech recognition frompossible effectsof psychoacoustic deficitsthat mayaccompany cochlear hearingloss,suchas abnormalfrequencyresolution, frequencydiscrimination, or temporalacuity.Oneapproach hasbeento comparethe performanceof hearing-losssubjectsagainsttheperformance of normal-hearing subjects listeningto speechmaterialsthat havebeendegraded(filtered or masked) to simulatea sensitivityloss. Severalexperimentsin whichexceptional carewastakento matchthe sensitivitythresholdsof the simulatedhearinglossesin normalhearing subjectsto the individual hearing-losssubjects' thresholdsmay reflectdirectly on the contributionof audibility deficitsto generalspeechrecognition.Milner's (1982) findingssuggestedthat usingmaskingnoiseto simulate hearinglossdid not accuratelymodelcochlearhearingloss; the maskednormal-hearingperformance(in termsof percent correct) was better than that of the hearing-lossears. FabryandVanTasell(1986) simulatedhearinglosses using filteringand maskingnoiseand alsomeasuredperformance in termsof both percentcorrectand error patterns.For onehalf of theirsubjects, bothmaskedandfilteredsimulationsof hearinglosssuccessfully approximatedthe effectsof the cochlearloss.For the remainingsubjects,eitheroneor both of the simulationsyieldedperformancethat wasbetterthan (percentcorrect) or differentfrom (in error patterns)the actual sensorineurallosses.In contrast,Zurek and Delhorne

(1986) showedthat usingmaskingnoisein normal-hearing subjectsto simulatehearinglosssuccessfully approximated the percent-correct performanceof many hearing-loss sub-

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jectsacrossa wide varietyof speech-in-noise conditions.Resuits from these studies do not lead to firm conclusions re-

gardingthe relative contributionof audibility deficitsto speechrecognitionby the hearingimpaired.They do, however,provideevidencethat underconditionsof limited audibility for speechmaterials(resultingeitherfrom an actualor a simulatedhearing loss), the sensitivityloss alone can sometimesaccount for the poorer performanceof some hearing-impairedsubjects. An alternateapproachto separatingthe effectsof sensitivitylossfromotherpsychoacoustic factorsin speech recognitiontaskshasbeento computea measureof audibilityfor the speechmaterialsusedin eachpresentationcondition. Thisaudibilitymeasure isthenusedto comparehearing-loss subjects' performanceto that of normalsat equivalentaudibility levels.Dugal et al. (1980), usingthe articulationindex (AI) (Kryter, 1962), analyzedthe speechrecognitionresuitsof Skinner(1980) in sucha manner.They foundthat, althoughthe AI provideda well-orderedpredictionof hearing-loss results,theperformance of thehearing-loss subjects waspoorerthan that of normalsat equivalentlevelsof audibility. Pavlovic (1984) and Kamm et al. (1985) reached similarconclusions, in that manyhearing-loss subjects performedmorepoorlythan predictedby the normal-hearing assumptions of the AI. In thestudiescomparingdegraded normalperformance

(viamasking orfiltering)tothatofhearing-loss subjects, the comparisons are generallymadeat low levelsof audibility for bothsubjectgroups.Thesestudies,therefore,do not ad-

dressthequestion ofwhether' hearing-loss subjects canaccuratelyrecognize individualstopconsonants at highpresentation levels when the entire consonant is made audible. While

the useof the AI is well suitedto investigatethe relation betweendegreeof audibilityandrecognition performance of many speechmaterials,it wasnot designedto predictthe audibilityof individualstopconsonants. Recallthat the AI calculations are baseduponthe long-termaveragesound levelsof speech,whichare primarilyrepresentative of the moreintensevowelsounds. The conceptof the audibilityof individualstopconsonants hasbeenlargelyignoredin the

nants.In particular,we wereinterested in determining if cochlearhearing-loss subjects with substantial sensitivity losses couldachievehighrecognition performance whenthe stop-consonant stimuliwerepresented at levelsyieldinga highdegreeof audibility.The presentexperiment deriveda measureof the audibilityof short-duration stop-consonant sounds to examinestop-consonant recognition performance as a functionof the audibilityof the individualspeech sounds.

I. METHOD

A. Subjects

Fournormal-hearing andfivesensorineural hearingimpaired adultsubjects participated in theexperiment. The normal-hearing subjects hadpure-tonethresholds of better

than20dBHL (ANSI, 1969)at audiometric testfrequenciesfrom250to 4000Hz. Thehearing-impaired subjects' losses rangedfrommoderate to severe. Sensitivity thresholdsofthefivehearing-impaired subjects areshownin Table I. The thresholds weremeasured usinga four-alternative forced-choice adaptiveprocedurethat converged uponthe 71%-correct level of detection (Levitt, 1971). Several

thresholddeterminations wereaveraged to yieldfinalvalues. All of the hearing-impaired listenerswereregularusersof hearingaids.Only oneearof eachsubjectwastestedand,in the hearing-impairedsubjects,the testear waschosenasthe ear with the most sensitive thresholds. B. Stimuli

Consonant plusvowel(CV) speech tokensweregeneratedvia digitalsimulationof a five-formant terminal-analog speechsynthesizer(Klatt, 1980). The CV syllableswere constructed by pairingthesixstopconsonants/b,d,g,p,t,k/

with the vowel/a/to producetokensof 305 msin duration. We designed the C¾ tokensto differonlyin the initial 40-ms consonant portionofthestimuli.For thelabialstops/p/and /b/, formant-transition startingfrequencies were200, 900, and2000 Hz for F1, F2, andF3, respectively. For the alveolar stops/t/and/d/, the startingfrequencies were400, previous literature, although Kentetal. (1979)didreport 1700,and2800Hz; andfor thevelarstops/k/and/g/, the that in normal-hearing subjects, the recognition of individ- starting frequencieswere 300, 1610, and 1750 Hz. For the ualconsonants in CV contextappeared to showconsiderable unvoicedstops,voicinginitiated40 msfollowingthestimudependence upontheintensityof that particularconsonant. lusonset, whileforthevoicedstops, voicing beganfollowing The presentexperimentexaminedthe relationbetween a 5-ms burst at stimulus onset. Bursts were wideband in stop-consonant recognition by normalandhearing-loss sub- spectrumfor/p/and/b/, centeredat 3850 Hz for/t/and jectsandthe degreeof audibilityof individualstopconso- /d/, andcenteredat 1750Hz for/k/and/g/. Eachstimulus

TABLE I. Sensitivity thresholds (indBSPL) forthefivehearing-impaired subjects. Thesmall letter following each subject's intitials refers totheappropriate

panelin Fig. 7 for that subject's data.

Thresholds (dB SPL)

Subject

Sex

HB (a) MK (b) PD (c) LS (d) BE (e)

M F F M F

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0.25

0.50

1.0

1.5

2.0

3.0

4.0(kHz)

31 27 38 62 98

10 29 20 50 98

18 53 36 52 96

33 54 55 60 85

61 58 72 59 67

71 63 73 66 45

74 72 73 74 35

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C.W.Turner andM.P.Robb: Audibility ofstopconsonants 1567


ended with a 265-ms vowel (F 1at 720 Hz, F 2 at 1240 Hz, and F 3 at 2500 Hz). Here F 4 and F 5 were setat a constant center

ted. This resultedin a scoringsystemin which a scoreof 0%

frequency(3600 and3850Hz, respectively)throughoutthe

the phonemeand a 100% scorerepresented perfectidentifi-

duration of the stimuli. All speechtokenswere stored on computerdisk, and for presentationwere directedat a 10kHz samplingrate into a low-distortion12-bit digital-to-

cation.

information

transmitted

reflected chance identification

of

In order to predict the audibility of the consonantportion of eachsyllable,an acousticanalysisof the initial portion of each CV stimuluswas obtained and subjectedto a analogconverter(Data Translation3366) followedby a transformationdesignedto simulatehuman auditoryproplow-passfilter (cutofffrequencyof 4800 Hz). erties.Thesepropertiesincluded:( 1) logarithmicfrequency representationand (2) summationof energy within each C. Procedures auditoryfilter or criticalband.This transformationincluded All subjectsweretestedindividuallyin a sound-isolated the followingsteps. booth. Prior to the actual data collection,all subjectsre( 1) A fastFourier transform(FFT) of the acousticsigceiveda minimum of 2 h of practicewith the test stimuli, nal developed in a 6-cm3 couplerfor eachstimulus wasobpresentedat comfortablelisteninglevels.A singleexperitainedusinga model3561 Hewlett-Packard spectrumanamental run consistedof the six CV speechitems presented lyzer. An exponential-decay weightingfunctionwith a time ten times each in randomizedorder. Subjectsidentifiedthe constantof 40 ms, beginningat stimulusonset,was usedto speechitems in a closed-setformat by pressinga response analyzethe initial stop-consonant portion of the CV syllabutton correspondingto that syllable.A 200-ms warning bles.This time window deemphasizedthe later portionsof light precededthe onsetof each stimulusand a response the stimulus(cf. Stevensand Blumstein,1978). Figure 1 is intervalof at least3 s followedeachresponse,servingasthe an exampleof the FFT for the stimulus/da/. The resulting interstimulusinterval. The subjectswereinstructedto guess FFT array isa constantbandwidth(12.5-Hz) analysisof the at the correct answer when they were unsureof their reinitial portionof eachstimulus. sponse. (2) A slidingfilter modelof the humanauditorysystem Data were obtainedfrom subjectsin 5- or 10-dB steps was applied to the FFT of each stimulusitem in order to acrossa wide rangeof presentationlevels,rangingfrom the accountfor the approximatelylogarithmicrepresentation of level at which the subjectobtainedno better than chance frequencyin theauditorysystemandthe summationof stimperformanceon the closed-settask (16.7% correct), to a ulusenergywithin a "critical band" (Green, 1958;Gfissler, maximumof 100dB SPL. The 100-dB-SPLpresentationlev1954). This was done in order to compare this "transel wasthe highestthat mostsubjectsfoundtolerablein this formed" versionof the stimulusto a subject'ssensitivity experiment.This nominal presentationlevel was expressed thresholds.The roex (p,r) exponentialauditoryfilter shape asthermsaverage ofacontinuous vowel,withspectral char- as describedby Pattersonet al. (1982) was usedas an apacteristicsidenticalto the vowelcomprisingthe final 265 ms proximationof the appropriateauditory filter shape.This of each CV stimulus. A subject'sdata were considered filter is describedby a weightingfunction W(g) that operasymptoticand final in a givenconditionwhen the subject atesuponthe stimuluspower;the total powerpassedby the showedno further improvementsin performancewith refilter is describedby: peatedruns,and whensuccessive three-runpercent-correct IV(g) = (l-- r)(l +pg)e -p• + r, averagesdifferedfrom oneanotherby lessthan 10%. D. Data analysis

We wereinterested in contrasting theaudibilityof individualstopconsonants for eachsubjectwith that subject's recognition performance for the samephoneme. Unfortunately,a simplemeasure ofpercent-correct identification for eachofthesixphonemes couldbecontaminated bysubjects' response bias.For example,if a subjectadopteda response strategyof simplyresponding/ba/following all presentations,thepercent-correct scorefor that phoneme wouldbe artificiallyhigh.For this reason,eachsubject's finaldata, based uponanaverage ofthefinal180trialsateachpresenta-

wheregisthenormalizedfrequencydistancefromthecenter of the filterto a signalcomponent, p is a parameterdescribing the in-bandwidthof the filterand r describes the filter "tail." Valuesofp = 25 and r = 0.0001 wereemployedin

1o

dB /OlV

tionlevel,werecastintosixindividual2 X 2 confusionmatri-

ces,contrasting the correctidentification of an individual phoneme versusincorrectidentification of that phoneme (e.g.,/ba/versus not/ba/). These2X 2 matrices werethen corrected for theresultingunequalcellfrequencies andsub- START• 100 Hz Bllh ] 2.5 Hz STOP, 5 100 Hz jectedto a bit analysis asdescribed by Miller and Nicely (1955). For the2 X 2 matrices,themaximumpossible inforFIG. 1.Example ofanFFT performed ontheacoustic stimulus/da/.A 40mationtransmittedwas 1.00bits;we thenexpressed the re-

msexponential timewindowwasusedto reflecttheinitialconsonant por-

suits in terms of relative identification information transmit-

tion of the stimulus.

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C. W. Turner and M.P. Robb:Audibilityof stop consonants

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II. RESULTS 11o

A. Normal-hearing subjects

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FIG. 2. Sliding-filtertransformation(seetext) of the FFT of the syllable /da/, resultingin an "audibilityspectrum"of the stopconsonant/d/. The presentationlevel was 110 dB SPL.

the presentcalculation,followingPatterson'sdescriptionof thosevaluesasbeingrepresentative for normal-hearingsubjects.The resultingfilterhasanequivalentrectangularbandwidth of 0.16 timesthe filter centerfrequency.The outputs of thisfilterwerecalculatedfor 200 pointsspacedlogarithmicallyin frequencyrangingfrom 100 to 5250 Hz. This resultedin what we havetermedan "audibility spectrum"of an individualstopconsonant.Sucha transformationis similar in principle to that describedby Searle et al. (1979), Carlsonet al. (1979), and Kewley-Port (1983), with the additionalfeaturethat absolutesoundpressurelevelswere determinedfor eachpoint.Figure2 showsan exampleof the audibilityspectrumfor the stopconsonant/d/at a presentation levelof 110dB SPL. It is interestingto notethat for this 110-dB-SPLstimuluspresentation level,the audibilityspectrum of the initial stop-consonant portion lies between70 and 90 dB SPL.

Figure 3 showsthe meanaudiometricthresholdsfor the four normal-hearingsubjects plottedalongwith severalrepresentations of the audibilityspectrumof the stop-consonant /g/at variousnominal presentationlevels.All valuesof the audibility spectrum and the subjects'thresholdswere expressedin termsof levelsdevelopedin a 6-½m 3 acoustic coupler.The percentageof informationtransmittedfor the phoneme,as describedpreviously,is noted to the right of each audibility spectrum.An orderly relation betweenthe degreeof stop-consonant audibilityand recognitionperformance is observed.Essentiallyperfect recognitionperformance occurred when the audibility spectrumfrom 2504000 Hz was at suprathresholdlevels. Note that performancewasessentiallyequalto chance( 1% ) whenthe entire audibility spectrumwasbelowthe subjects'thresholds.Similar resultswereobtainedfrom the normal-hearingsubjects on the other five stop consonants.On the basisof theseresuitsfrom the normal-hearingsubjects,we concludedthat our calculationsof an audibilityspectrumfor eachstopconsonantprovided a meaningfuland orderly expressionof stop-consonantaudibility. In order to condensethe resultsinto a more manageable form, a "single-number"measureof audibilitywascomputed for each audibility spectrumin relation to a subject's thresholds.The percentageof logarithmicallyspacedpoints on the audibility spectrumbetween250 and 4000 Hz that were abovethe subject'sthresholdcurvewas usedas a measure of overall audibility for that stimulus.Portions of the audibility spectrumthat were at low sensationlevelswere assigneda reducedimportance.Each point lying 10 dB or more above the thresholdcurve was given full weighting (i.e., 1.0), while thosepointsbetween0 and 10 dB abovethe thresholdcurve were given interpolatedweightsbetween0

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J. Acoust.Soc.Am.,Vol. 81, No. 5, May 1987

C.W. Turnerand M.P. Robb:Audibilityof stopconsonants

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functionof the meanpercentage of audibilityfor eachof the sixstopconsonants. The functionsaremonotonicandorderly in eachcase.Figure 4 showsthat when70%-80% of the stimulusis calculatedto be audiblefor the normal-hearing subjects,recognitionperformanceis essentially perfect. We should mention that the data were also analyzed usingseveralalternativeschemesfor calculatingstimulus audibility. For example, the present 250-4000-Hz bandwidth wasreducedto 250-3000 Hz, and alternativeweighting schemes rangingfrom a 0-20-dB sensation levelfor the audibilityof a singlespectrumpointwerecalculated.Modificationsof the weightingschemeresultedin shiftsof the functionsin Fig. 4 to the left or the right, but causedno changein the relativeorderingor monotonicityof the functions.Minor variationsin the analysisbandwidthresultedin little changein the form of the PA functions.

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FiG. 4. Percentageinformationtransmittedfor the six stop consonants plotted asa functionof phonemeaudibility (seetext) for the normal-hearing subjects.The consonantsdepictedare: (ß)--/b/, ([])--/d/, ( ß )• /g/, (A)•/p/, (ß)•/t/, and (¸)•/k/.

and 1.0. This partial weighting of the low-sensation-level points on the audibility spectrumwas usedin order to accountfor the poorerpsychoacoustical abilitiesthat subjects generallyshowfor low-sensation-level stimuli, and also to reflectthe generalphilosophyof deemphasizing low-sensation-levelportionsof the speechspectrumthat has proven successfulin Articulation Index calculations (Kryter, 1962). The resulting percentageof audible spectra was termed the "percent audibility" of the stop consonantfor eachconditionand subject.Figure 4 displayssix "performance-audibility (PA) functions,"plottingthemeanrecognition performanceof the four normal-hearingsubjectsasa

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B. Hearing-impaired subjects

Figures5 and 6 are examplesof audibilityspectraplotted in relationto the sensitivitythresholdsfor two hearinglosssubjects.In Fig. 5, the subjecthas a moderatehearing lossand it is evident that when the stop consonant/d/is fully audible, the recognitionperformanceis accurate.In this particularcase,overcomingthe attenuativeeffectof the hearinglossthrough simpleincreasesin presentationlevel restorednormal performance. In Fig. 6, such is not the case,as this subjectfails to achievecompletelyaccuraterecognitionfor the stopconsonant/p/, even when the calculatedaudibility of the stop consonantis complete.We observedboth typesof eventsin our analysisof the fivehearing-loss subjects'data.That is, in somesubjectsfor somephonemes,accuraterecognitionoccurred when the stimuli were made audible, while in other

casesaudibilitywas not sufficientfor accuraterecognition.

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C. W. Turner and M.P. Robb: Audibilityof stop consonants

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In order to summarizethe data from the five hearinglosssubjects,PA functionsfor each subjectand phoneme conditionwerecalculated.Thesefunctions,relatingaudibil-

theaudibilityof stopconsonants to recognitionperformance for the normal-hearingsubjects,asillustratedin Figs. 3 and 4. Usingthis samemethodof expressing audibilityfor the ity of thestopconsonant to therecognition performance,hearing-impaired subjects allowedfor comparisons with the were comparedwith the correspondingdata from the nornormalsubjects.We foundthat, in mostcases,comparisons mal-hearingsubjects.Figure 7 displaysthe six PA functions at equivalentlevelsof calculatedaudibilitybetweenthe two from eachhearing-losssubjectin comparisonto the normalgroupsrevealedpoorer performancefor the hearing-imhearing subjects'functions.In general,the PA functions pairedsubjects thanfor thenormals.Thus,in mostcases,the from the hearing-impairedsubjectstendedto lie below the hearing-impaired subjectswereunableto utilizethe audible normal functions,indicatingthat, in mostcases,equivalent portionsof stopconsonants asefficientlyasdid the normals. levelsof stimulusaudibilityfor the two groupsdid not result This evidencesupportsthe existence,and importanceto in equivalentlevelsof recognition.For the four subjectswith speechrecognition,of other psychoacoustical deficitsbesensitivitylossesat high frequencies[ Fig. 7 (a), (b), (c), yond the simplesensitivitylossof a sensorineural hearing and (d) ], only four instanceswere found where their PA impairment.Our results,therefore,supportthe conclusions functions reached the same levels as the normals (/b/for of Dugal et al. (1980), Pavlovic (1984), Kamm subjectHB,/d/for subjectMK, and/k/and/d/for subject et al. (1985), and Fabry and Van Tasell (1986), who found PD). The remaining20 comparisonsrevealedthat equivathat the lack of audibility of speechmaterialsfor hearinglent levelsof audibility for thosesubjectsdid not yield norimpairedsubjects is only partiallysuccessful in accountin. g mal levelsof performance.In contrast,for the low-frequency for their reducedperformance. hearing-loss subjectBE in Figure7 (e), the functionswerein On the other hand, despitehigh nominalpresentation many casescloseto thoseof the normals,althoughthe severe levels,mostof our impairedsubjectsdid not receivefull auhearinglossof this subjectlimited the availableaudibility of dibility of the availablestop-consonant spectralcuesat the the spectralcuesevenat the highestlevels. highesttolerablepresentationlevels.We hypothesizethat It isinstructiveto keepin mindthat the right-mostpoint thisoccurredbecausethe moreintensevowelportionof each on eachhearing-impaired subject'sfunctionin Fig. 7 repre- stimulusservedto limit the acceptablepresentationlevelof sentsthe 100-dB-$PLpresentationlevel.In nearlyeveryinthe CV stimuli for the subjects.This lendssupportto the stance,the stopconsonantdoesnot reach 100% audibility notion that incompleteaudibility of stop consonantsfor for the hearing-impairedsubject.This suggeststhat few hearing-loss subjectscan serveas a limiting factor in their hearing-loss subjects wereeverableto perceivethefull range recognition performance. In manypanelsof Fig. 7, thehearof audiblespectralcuesfor the stopconsonants(audibility ing-impairedsubjects'PA functionswerestill risingas the nearor at 100%), evenat presentation levelsof 100dB $PL. functionapproached the 100-dBlevel,leadingto the speculation that further increasesin the audibilityof the stopconiii. DISCUSSION sonantswould have resultedin better recognitionperforThemeasureof stop-consonant audibilityderivedin this mance. The presentstudyis thereforeunableto offerunqualistudyprovidedanorderlyanddescriptive methodof relating 1571


C. W. Turner and M.P. Robb: Audibilityof stop consonants

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FIG. 7. Performance-audibility (PA) functionsfor all fivehearing-impaired subjects.Eachbrokenline represents a hearing-impaired subject'sperformance for a particularstopconsonant,while the solidlinesrepresentthe meanresultsfrom the normal-hearingsubjects'group.The small letter identifyingeach panel [ (a)-(e) ] refersto that subject'ssensitivitythresholdsaslistedin Table I.

fledsupportforor against thehypothesis thataudibilitydeficitsalonecanaccountfor thepoorerrecognitionof stopconsonantsin sensorineuralhearing-losssubjects.Instead,we concludethat both audibilityand otherpsychoacoustic fac1572


tors are mostlikely responsible. The methodsof the present study might offer a meansof separatingthesefactorsfor individual stop-consonant sounds. There are, however,somecriticismsthat may be directC.W. Turner and M.P. Robb: Audibilityof stop consonants

1572


edat theuseof thepresentmethodof analyzingthesedata. There is evidencethat cochlearhearing-loss subjectsoften have critical bandwidths that are wider than normal (e.g.,

Margolisand Goldberg,1980;Tyler et al., 1982).These wider bandwidths(or filters) in the hearing-impairedsub-

jectswerenotincorporated intothepresent model,although if theywere,thecalculated audibilityof thestimulifor the hearing-impaired subjects wouldhavebeengreaterthanthat reported here.In thiscase,ourconclusion thatmanyhearing-impaired subjects do not recognize stopconsonants as wellasnormals,whencompared at equivalent levelsof stimulusaudibility,wouldremainunchanged. It might,however,tendto argueagainsttheconclusion that stopconsonants tend to be inaudible for many hearing-impaired

subjects, evenat highnominalpresentation levels. Another considerationin the data analysisis that the

frequency rangeusedto compute audibilityof thestimuli (250-4000Hz) mayhavebeeninappropriate andshouldbe modified.Dubnoet al. (1986) havepresented evidencethat therecognition of individualstopconsonants is dependent upondifferent frequency regions for particularconsonants. We notedearlier,however,thatsimplychanging thegeneral bandwidthof the analysis did notaffectthetrendsobserved in thedatatoanygreatextent.A relatedapproach wouldbe to calculate an importance function,or information-by-frequencyweighting,for the presentstimulusset.Theseapproaches mayperhaps yielddifferentconclusions for some of the questions of the presentstudyand deservefurther investigation. Asforchoosing different frequency regions or frequency-importance weightings for theanalysis of differentindividualstopconsonants, wesuggest thatsuchmodificationsof the presentschememaybe helpfulin futureexperiments that seek to identify specific acoustic characteristics or frequency regionsthat areresponsible for the identificationof specificphonemes. ACKNOWLEDGMENTS

Thisresearchwassupportedin partby a grantfrom the Deafness Research Foundation. Thanks are due to James

Hillenbrand for assistancein constructingthe synthetic

speech stimuliandto DianneVanTasellfor severalhelpful discussions regardingthe scoringof speechrecognitionresultsthat werehelpfulin our dataanalysis.EdwardConture

providedhelpfulcommentson an earlierversionof this manuscript. We alsothankLenoreHolteandSwatiLotlikar for assistance in collectingthe data.

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C.W. Turnerand M.P. Robb:Audibility of stopconsonants

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