acoustic and perceptual correlates of the non-nasal-nasal

7/25/2019 Acoustic and Perceptual Correlates of the Non-nasal-nasal

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Acoustic and perceptual correlates of the non-nasal-nasal

distinction for vowels

Sarah Hawkins

Haskins Laboratories, New Haven, Connecticut 06511

Kenneth N. Stevens

Departmentof ElectricalEngineering nd Computer cience, nd Research aboratoryof Electronics,

Massachusettsnstitute of Technology, ambridge,Massachusetts2139

{Received 9 June 1984;acceptedor publication28 December1984)

For eachof fivevowels i e o o u] following t],a continuum rom non-nasalo nasalwas

synthesized. asalizationwas ntroduced y insertinga pole-zeropair in the vicinity of the first

formant n an all-pole ransfer unction.The frequencies nd spacing f the pole and zero were

systematicallyaried o change he degreeof nasalization. he selection f stimulusparameters

was determined rom acoustic heory and the resultsof pilot experiments. he stimuli were

presentedor identification nddiscriminationo listenerswhoseanguagencluded non-nasal-

nasalvowel opposition Gujarati,Hindi, and Bengali)and to American isteners. here were no

significant ifferences etweenanguage roups n the 50% crossover ointsof the identification

functions.Somevowelsweremore nfluenced y rangeand contexteffects han wereothers.The

language roups howed omedifferencesn the shapeof the discriminationunctions or some

vowels.On the basisof the results, t is postulated hat (1) there s a basicacoustic ropertyof

nasality, ndependent f the vowel, o which the auditory system espondsn a distinctiveway

regardless f language ackground; nd {2) hereare oneor more additionalacoustic roperties

that may be used o variousdegreesn different anguageso enhance he contrastbetween nasal

voweland its non-nasal ongener.A proposed andidate or the basicacousticproperty s a

measure f the degreeof prominence f the spectralpeak n the vicinityof the first formant.

Additional secondary ropertiesncludeshifts n the centerof gravityof the low-frequency

spectralprominence,eading o a change n perceived owelheight,and changesn overall

spectralbalance.

PACS numbers:43.71.Es, 43.70.Fq, 43.70.Hs, 43.71.An

INTRODUCTION

A. Review of the acoustics of nasal vowels

The phonemicoppositionof oral and nasal vowels s

widespread mong anguages, ut the acoustic ropertyun-

derlying his oppositions not well understood. nasalvow-

el is produced y introducing coustic ouplingbetween he

oral and nasalcavities t a point that is abouthalfway along

the vocal ract between he glottisand the lips.The effectof

this acousticcoupling s to shift the natural frequencies f

the vocal ract (i.e., he formant requenciesor the non-nasal

vowel),and to add pole-zeropairs o the vocal-tract ransfer

function Fant, 1960;Fujimura, 1960, 1961;Fujimura and

Lindqvist, 1971; House and Stevens,1956; Mrayati, 1976;

Lonchamp,1979).The principaland mostconsistent onse-

quenceon the acoustic pectrumof the vowel appears o be

at low frequencies,n the vicinity of the first ormant. When

the cross-sectionalrea of the velopharyngeal pening s

gradually ncreased, here is usually a shift in the first for-

mant frequency, nd there s an increasen its bandwidth.A

pole-zeropair is introduced n the vicinity of the first for-

mant and the spacing etween he poleand zero ncreases s

the velopharyngeal pening ncreases,with the result that

the additional pole shows ncreasedspectralprominence

with largeropenings. heseobservationsrebasedon calcu-

lation of the transfer unctionof a systemof coupled ubes,

and are confirmed y data from the acoustic pectraof natu-

rally spoken asalvowelsand rom studies f thebehaviorof

articulatory models.

Sometimes more than one additional resonance in the

vicinityof the first formant canbe observedn acoustic pec-

tra of nasal vowels. These additional resonances must arise

becausehe mpedance f the nasalcavityhasmore han one

low-frequencyesonance elowabout 1.5kHz. These eson-

ancesare presumably consequencef the sinuses,which

are oftenof appreciable olume,and which are coupled nto

the nasalcavity proper throughrelativelynarrow openings

(Maeda, 1982a;LindqvistandSundberg, 972). n any event,

the end esult s that the ntroduction f acoustic oupling o

the nasalcavity modifies he spectrumof a vowel n the vi-

cinityof the irst ormant,so hat the narrow spectral romi-

nence ssociated ith the first ormant s replaced y a pole-

zero-polecombination possiblywith additional pole-zero

pairs).The additionalpole-zeropairscan result n a "filling

in"of the valleys n the spectrum boveor below he frequen-

cy of the original first formant. Consequently here is a

broader requency egion n the vicinity of the first formant

overwhich he spectral nergy s distributedor a nasalvow-

el, asopposedo a relativelynarrowspectralprominenceor

a non-nasal owel.This typeof modification f the spectrum

was irst notedby Hattori et al. (1958).

1560 J. Acoust.Soc. Am. 77 (4), April 1985 0001-4966/85/041560-16500.80 1985 AcousticalSociety of America 1560

distribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 129.174.21.5 On: Mon, 22 Dec 2014 22:04:44


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In addition o modification f the spectral hape n the

vicinity of the first formant, nasalization n natural speech

can alsogive rise to changesn the spectrumat higher fre-

quencies. here may be shifts n the frequencies f higher

formants,modificationsn the amplitudesof the spectral

peaks,and ntroductionof additionalspectralpeaks.These

changesn the acoustic pectrum t high frequencies o not

seem o be asconsistent cross ifferentspeakers nd vowels

as hose n the vicinity of the first formant.This lack of con-

sistencyn the effects f nasalization t higher requenciess

predictable n view of the substantialndividual differences

in the anatomyof the nasalcavities.

Spectrograms nd spectracontrastingnasal and non-

nasalvowelsproducedby a male speakerof Gujarati (a lan-

guagespoken n India which has a nasal-non-nasal hone-

mic oppositionor eachof its vowels) re llustrated n Fig. 1.

The frequency esolutionof the analyzingsystemdoesnot

alwayspermit clear observation f a zero in the spectrum,

but at leastone additionalpole and consequent roadening

of the ow-frequencypectral rominenceanbeseenn each

of the nasalvowels. n all the nasalspectra he first formant

peak s rendered essprominentby the filling n of the valley

in the spectrum ither aboveor below he first formant.

B. Perception of nasal vowels

Experiments ndicate that nasal vowels can function

perceptually s a class n distinction rom oral vowels,even

for speakersf languagesacking he non-nasal-nasalppo-

sition sa phonemic ontrastButcher, 976;Wright, 1980).

Several tudies aveattempted o determineheacoustic nd

articulatory ttributes hat giverise o listeners' erception

of nasal vowels as a class and as distinct from their non-nasal

counterparts.hesestudiesall roughly nto two categories

depending pon how the stimuli are manipulated: tudies

searchingor the acoustic rerequisitesor the perception f

nasality y directmanipulation f thespectrum f thesound,

and studieswith articulatory ynthesizersn which he per-

ceptionof differentdegrees f nasalizations examined y

varying he areaof the velopharyngealort.

Studiesnvolvingdirectmanipulation f the spectrum

indicate hat vowels re udged o be nasalwhen he spec-

trum is modifiedn the ow-frequencyangeand sometimes

when heamplitude f the hird formant s ncreased. sing

the Haskinspattern-playbackystem, elattre 1954)pro-

ducednasalvowelsby addingenergy n the vicinity of the

fundamentalrequency. akeuchi t al. (1974,1975)added

pole-zero airs o spectra f naturalspeech t various re-

quencies nd found hat the greatest umberof nasal udg-

ments ended o occurwhena pole-zero air was ntroduced

in the vicinityof the first ormant.Raising he amplitude f

the hird ormant urther ncreasedudgments fnasality or

somevowels.For mostvowelsnasal udgmentswere n the

range40%-60%, although heywerehigher or [u] and [a].

In studieswith articulatorysynthesizers, ouse and

Stevens1956)and Maeda (1982a)produced everal teady-

state owelswith various egrees f opening f thevelophar-

yngealport.Their results howedhat a greatervelopharyn-

gealopeningwasneededo produce given evelof nasality

(as udgedby listeners)or the low vowels han for the high

--- .

.

dB .7.; ?7 ...."-... ...

o t 2 n 5

5

0 '"-.-

.. .

-,.-z::. .......,,- ;:;:.: .:...

: ....::'] :::: ?,---- ' [':[-.:'

:'' '0

...

0.2 s

6o. ::t.................'....i.tTpi"........,

o I 2 3 4 5 kHz

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vowels. Observationsof the articulators using cineradio-

graphy Delattre, 1968), iberopticsBenguerel nd Lafar-

gue,1981), nd iberopticsith lectromyographyHender-

son, 1984) also show that there are in fact systematic

differencesn velopharyngeal ort area dependingupon

vowelheight. n both Houseand Stevens' 1956)and Mae-

da's 1982b) tudies,he spectra f stimuli hat were udged

to be nasalwereobservedo be flatter than non-nasal pectra

in the vicinityof the first ormant.That is, the prominence f

the spectralpeak or the formantwas reduced,with addi-

tionalpeaks ometimesresentn thevicinityof the ormant.

Theseobservationsupport he findings f studies hat ma-

nipulated he spectrumdirectly.

Beddorand Strange 1982)examined he identification

and discriminationof stimuli varying along a continuum

from [ba] to [bill. These timuliwereproduced y systemati-

cally varying the velopharyngeal ort area in articulatory

synthesis.dentification unctionsweresteeperor Hindi lis-

teners, or whom he non-nasal-nasal owelcontrasts pho-

nemic, than for American English isteners, or whom it is

not. The two language roupsalsodifferedslightly n their

placementof the 50% crossover oundary.Discrimination

functionswere similar for the two groupsof listenerswhen

the differencebetween stimuli was small: responseswere

aroundchance t the endpoints nd peaked round he cate-

goryboundary. n a simplified iscriminationask,however,

the American but not the Hindi listenersappearedbetter

able to distinguishbetweennon-nasal han betweenna_sal

vowels.These data indicate hat linguisticexperiencemay

affect he perceptionof vowelnasality n relativelycomplex

ways.

The literature suggests,hen, that vowelswill be per-

ceivedasnasal f the acoustic pectrumof a non-nasal owel

is modifiedso hat the prominencen the vicinityof the first

formant is broadened and flattened. For some vowels, addi-

tional consequencesf the low:frequency pectralmodifica-

ion, which s the primary and mostconsistentmodification,

are changes n the amplitudeof higher-frequency ompo-

nents,and/or changesn the perceivedheight of the vowel

due to a shift in the centerof gravity of the low-frequency

prominence.Beddor and Strange's 1982) results ndicate

that responseso stimuli varying along a non-nasal-nasal

continuum renot stronglydependent n whether he isten-

er'snative anguage istinguisheshonemically etween a-

sal and non-nasal vowels. There are, however, behavioral

differenceshat do appear o be dependent pon inguistic

experience.

A possible onclusionrom thesedata s that nasality n

vowels nvolvessomeacousticproperty hat is common o

all nasal vowelsand that may be perceivedas nasalityre-

gardless f linguistic xperience,ogetherwith otherproper-

ties hat may differ across owelsand acrossanguages. he

purpose f thispaper s to attempt o identifyandspecifyhe

parameters f sucha universal asalproperty,using ynthet-

ic speechn order to minimize he occurrence f the more

language-or vowel-specific oncomitantproperties.The

first experimentdescribed ere sought o producegoodex-

emplarsof nasalvowelsby insertingan additionalpole-zero

pair n the spectrum f non-nasal owels,n accordance ith

predictions rom acoustic heory and analysisof natural

speech.Subsequent xperiments sed he "best" exemplar

for each nasal vowel as endpoints n acousticcontinua.

Thesecontinuaweredesignedo testwhether he presence f

a low-frequency ole-zeropair produces imilar responses

from listenerswith different inguistic ackgrounds.

I. GENERAL DESCRIPTION OF STIMULI

In all heexperimentshestimuli ere onsonant-vow2

el syllablesbeginningwith a voiceless naspirateddental

stop, t],and the vowelwas one of [i e a o u] or their nasal

counterparts, fi fi ill. Parametervalues or thesestimuli

were selected in accordance with measurements of words

produced y a malenativespeaker f Gujarati. The syllables

weregenerated n a Klatt synthesizerKlatt, 1980)with the

formant resonators connected in cascade. The duration of

the vowel was 325 ms, and the fundamental requencywas

givenan appropriate ising-fallingcontour.The initial stop

burst was identical for all stimuli, and the first 40 ms after

voicing onset were identical for both non-nasaland nasal

vowelsof the samevowelquality.

A spectrogram f a typical nasalstimulus s shown n

Fig. 2, and trajectories f variousparametersor this stimu-

lus are displayed n Fig. 3. The time from stop consonant

releaseo onsetof voicingwas 10ms. mmediately ollowing

the onsetof voicing, herewere appropriate ormant transi-

tions from the dental consonant o the steady-state owel

with whicheachsyllable erminated. The example n Figs.2

and 3 for the vowel ] is slightlyatypical, n that therewas

somediphthongization f the vowel, as evidenced y the

slowly risingF2, in accordancewith measurements n the

Gujarati okens. herewasno suchdiphthongizationor the

other vowels.)Detailed characteristics f the initial transi-

tions and the non-nasal owelsare listed n Table I. Slight

modifications f the measured ormant frequenciesor the

natural Gujarati tokenswere made n order to obtain a best

matchof vowelquality based n nformal istening) etween

the synthesized owelsand the naturally produced' owels.

5 , '

0 '

I I I I I

o 0.2 0.4

TIME (s)

FIG. 2. Spectrogramf a typical ynthetic onsonant-voweltimulust].

Introductionof a pole-zeropair to simulatenasalization egins t 40 ms,

and nasalizations complete t 80 ms. For this vowel,somediphthongiza-

tion s ntroducedhrough he ising econdormant n the nitialpartof the

vowel

1562 J. Acoust.Soc. Am., Vol. 77, No. 4, April1985 S. Hawkins nd K. N. Stevens:Correlatesof vowelnasality 1562



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N

>-

z

o I

0 I

I

F

F2-

FNZ FNP

....... FI

I I

IOO 200 300

TIME (ms)

FIG. 3. Trajectories f the first three ormant requenciesnd of the fre-

quenciesf headditional ole FNP)andzero FNZ) for hesynthetic asal

stimulustE], hose pectrogramsshownn Fig. 2. Time smeasuredrom

the onset of the burst, whose duration was 15 ms.

For all vowels, he fourth and ifth formant requencies ere

3500and4500Hz {except 4 = 3700Hz for [i]},andband-

widthsB3, B4, and B5 for the higher ormantswere 150, 170,

and 250 Hz, respectively.

Each syntheticnasalvoweldiffered rom its non-nasal

congenern either one or two ways:all nasalvowelscon-

tainedan additionalpole-zeropair, and in somecases he

frequency f F 1 differed rom that of the equivalentnon-

nasalvowel.The additionalpole-zeropair for a nasalvowel

began o separate 0 ms aftervoicingonset, nd during he

next40 ms he requenciesf thepoleandzerounderwent

piecewise-linearotions owardvalues ppropriateor the

intendednasalvowel.The beginning aluesof the frequency

of the pole{FNP) and the frequency f the zero {FNZ) at 40

msaftervoicingonsetwerealways 00 Hz. The bandwidths

of FNP and FNZ were always100 Hz. In addition o intro-

ducing localperturbation f the spectrumn thevicinityof

FNP andFNZ, the pole-zero air alsohasa small nfluence

on the amplitudes f the formantpeaksat higher requen-

cies.For the vowels n which he frequency f the first for-

mant {F 1} n the nasalvowelwasdifferent romF 1 for the

correspondingon-nasal owel,F 1 underwent piecewise-

linear rajectory rom henon-nasalo thenasal alueduring

the time interval from 40-80 ms after voicingonset. n the

TABLE I. Characteristics f the non-nasal owels hat wereusedasstarting

points n developinghenasal owels nd henon-nasal-nasalontinua. 1

to F3 representhesteady-stateowel ormant requencies; li to F3i are

the startingrequenciesf the irst hree ormantsor the ransitionsollow-

ing the release f the burst or the nitial Itel; ndB 1 andB 2 are the band-

widthsof the first two formants.All frequenciesre n Hz. Other param-

eters of the stimuli are described in the text.

i e o o u

F 1 270 400 700 430 270

F 1 200 270 350 270 200

F2 2300 2025 1150 850 850

F 2i 1800 1700 1500 1350 1350

F 3 2900 2915 2500 2500 2500

F3i 3000 3100 2800 2800 2800

B 1 60 60 80 80 80

B2 80 80 100 100 100

examplen Figs.2 and 3, the non-nasal 1 s 400 Hz and he

nasal F 1 is 350 Hz.

The selection of 40 ms as the duration of the initial non-

nasal ime nterval ollowing he obstruent onsonant efore

the onset of nasalization was based on informal observation

of this time interval for a number of CV utterances n several

languages.here s considerableariability n this time in-

terval,but 40 ms s within the observed ange.The selection

of 400 Hz as he frequency t which he poleand zerobegin

to separateepresentsnestimate f thenatural requency f

the nasalcavitywith a closed elopharyngeal ort. This val-

ue is consistentwith estimates eported n the literature

(House nd Stevens, 956;Fant, 1960;Maeda, 1982a;Lind-

qvistandSundberg, 972)although, gain, here spresuma-

bly considerablenterspeaker ariability,and there are no

satisfactory easurementsf this requencyor a numberof

talkers.

II. PRELIMINARY EXPERIMENT

The aim of the preliminaryexperimentwas o establish

a configurationof F l, FNP, and FNZ that would lead to

acceptable ersions f eachof the nasalvowels fi fi] for

nativespeakers f Gujarati, which hasall thesevowels n its

phonemicnventory.Our strategywas o generate number

of possible ersions f eachof thenasalvowels y manipulat-

ing F l, FNP, and FNZ, and to ask listeners o rate these

stimuli n termsof their acceptability s nasalvowels.

A. Procedure

Informal observations,as well as theoretical consider-

ations,edus o expect hat thebestversions f nasalvowels

wouldbe obtainedwhen he frequency f the nasalzero was

placed pproximately idway etweenheoriginal irst or-

mant frequency nd the frequency f the nasalpole. The

absolute ifferencen frequency etween he two poleswas

in question, owever.We werealsoaware hat nasalization

of the vowel might causesomeshift in F 1 relative o its

frequencyn the non-nasal owel,especiallyor nonhigh

vowels.For eachvowel, then, we generated 2-24 stimuli

with variousvaluesof FNP and FNZ (with FNZ always

between NP andF 1) and, n somecases,with more han

one value of F 1.

The parameter aluesor theconstellationsf teststim-

uli for eachvowelareshownas he pointsplotted n Fig. 4. In

this figurewe plot the valuesof FNP and FNZ for each

stimulus. igure4 alsodisplaysheF 1 valuesor thestimuli

(horizontalines} xcept or [], whereF 1 is too high o be

displayed, nd gives oci of pointscorrespondingo FNZ

midwaybetween 1 and FNP (diagonalines), .e., FNZ =

(FNP d- F1)/2.

There were 86 stimuli n all. Three repetitions f each

stimuluswere generated, nd the resultingstimuliwere ar-

rangedn random rderand ecorded n our est apes. he

test apeswerepresentedo threegroups f listeners:hree

nativespeakersf Gujarati,sixnativespeakersf American

English, nd ivenativespeakersf other anguagesf north-

em India (Punjabi,Hindi, andBengali)whichhavea nasal-

non-nasal ppositionor vowels. isteners eregiven n an-

swer sheet on which the intended nasal vowel was indicated

1563 J. Acoust. oc.Am.,Vol.77, No.4, April1985 S. Hawkins ndK. N. Stevens: orrelates f vowelnasality 1563



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iooo

800

600

400

00

[r]

'"" FN- FNP+ I

I I I I

400 800 1200

4oo

[]

400 I I , JO0 800

ADDITIONAL POLE. FNP (Hz)

6OO

4OO

200

OO 6OO

[]

400 FI

400 600

[,] FI820

600

400 )0 J

00 4

600

400200

ADDITIONAL POLE, FNP ( Hz )

FIG. 4. Stimuli used n the preliminaryexperiment o select goodnasal

vowel or eachvowelquality.Each stimuluss representedn termsof the

frequency f theadditional ole FNP) andadditional ero FNZ). Note that

the axes or [i] are half the scaleof thoseof the othervowels. he frequency

of F l is representeds a horizontal ine (except or [], where t is given

numerically). he diagonaline on eachgraph s the requency t which he

nasal ero s exactlyhalfwaybetweenhe frequenciesf F 1 and he nasal

pole [FNZ = (FNP + F 1)/2]. The arrowsand the dotted,hatched, nd

cross-hatchedegionsdentifystimuliwhose esponsesell into particular

categories s ollows.Stimuli outside he dottedareaswereexcluded rom

consideration ue o inconsistenciesn the responses,r due o poorvowel

quality.Stimuli within the hatched reaswere hosewhichsubjectsudged

most nasal.Stimuli within the cross-hatchedreaswere thosemost pre-

ferredwithin this ast mostnasal)group. See ext for details.) he arrows

point to the stimulichosen s he nasalendpointof a non-nasal-nasal on-

tinuum for eachvowel.Within a givenvowel set,all stimulusparameters

other han FNP and FNZ remained he same,with theseexceptionsindi-

catedby opencircles):or [e], of the two stimuliwith FNP -- 750 Hz and

FNZ ---- 25 Hz, F l was400 Hz in onecase nd430 Hz in the other; or [o]

withF l ----820 Hz, oftwo stimuli with FNP ----300 Hzand FNZ ---- 70 Hz,

F 2 was 150Hz (as or all the otherstimulion this graph) n onecase, nd

uniquely etat 1300Hz in the othercase.

for each stimulus tem. For each stimulus, the listenerswere

asked o respond o two questions:1) ndicate f the vowel

quality s unlike hat of the intended owel; 2) ndicate he

adequacy f the nasalquality of the vowel on a four-point

scale anging rom "poor" to "good."

B. Results

Analysisf he udgmentsfvowel ualityndicated

that the American istenersudged he vowel quality to be

inadequatemore often than did the Indian listeners, nd

were alsomore consistentn their judgments.For this rea-

son,a stimuluswasexcluded rom further considerationf it

accrued total of four or more "poor" udgments rom the

American isteners. Stimuliexcludedon this criterionwere

as ollows:13 tokensof [];wo of []; and six of [fi]. These

numbers eflect he difficultywe had n synthesizing range

of highnasal owels f goodquality}t wasassumedhat he

remaining timuliwereall adequaten vowelquality.

Some of the remaining stimuli elicited inconsistent

judgmentsof degreeof nasality or certainsubjects. n in-

consistentudgment' asdefined sonewhere he rangeof

ratingswasgreater han one{on he four-pointscale}or the

three epetitions f eachstimulus. or example, ratingby a

single ubject f 2, 3, 2 for a givenstimuluswasconsidered

to be a "consistent" esponse;atingsof 2, 3, 1 or 2, 4, 4,

however,wouldbe considerednconsistentesponses.tim-

uli eliciting hree or more such nconsistent etsof ratings

from the American listeners,or four or more from the Indian

listeners,were excluded rom the analysis. Thesestimuli

were two tokensof [] and two of [].) In all, a total of 25

stimuliwereexcluded n the basisof poorqualityor incon-

sistent esponse. he followingnumbers f stimuliwere eft

in eachcategory: 1 okens f[i], 12of[], 19of[a], 13of[6],

andsixof[a]. These re hosestimuliwithin the dottedareas

of Fig. 4.

The remainingstimuli were then rankedaccording o

their nasality atings.The rangeof ratingswassmall or all

languagegroups,especially or the Indian listeners.The

maximumpossibleangewas3.0 points.For eachvowelset,

the ndian isteners' verage angeof ratingswasalways ess

than one point, whereas he American listeners'average

rangeof ratingsvaried rom ust less han onepoint {for a])

to almost wo points for []}. hat is, the Indian listeners

werenot discriminating etween he stimuli n termsof na-

sality, as reinforcedby variousstatisticalanalyses. or ex-

ample,Kendall coefficientsf concordance etween atings

for eachvowelby the threeGujarati listenersell far shortof

significance.n contrast,Spearman ank-ordercorrelations

between he American isteners, andomlydivided nto two

groups f three,weresignificant t the 0.05 evelof probabil-

ity or better or all vowels xcept a], for which herewerea

largeproportionof tied ranks. Spearman orrelations ere

used with the American listeners since a Kendall coefficient

of concordance n six ndividuals s clumsy o compute.}

The fact that the Indian listeners ppeared o differen-

tiate very little between he stimuli in terms of nasality s

consistent ith the resultsof discrimination xperimentse-

ported ater in this paperas well as by Beddorand Strange

{1982}.t did not helpus o select he "idealnasal owels" or

our subsequent xperiments,however. t was decided o

identify he mostacceptable asalvowels rom the rankings

of the American isteners, s long as thosevowelsselected

wereamong hosemost avoredby the Gujaratis.That is, he

wider angeof rankings ivenby the Americanswas akenas

evidencehat their rankings robably ave he most eliable

measure f adequacy, hile he validityof the chosenokens

as acceptable ujarati nasalvowelswasensured y requir-

ing hese okens o beamong he hreeor fourmostpreferred

by the Gujaratis.The mostacceptable ole-zero-poleombi-

nationsas defined n this way are those alling within the

1564 J. Acoust.Sec. Am., Vol. 77, No. 4, April 1985 S. Hawkinsand K. N. Stevens:Correlates of vowel nasality 1564



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hatched reas n Fig. 4. For all the stimuli, he average ating

by American istenerswasat least2.9 on the four-pointscale

from 1 (poor} o 4 (good}. or the vowels ], ], and [5],

certainpole-zero-pole ombinations toodout as mostpre-

ferredwithina wider rangeof preferred ombinations. hese

stimuli are identifiedby the crosshatching n Fig. 4.

It canbe seen rom Fig. 4 that all of the preferredpole-

zero combinations of each vowel lie on or close to the line

labeledFNZ -- (FNP + F 1 /2, and there s at least 100 Hz

between he zero and each pole. This finding s consistent

with predictionsrom acoustic heoryand provides syste-

maticbasis or selecting arametersor a setof"ideal" nasal

vowels.

Parameter aluesof the stimulus hat received he high-

estnasality ating within the preferredgroup or eachof the

fivevowelsetswereselected s he nasalendpoints f the five

continua used in the identification and discrimination ex-

periments hich ollow.The sti muli re dentified ith an

arrow in Fig. 4. The characteristicsf thesemostpreferred

nasalvowels re summarizedn Fig. 5. Figure 5 shows gain

that the bestnasalvowelswereproducedwith a zero that is

about midway between he first formant and the additional

resonance. It indicates the shift in the first formant for the

nasal vowel in relation to that for the corresponding on-

nasal vowel. It also shows that there is not much variation in

the locationof the low-frequency ole-zero-poleombina-

tion from one nasal vowel to another comparedwith the

variation n F 1 for non-nasal owels.This configuration f

two polesand a zero s similar o that predictedon the basis

of acousticheory{Stevenst al., in press}.

III. IDENTIFICATION TEST

Havingestablishedn appropriate etof synthetic asal

vowels in a LtV] context) hat wereacceptableo a groupof

2.5

2.O

1.5

ol.0

0.5

i e a o u

VOWEL

FIG. 5. Frequencies f poles nd zerocorrespondingo the nasalendpoint

stimulus or eachvowel set, dentifiedby the arrows n Fig. 4. Also shown

(/) is the frequency f the first ormant or eachnon-nasal owel.The

crossesX } representhe shifted}irst-formantrequencyF 1 , solidcircles

(e} representhe frequency f the additional ole FNP), and opencircles

(O}representhe frequency f the zero FNZ}.

listeners,we proceeded ext to produce, or eachof the five

vowels, series f stimuliranging rom a non-nasalo a nasal

extreme. Vowels in these series were identified as nasal or

non-nasal y listenerswhose anguagencludeda non-nasal-

nasaldistinction or vowelsandby listenerswhose anguage

did not use hat distinction.The principal aims of theseex-

perimentswere o determine: 1 whether isteners ield simi-

lar identification esponseso the stimuli independentof

their experience ith thenon-nasal-nasal istinction, nd (2}

whether someacousticproperty or processingmechanism

can be defined, ndependent f the vowel, hat characterizes

vowels dentifiedas nasalas opposed o vowels dentifiedas

non-nasal.

A. Stimuli

For each of the five series of stimuli, the vowel at the

non-nasal ndof the continuumhad the formant frequencies

listed n Table , and he pole-zeropair was ocatedat 400 Hz

(i.e., herewasno effectof the additionalpoleand zero}.For

the vowel at the nasal end of the continuum, values for F 1,

FNZ, and FNP defined stimuli that were rated as the most

acceptable asal vowels n the preliminary experiment,as

summarized n Fig. 5. Intermediatestimuli on the contin-

uum were obtainedby interpolating n equalstepsbetween

values of F 1, FNZ, and FNP for the non-nasal and nasal

extremes. There were nine items in the continuum for the

vowels ie a o] andeight or [u]. Figure6 shows alues ff 1,

FNZ, and FNP for the continua or eachof the vowels.For

the high vowels i] and [u] there s no shift n F 1 along he

continuum,whereas here s a change n F 1 for the vowels

[eao].

In Fig. 7 we showspectra f threeof the stimuli n each

continuum: he two endpointstimuli and an intermediate

stimulus'.While the stimulus escriptionn termsof poleand

zero ocations n Fig. 6 indicates hat there s a regularpro-

gressionn the stimulus ropertiesor eachcontinuum, x-

amination f Fig. 7 suggestshat theremaybe otherwaysof

describinghe stimuli.Stimulus hangesmight be specified

in termsof amplitudes f particularspectral omponents r

the ratio of the amplitude f a spectral eak o the amplitude

of a valley,or in termsof the center requency r width of a

gross pectral rominencehat may consist f two or three

closelyspacedpeaks.

For example,n the caseof the stimulion the [i-i] con-

tinuum, he spectrums changingn severalways hat could

potentially be used by a listener either to discriminate

between timulior to identifynasality.These nclude: 1 the

frequency f the additionalpole, 2} he relativeamplitudeof

the spectral eakarising rom this pole,and (3} he relative

spectral amplitude at high frequencies,which changes

graduallyover he entirecontinuum.Similar changesn the

amplitudesof spectralpeaksoccur for stimuli on the other

continua.For example, here s an increase f 8 to 12 dB in

the relative amplitudeof the high-frequency eaks ampli-

tude of F 3 peak and above n relation o F 1 peak} rom the

non-nasal nd of the continuum o the nasalend for [ie o u],

for which FNZ FNP, there is a decreaseof about 5 dB in the absolute

high-frequencyamplitude over the continuum, but the

1565 J. Acoust.Soc. Am., Vol. 77, No. 4, April 1985 S. Hawkinsand K. N. Stevens:Correlatesof vowel nasality 1565



7/16

IOOO

800

600

400

I I I

0 0 0

200

O' I

o o

o o -

1 x 1 x x x x x x

I I I I

$ 5 7 9

IOOO

800

600

400

200

i i

_

0 0

0 0 0 0

-

-d x x x x x x X

i :5 5 7 9

IOOO

800

600

400

200

0

IOOO

800

600

400

200

I I 1 I I

x x x x x x x x x_

o

o

o

o

i 3 5 7 9

i i I i 1

_

--

x x x x x x x x

_

I 3 5 7 9

STIMULUS NO.

IOOO

800

600

400

200

I I I I I

I 3' 5 7 9

STIMULUS NO.

x FI

o FNZ

* FNP

FIG. 6. Frequenciesf the two poles X andO) andzero 0) for the stimuli

comprisinghe non-nasal-nasal ontinua or eachvowel.For the non-nasal

vowel n eachcase stimulus1) the additionalpole and zero wereset at 400

Hz, and canceled each other.

[i o u ], the frequency t which he additional ole-zero air

is ntroduced 400 Hz) early n eachcontinuum s well sepa-

rated from the original first-formant requency.For these

three vowels, f the additionalspectralpeak due to FNP

causes shift in the low-frequency enterof gravity, this

effect s expected o besmall. t wouldbe n the directionof a

highereffective 1 or more openvowelquality for the high

vowels i] and [u], and a lower effectiveF 1 or more close

quality or the low vowel o]. It shouldbe noted hat, if the

spacing f the pole-zeropair were ncreased eyondcertain

critical limits the quality of the vowel is likely to change

dramatically.For example n the [o-fi] continuum,vowels

with a wider pole-zero pacinghan that of stimulus begin

to sound ike [3].

In summary, hen, there are severalacoustic ttributes

that change s the pole-zero air is manipulatedn the var-

iousstimulus ontinua; hesehavepotentialconsequencesn

the way the stimuli are dentifiedor discriminated y listen-

ers.The shapeof the low-frequency pectral rominences

modified y the additionof a subsidiary eak hat s aboveor

below he requency f theF 1 peakor that splits heF 1peak

into two nearbypeaksof about the sameamplitude.There

may be a shift n the perceived owelheight,and theremay

bea shift n overallspectral alance f high-frequencympli-

tude in relation to low-frequency mplitude hat could be

interpretedas a change n voice quality (of the type that

arises rom laryngealmanipulations). hesechangesn the

stimuli are automatic consequencesf inserting he addi-

tional pole-zeropair, and assuch hey cannotbe manipulat-

ed independentlyn a speech ynthesizer ithout ntroduc-

ing other sideeffectsnto the stimuluscharacteristics.

change n relativeamplitude ollows he samepatternas for

theothervowels, he ncrease eingust 4 dB between timuli

1 and 9.

For all of the continua here is a spectralminimum,

arising rom the zero, and this minimum can be seen n the

spectrum f the mostnasalstimulus nd sometimesn the

intermediate timulus. t is probable, owever, hat a change

in the depthof this minimum contributesittle to a listener's

identification r discrimination f the stimuli Malme, 1959).

The interactionof the pole-zero-pole ombinationof

F 1, FNZ, andFNP gives ise o a broad ow-frequency pec-

tral prominencewith differentshapes epending n the vow-

el. In the caseof the back vowels,F 2 alsocontributes oward

shapinghis ow-frequency rominence. or example,n the

[o-0] continuum, s he spacing etween NP and FNZ in-

creases,he amplitudeof the spectral eakcorrespondingo

F 1 decreases hile the amplitudeof the F 2 peak ncreases.

This changen balance etween he two peaks an ead o an

increasen the frequencyof the centerof gravityof the low-

frequency pectralprominenceelative o that for the nor/-

nasal stimulus Chistovichand Lublinskaya, 979; Chisto-

vich et al., 1979).A possible onsequences a shift to a more

openvowel quality, as will be discussedater when we exa-

mine the results.A similarbut smallereffectcanbe expected

for the [e-] continuum, or which there s alsoa symmetri-

callyspaced ole-zero-poleombinationn theF 1 region. n

the case of the non-nasal-nasal continua for the vowels

B. Procedure

An identificationestwasprepared or eachvowelcon-

tinuum.Each stimuluswasrepeated ine times eight imes

for ([u-fi]) and orderof presentation ascounterbalancedo

that a givenstimulustem ollowed achotherstimulustem

(includingtself)exactlyonce.Otherwise,he ordering f the

stimuli was random. With this construction of the tests, we

wereable o counterbalanceny effect he previous timulus

mighthaveon the identification f any particularstimulus

item. An item consisted f heating a particular stimulus

twice, ollowedby a 4-s-pause uringwhich hesubjectdefa-

titled he vowel n the syllable seithernasalor non-nasal y

marking he appropriate olumnon a sheet f paper.A prac-

tice estconsisting f six tems rom eachcontinuum includ-

ing the extreme timuliand some ntermediate timuli)was

heardbefore he test proper;Restswere givenas requested

between eparate owel ests,but not within a test. Stimuli

were presentedree field n a sound-treatedistening oom.

The subjects, ho were ested n groupsof from one o five,

had no difficultyunderstanding hat was requiredof them

after non-nasal and nasal vowels were demonstrated for

them by the experimenter--inminimal pairs n their own

languageor Indian subjects,nd as solated owels r pre-

cededby Lt] or Americansubjects.

Table II lists the native anguages nd the numberof

subjectsor this experiment nd the later discrimination x-

periment.There were native speakers f Gujarati, Hindi,




8/16

"::60[ i ; i ]

40 .H IAIiJ I

i i i i i

' : : I ] I I I i I J ' I i s , , ,

I i Io,, ,,, ,, u, , I i j i I"

' ' .... ' '-'' I io9,

i i i . :,;,. , i i : i I :

i i i i i I i i i i i i i i I

FREQUENCY (kHz)

FIG. 7. Spectra f a numberof the stimuli rom the continuaused n identificationests.Each row gives xamples f spectra rom oneof the fivevowels s

shown.The left panel n a row representshe stimulus t the non-nasal nd of the continuum,and the right panelrepresentshe mostnasalstimulus.The

middlepanel s the spectrum f the ntermediate timulus hat s closesto the average 0% crossover oint n the dentificationunction or that vowel.The

stimulusnumber s indicated n eachpanel.Spectraare discreteFourier transforms alculated rom the waveformweightedwith a Hamming window of

duration 26 ms.

Bengali,and English.The Indian subjectswere faculty and

students t universitiesn the Bostonarea, or their spouses,

all of whomwerehighly educated ndheld or werequalified

to hold positions n the professions. he "naive English

speakers"werestudents t MIT who wereuntrained n pho-

netics,unused o heatingsynthetic peech, nd knewno lan-

guageother han English.Except or their monolingualism,

thesesubjectswere similar to the Indian subjects n their

linguisticsophistication. he "non-naiveEnglishspeakers"

were rainedphoneticiansnd/or members f the Speech

CommunicationGroup at MIT, whosenative anguagewas

English.They were included o help assesshe effectof ex-

perienceor training on perception f nasalvowels.

English s considered ot to possess non-nasal-nasal

opposition or vowels, although contextually determined

vowel nasalization s strong n somedialectsof American

English. Indeed, speakers rom the southeasternUnited

Statesmay produce sequence f voweland nasalconsonant

as a nasalvowel withouta nasalmurmur)as n ['m:ki] or

monkey.Ma16cot 1960)hasargued hat all American isten-

ersneed o distinguish asal rom non-nasal owels n some

phoneticenvironments, s n the pair cat versus an't. The

occurrence f such minimal pairs s much lesssystematic

and widespreadn AmericanEnglish han n the Indian lan-

guagesnvolved n theseexperiments, owever.None of our

"naive Englishspeakers" pokea dialect employingstrong

vowel nasalization, but two of our "non-naive English

speakers" id; their experimental esponsesell solidly n the

middle of thoseof the rest of their group.

Hindi and Bengali, like Gujarati, possesshe nasal-

non-nasalopposition or vowels.Hindi vowelsare relatively

similar to those of Gujarati in quality and distributional

characteristics.he Hindi speakers'esponses ere husex-

pected o be similar o thoseof the Gujarati speakers. ow-

ever, here s somequestion s o the statusof the non-nasal-

nasaldistinction n Bengali see, or example,Ferguson nd

Chowdhury, 1960). Although usually cited as having an

equalnumber seven) f oral and nasalvowels, he oral vow-

els are much more frequent n standardcolloquialBengali

than are the nasalvowels.Fergusonand Chowdhury state

that the degreeof nasality"is relativelyweak and at times

may evenbe a kind of breathinessather than nasality n the

strict sense" 1960,p. 37). n addition o these actorscloud-

ing the statusof distinctivelynasalvowels n Bengali, here

appear o be many dialects hat are even essconsistenthan

standardcolloquialBengali n their separationof oral and

nasalvowels.These actorscouldcause he Bengalispeakers

to respond o identificationand discrimination estsdiffer-

ently from Gujarati and Hindi speakers, lthoughnot neces-

sarily in the sameway as American listeners.

1567 J. Acoust. Soc. Am., Vol. 77, No. 4, April 1985 S. Hawkins and K. N. Stevens: Correlates of vowel nasality 1567



9/16

TABLE II. Native languages f subjects sed n identification nd discrimi-

nationexperiments, nd numberof subjectsested n eachgroup.

Language roup Number

Gujarati 10

Hindi 5

Bengali 5

Naive American English 10

Non-naiveAmericanEnglish 10

The characteristicsf the fivegroupsof subjectsmade t

reasonableo analyze he data of this and subsequentxperi-

ments n two ways'comparing oth Americangroupswith

the Gujarati andHindi groups excludinghe Bengalis ue o

the questionable tatusof vowel nasality n their language),

and comparing ll five anguage roupsseparately.

C. Results

Figure 8 shows identification functions for all five

groups of listeners or all the vowels. All of the listener

groups dentified he nasalend of the continuumwith essen-

tially unanimousesponses,xcept or [i], for which he Ben-

gali and Hindi listenersgave less than 90% "nasal" re-

sponses.

Regression ines were calculated on the two points

above and the two points below the 50% crossover f the

identification unction or eachsubject, hat is, on the most

linearportionsof the functions.Slopes f the regressionines

were calculated on these same portions and were used to

determineprecise 0% crossover oints. n this and all sub-

sequentstatisticalanalyses, he data for [u-a] were trans-

formed o make them comparableo the other (ninestimu-

lus) continua.Figuresare plotted with the untransformed

data.

The 50% response oints or eachsubjectweresubject-

ed to a repeated-measuresnalysisof variancewith two lan-

guage amilies AmericanEnglishversusGujarati and Hin-

di)X 5 vowelsX20 and 15 subjectsper group. The only

significant effect was for vowels [F(4,132)--31.82,

p0.0001], with the difference etween he American and

Indian listeners falling far short of significance

[F (1,33) 0.28,p < 0.6]. The difference cross owelsn the

50% crossover oint was expected, ince he effectof the

additionalpole-zero air began o be perceptible t different

points n eachstimulus ontinuum,dependingargelyupon

the stepsizebetweenstimuli.

We carried out a secondanalysisof variance hat was

identical to the first except that the two-level contrast

between ndians and Americanswasreplacedby a five-level

contrastbetweeneach anguagegroup separately. Hence

there were 5 languagesX 5 vowels X 10 and 5 subjects er

group.)The vowelX languagegroup nteractionwas signifi-

cant F (16,140) = 2.32,p < 0.005].Although herewereno

consistentrends hat characterized ne groupas beingdif-

ferent rom anotheramongvowels, he Bengalisended o be

more variableamongvowels n their crossover oints ela-

tive to the other anguagegroups.This observations borne

out in other testsnot reportedhere. The main effectof vow-

els was again highly significant [F(4,140)=33.15,

I00

iz 80

6(

40

2.0

0

i $ 5 7 9 i $ 5 7

_1 I I I I I ._

[.-] ,'-

- J'?

i $ ,5 3' 9

STIMULUS NUMBER

I00

BO

60

40

20

0

/ to-] ....

BO // ' -

Y. GUJARATI

-'o'....-.NAIVENGLISH 0 ' -

NON-NAIVEENGLISH 40 -

-t---- HINDI

-.....BENGALI

i 3 5 7 9

STIMULUS NUMBER

FIG. 8. Average dentification unctions or eachof the stimulus ontinua

for the fivegroups f listeners s ndicated. he numberof subjectsn each

group s given n Table I.

p0.0001 ], and herewasagainno significantmaineffectof

language roup F(4,35} = 1.01,p


10/16

< 0.4] nor did the slopes f the vowelsdifferamong hem-

selves F(4,140)= 1.80,< 0.1]. The language roupsdid

differamong hemselvesoweverF(4,35)= 3.09,p 0.03].

Protected -tests Winer, 1971,.199)between airsof indi-

vidual languagegroups ndicated hat the only significant

differencewasbetween he two extremes, he Gujarati and

the naive American groups [t(35)= 1.94, p


11/16

z

IOO

8O

6O

o 40

20

FOLLOWING THE 2 MOST --

ORAL STIMULI

FOLLOWING THE 2 MOST

_

NASAL STIMULI

i e a o u

VOWEL

FIG. 10. nfluence f the nasalityof an mmediately receding owelon the

identification f a vowelasnasalor non-nasal.O: "nasal" esponseso stim-

uli which immediately ollowed either of the two most oral stimuli in the

identificationexperimentstimuli1 or 2). O: "nasal" esponseso stimuli

which mmediately ollowedeither of the two mostnasalstimuli n the ex-

periment stimuli8 and 9, or 7 and 8 for [u-ill). The interactionbetween

vowels nd nasalityof the preceding timuluswassignificantn an analysis

of variance see ext). Subjects ere 10 Gujaratisand 10 naiveAmericans.

The presence nd sizeof a rangeeffectwasassessedy

repeating he identification xperimentwith all instances f

the two mostnasalstimuliexcisedrom the tape. f stimulus

range nfluenced he subjects'udgments, he identification

boundary should shift one stimulus towards the non-nasal

end in this truncated continuum. Five of the naive American

subjects nd iveof the Gujaratisservedn bothexperiments.

Figure 11 shows he crossover oints or each vowel,

with two anguage roups ooled.The shiftwasgenerallyn

the expected irection--towards he lower numbered timu-

lirebut its sizevariedamongvowels.The boundaryshifted

by ess hanonestep or all continua xcept o-G].A repeat-

ed-measuresnalysis f variance 2 anguage roups 2 con-

tinua (truncated r complete) 5 vowelsX 5 subjectsn each

group]confirmed heseobservations,ince he nteractionof

vowelswith continuum, hown n Fig. 11, was significant

[F(4,32) = 6.03,p < 0.001], while the main effectof contin-

uum wasnot [F(1,8) = 2.86,p < 0.1]. One-tailed rotected

tests proved only the shift for [o-G] to be significant

[t (8) = 2.53,p 0.025],althoughhat for [o-U]wasalso ela-

tively large.

There was no overall difference between the two lan-

guage roupsn the extentof the rangeeffect F(1,8) = 0.36,

p0.57], but there were differences etween anguage

groups ependent pon hevowel F(4,32) = 3.09,p 0.03].

No interpretation s placedon the significance f this nter-

8

I I I i i

__ COMPLETE CONTINUA

-O-O- TRUNCATED CONTINUA

-

i e a o u

VOWEL

FIG. 11.Effectof therangeof the stimulus ontinua n udgments fnasal-

ity. Fifty-percent rossoveroints n the dentificationunctions re shown

for hecompleteontinuaO),withnine timuli ervowel ontinuumeight

for [u-ill), and orcontinuaruncated y removinghe womost asal tim-

uli (O).The nteraction f vowelwith complete r truncated ontinuum as

significantn ananalysisf varianceseeext).Subjects ere5 Gujaratis nd

5 naive Americans.

action,since t depends n a difference mongvowels n the

Gujaratis' and Americans' esponseso the two continua,

and this difference id not itself achievesignificance.

In summary, he data were subject o both immediate

contextand rangeeffects, ut the effects ended o be small,

and differences etween anguagegroupswere not statisti-

callysignificant. he two effects ppear o be ndependent f

eachother in that the nasalityof the immediatelypreceding

stimulus affected identification of front vowels but not back

vowels,while the rangeof the stimulus ontinuumonly sig-

nificantlyaffected o-G], and did not affect ront vowelsat

all. Conclusions oncerninghe rangeeffectare necessarily

tentative,since here were only five subjectsn eachgroup.

IV. DISCRIMINATION TEST

Having established hat listeners rom different an-

guagebackgrounds pparentlybase heir identification f

the non-nasal-nasal distinction on similar criteria, we

turned next to an examination of the behavior of the listeners

when hey are asked o discriminate etweenstimulion the

continua. n particular,we sought nswerso two questions.

(1) Given that the items on any one of thesestimuluscon-

tinua are equallyspaced, t leaston onesetof physicalmea-

sures, do listeners discriminate between stimuli better over

onepart of a continuum han another?More specifically, re

the stimuli hat are dentifiedconsistently sbeingwithin the

non-nasalor the nasal region esswell discriminated han

those ocatednear he dentification oundary?Sucha result

could ndicatea nonlinear elationbetween he simplephys-

ical parameters escribinghe stimulus nd he auditory ep-

resentation f thestimulus. 2) s thediscrimination ehavior

different for the different languagegroups?A difference

between anguagegroupswould suggesthat linguisticex-

perience nfluenceshoseattributesof the stimuli the listen-

ers are able to attend to when they make a discrimination.

Resultsof Beddorand Strange 1982)suggesthat this could

be the case.

1570 J. Acoust. Soc. Am., Vol. 77, No. 4, April 1985

S. Hawkinsand K. N. Stevens:Correlatesof vowel nasality 1570



12/16

To answer hesequestions, e prepared or eachvowel

series 4IAX discriminationest or pairsconsistingf stim-

uli that are oneand two steps part on the continuum.Pairs

were andomized, ndeachpair was epeated total of eight

times.The intervalbetween timuli n a pair was0.26 s, and

between airs t was0.42 s. The responseime between rials

was 2.3 s. The subjectswere the sameas thoseused n the

identificationests.The discriminationestwasalwaysdone

after the identification test, with at least one week between

the two. Stimuli were presented ver high quality head-

phonesn the samesound-treatedistening oom as for the

identification xperiment. ubjects ere ested n groupsof

from one o four, with restsgivenasnecessary, ut not with-

in individual vowel tests.

The resultsor two-step iscrimination resummarized

in Fig. 12. For clarity of presentation, nd consistentwith

the analysis n terms of two language amilies, he figure

shows discrimination functions for the two American

groups ogether, nd for the Gujarati and Hindi groups o-

gether.The Bengalidata are omitted rom Fig. 12, but they

were roughly similar to those of the Gujarati and Hindi

groups, lthougha little morenoisy.The tickson the curves

representmean identificationboundaries or thesegroups

from 50% crossover ointsdeterminedrom the data n Fig.

8.

tO0 I00

90

8O

7O

I-- 60

o

I I I I 50 I I I I

i $ 5 7 9 i $ 5 7

I--I00 I00 ' ,

z

uJ

90 90

i1. 80 80

70 70

50 I I I I I I I I I 50

I ? 9 i 3 $ ? 9

STIMULUS NUMBER

AMERICAN

INDIAN

I00

o -

80

70

60

50 I

I $ 5 7 9

STIMULUS NUMBER

FIG. 12. Average wo-stepdiscriminationunctions or two groupsof lis-

teners.Data from the Gujarati and Hindi listenerswere combined o form

one setof functions O), and data from the naiveand non-naiveAmerican

groupswerecombined o form the otherset ). The shortvertical ines n

each unction ndicate he 50% crossoveroundaries f the corresponding

identification functions.

A repeated-measuresnalysis f variancewasconduct-

ed on thesedata, with 2 language amilies X 5 vowels X 7

pairsof stimuliand 20 and 15 subjects ithin groups. The

analyses f varianceusing iveand our language roups, .e.,

with and without the Bengalis, aveessentiallyhe same e-

sultsas he analysisn termsof two language amilies. nclu-

sion of the Bengalis ended o increase he significance f

terms,but did not change erms rom significanto nonsigni-

ficant, or vice versa.The only major difference etween he

analyses n terms of separate anguagegroups compared

with two languageamilieswas hat the pair X group nter-

actionwasnot significantn the former,whereas he (equiva-

lent} pair X language amily interactionwas significant n

the latter.]

In the analysis sing wo languageamilies, herewere

significantmain effects f language amily [F(1,33} = 4.05,

p


13/16

particularvowel and upon whether heir native anguage

includes he non-nasal-nasal ppositionor vowels.

v. DISCUSSION

A. An acoustic correlate of the feature [ + nasal]?

Our experimentsonfirm hat reliablepercepts f nasal

vowels anresultwhena pole-zero air is added o the spec-

trum of an oral vowel in the vicinity of the first formant.

Informal comments y speakers f Gujarati and by Ameri-

canresearchersn speechndicated hat the majorityof our

vowelssounded ery natural. The nasalvowelswhosenatu-

ralness as east atisfactoryen,ledo be hehighvowels]

and fil. It ispossiblehat hequalityof thesynthesizedasal

vowels ouldbe mproved urtherby otherchanges, uchas

shiftsn the requenciesr amplitudes f thehigher ormants

or the introductionof additionalpole-zeropairs at higher

frequencies r in the vicinity of F 1.

The identification functions for the different non-na-

sal-nasalcontinuashowa substantial egreeof agreement

for the groupsof listenerswith different anguage ack-

grounds. he agreementsparticularly otablen viewof the

fact that some istenergroups particularly he Americans}

are not normallyconsideredo use he nasal eaturedistinc-

tively for vowels n their language,whereas thersdo have

phonemic asalvowels.One is led to concludehat, what-

everacoustic ropertyor properties ive ise o the identifi-

cation esponses,ll listeners se oughly thesameproper-

ties rrespective f whether heir native anguagencludes

phonemicnon-nasal-nasal owel distinction cf. Wright,

1980).

The questionof whether there is a commonacoustic

property, ndependent f the vowel, hat gives ise o identifi-

cation of the feature [ q- nasal] s more difficult to answer.

The fact that listenerswith different languageexperience

show similar crossoverpoints in their identification unc-

tions or eachvowelcould ead one o conclude hat they are

respondingo the sameacoustic roperty or eachvowel.A

strongerhypothesis ould be that there s a commonacous-

tic property for all nasal vowels,and hencea commonat-

tribute in the pattern of auditory response.f a different

propertywere o signal he presence f the feature q- nasal]

for eachvowel, t is unlikely that listenerswould respond n

essentiallyhe sameway to the differentvowels ndependent

of whether their languagecontained the non-nasal-nasal

distinction.

Can we identify sucha commonproperty or all nasal

vowels?Or, equivalently, an we postulate n auditorypro-

cessingechanismhtathowspatternf esponseith

commonattribute or any vowel hat is identifiedas [ q- na-

sal]?Oneway of describinghe vowelsn thepresent tudy s

in termsof the spacing etweenhe additionalpoleand zero.

In Fig. 9 we have shown hat the pole-zerospacingcorre-

spondingo the 50% crossover ointsdoesnot showa great

deal of variation rom onevowel o another 75 to 110 Hz).

Associatedwith a givenpole-zerospacing here s a particu-

lar maximum deviation in dB) of the spectrum rom the

shape or the non-nasalvowel. It is to be expected hat a

givenmaximumdeviation elative o the non-nasal pectrum

wouldbe perceptuallymore salient f it occurredn a fre-

quency egionneara spectral eak. n fact, thereare percep-

tual data hat support hisexpectationKlatt, 1982}. igures

6 and 7 show hat at the crossover oint the pole-zero air is

in a frequencyegion hat ssomewhatemovedroma spec-

tral peak or the vowel i], and, o some xtent, or [a], but s

closer o the F 1 spectralpeak for the remainingvowels.

Consequently ne might expect hat the pole-zero pacing

required o elicit a [ q- nasal] esponse ouldbe greater or

[i] (andpossiblyor [a]} han or theothervowels. his rend

can be seen n the data in Fig. 9, both for the stimuliat the

crossovernd, morestrongly, or the endpoint timuli.Thus

at leastonemeasure hat seems o relateclosely o the listen-

ers' responsess the maximum deviationof the spectrum

from the originalnon-nasal pectrumn theF 1 region,with

someweighting pplieddepending n the requencyocation

of thisdeviation n relation o the originalspectral eaks nd

valleys.

Although a measure f this type may describehe data

well, we do not consider t particularlysatisfactory s a hy-

pothesis bout the perceptionof nasalvowels.A major ob-

jection s that it requires he listener o compare he heard

spectrum f a potentialnasalvowelwith the memoryof the

spectrumof a non-nasalvowel. Even though nasal vowels

are linguisticallymarked, t seems nlikely hat the listener

uses ucha cumbersome rocedure o identifynasality.We

therefore search for alternatives.

Anotherway of describinghe stimuli s n termsof the

degree of low-frequency prominence in the spectrum.

(Maeda, 1982b,hasattempted o definesucha measure.) s

we introduce he pole-zeropair with an increased pacing,

we are reducing he degreeof prominence f the F 1 peak n

the spectrum, o hat a singlenarrowspectral eakno onger

dominates he low-frequency ange. This reducedpromi-

nence s achievedby creatingan additional spectralpeak

nearF 1 or by splittingor broadening he F 1 peak.

We do not know how the patternof auditory esponse

changes s the degree f prominence f a spectral eak s

manipulated.t is ikely hat the physiologicalrocessesn-

volved n makinga phoneticdistinction ccuroverseveral

stagesn the auditorypathway.Somethings known,how-

ever,aboutresponsesf the auditorynerve o vowel-like

stimuli.We mightaskwhether he patternof auditory-nerve

responsesouldbegin o reflecta groupingnto two classes

correspondingo nasal ndnon-nasal owels. uch tenden-

cy wouldbe provocative, ut clearlycouldnot provide he

entirebasis nwhich hephonetic istinctionsmade. n this

spirit, we consider possible attern of auditory-nervee-

sponsesor the feature q- nasal].Specifically,t maybe ap-

propriateo interpret heconcept f prominence ith refer-

ence o the synchrony f firingsof primary auditoryneurons

rather han to attributes f the spectruman approach on-

sistentwith some heories f pitch perceptioncf. Srulovicz

and Goldstein, 1983).

It is known that a stimuluswith a prominent ow-fre-

quency pectral eak eads o responsesf auditoryneurons

that tend to be synchronouso the frequencyof this peak

(Sachs nd Young, 1980;Delgutteand Kiang, 1984}.This

synchrony xtends verauditoryneurons hat covera range

1572 J. Acoust.Soc. Am., Vol. 77, No. 4, April1985 S. Hawkinsand K. N. Stevens:Correlatesof vowelnasality 1572



14/16

of characteristicrequenciesn the vicinity of the stimulus

frequencyf the spectralprominences sufficiently arrow

and well separatedrom otherprominences.f this ow-fre-

quency pectral eak s broadened, r if an additional pec-

tral peak s introducednearby, here will be a reduction n

the rangeof characteristicrequencies verwhich he audi-

tory neurons ire synchronously ith the F 1 prominence.

Introductionof a pole-zeropair with a gradual ncreasen

spacingwill monotonicallyncreasehe amplitudeof the ad-

ditional spectralpeak. t is to be expected, hen, that at a

particularspacing he additionalpeak will be of sufficient

amplitude hat it will extinguishhe synchrony f firings o

F 1 or auditoryneuronsn its requencyange.That is, here

will be an abrupt reduction n the rangeof neurons hat re-

spondsynchronouslyo F 1.

As we haveseen, he pole-zero pacing ecessaryo ac-

complishhechangen identificationesponsehat may ol-

low a changen auditory esponses n therange75-110 Hz.

This criticalspacings somewhat reater or the vowel i]

than for the other vowelspresumably ecause,or a given

pole-zero pacing,heFNP peakdue o theadditional ole s

weaker for this vowel than for the others, as discussed bove.

Consequentlygreater ole-zero pacings neededor [i] if

the FNP peak s to eliminate he synchronyo F 1.

An alternativeway of achieving reductionof the F 1

prominenceimilar o that produced y theadditional ole-

zero pair would be to broaden he bandwidthof the first

formant,without addinga pole-zeropair. There are indeed

data to indicate hat broadening 1 leads o the perception

of nasality Delattre,1968;Hawkinsand Stevens, 983}. n

naturalspeech,he bandwidthof F 1 is almostcertainly n-

creased y the additionalacousticossesntroduced y cou-

pling o the nasalcavity; his wouldcontribute dditional

reduction f prominence ver hat resulting rom the pole-

zero pair.

In summary, hen, heremay be somebasis or suppos-

ing that the patternof responset the levelof the auditory

nerve s such hat a rathersimpledetection rocedure ould

be used o identifya vowelas a memberof the class + na-

sal].Further studies f auditory esponseso these ypesof

sounds reclearlyneeded, owever, o support hisassertion

and to provide a more quantitativespecification f the

acoustic correlate of this feature.

B. Language- and vowel-dependent effects

While there s some videncehat listenersespondn a

distinctiveway to a simpleacousticpropertyof nasality n

the dentificationests, hereare aspects f the identification

and discrimination atasuggestinghat other actorsare n-

fiuencing he responses.We turn now to an examinationof

these factors.

A consistent ifference etween anguagegroupswas

that both Americangroups howed patternof discrimina-

tion that differed rom that of the Indian groups. hat is, the

Americans discriminated better between stimuli at the non-

nasalendof the continua or [o u] {andpossiblyor [i]} than

at the nasalend. There were also differencesn the way lis-

teners espondedo particularvowelcontinua.For example,

thereappeared o be somedifferences etweenvowels n the

extent o which hey weresubjecto rangeeffects:he place-

ment of the identification oundarywasparticularlydepen-

denton herangeofnasality or the vowel o-], and, articu-

larly insensitiveo the rangeof nasality or [u-fi]. Also, the

effectof immediate ontextwasgreatestor the front vowels

and was small or nonexistent for the back vowels.

In order o interpret hese indings,we proposehat the

basicnasalityproperty s accompanied y oneor moreaddi-

tional acousticproperties,perhapsdifferent for different

vowels, hat changewithin the variousstimuluscontinua,

and can influence he reponses f the listeners.Among the

most mportant of theseadditionalproperties,we suggest,

arechangesn vowelquality,caused y shifts n the centerof

gravityof low-frequency pectral rominences,nd changes

in overallspectral alance aused y the presence f the pole-

zeropair (asdiscussedn connectionwith Fig. 7).

We consider irst a possible xplanation or the influ-

enceof the preceding timuluson the identification f nasal-

ity for the front vowels Fig. 10). The principal acousticat-

tribute distinguishingront vowels rom back vowels s the

relatively arge amplitudeof the spectrum t high frequen-

cies, n the rangeoff 2 andF 3. Thus or the front vowels,we

might expect hat the changesn high-frequency pectral

amplitude hat occur hrough he continuaare perceptually

more salient han they are for the back vowels.This change

in spectral alance ould ead o the perception f a change

in voicequality that listenersmight associate ith a change

in nasality,particularly or ambiguous timuli.Perhaps is-

teners'assessmentf theseaspects f voicequality is more

prone to contextualeffects han is their assessmentf the

primary propertyof nasality.

Another exampleof the influenceof changesn vowel

quality on the perception f nasality s in the rangeeffect,

whichwas argestor thecontinuumo-6] for bothgroups f

subjects. everalphoneticiansudged that there was a dis-

tinct change n quality of the vowel at or near stimulus4,

where [o] became a] (a finding hat is consistentwith the

description f this stimuluscontinuum n connectionwith

Fig. 7). Apparently, n the absence f any unambiguously

nasal owelsn the runcated ontinuum,ubjectsistening

to stimuli n this setadjusted heir criterion o one of vowel

quality. That is, the listeners eemedo be willing to make

judgmentsof vowelnasalitybased argelyupondifferences

in perceived owelheight. n the nine-stimulus ontinuum,

which ncludedunambiguously asalstimuli, hey presuma-

blyheardheshift n vowel eight, utplaced oreweight

on the percept ssociated ith the widerpole-zero-pole epa-

ration aroundstimuli 5 and 6. Further support or the idea

that vowel height can influence udgmentsof nasality is

found n the data of Lonchamp 1978) or French.

The performance f listenersn the discrimination x-

periment s alsoconsistent ith the hypothesishat listeners'

responsesred.eterminedyadditionalecondaryroperties

as well asby a primarypropertyof nasality. n general, he

Indian and American listenersgive similar discrimination

functions or the vowels e] and [o]. Both groupsshowre-

duced discrimination at the two ends of the continuum, with

a rathernarrowpeak or [o] anda broaderplateauor peak n

the middle range for [e]. For thesemidheightvowels, he

1573 J. Acoust. Soc. Am., Vol. 77, No. 4, April 1985

S. Hawkins and K. N. Stevens: Correlates of vowel nasality

,1573



15/16

introductionof the pole-zeropair results n a splittingof the

F 1 prominence.When the two componentpeaksare close

together or the first three or four stimuli n the continuum,

discriminationbetweenstimuli is relativelypoor. Discrimi-

nation approaches maximum when the spacingbetween

thesepeaks s about 100Hz for [e] (stimulus -4}and about

80 Hz for [o] (stimulus -5}. Thesestimulus angesdo not,

however,correspondo the boundaries f the untruncated

non-nasal-nasaldentification unctions.Apparently he lis-

tenersaredistinguishing erea changen vowelqualityrath-

er than a change n nasality.We note, however, hat the

identification oundariesor the truncated o-] continuum

are in the rangeof the discrimination eak (nearstimulus ,

from Fig. 12}. This correspondencerovidessomesupport

for our hypothesishat, with the truncated o-] continuum,

the listenersare basing heir identificationudgmentsmore

on vowel quality than on a nasalityproperty.

For the high vowels i u] and the low vowel [a], the

introductionof the pole-zeropair is at a frequency hat is

well separatedrom F 1: aboveF 1 for [i] and [u] and below

F 1 for [a]. The discrimination unctions or thesevowels

havedifferentshapesor the American and Indian listeners.

Both groupsshowa decreasen accuracyof discrimination

as the nasalend of the continuum s approached although

this trend doesnot apply to [i-] or the Americans}. his

decreasen discriminabilitymay bea consequencef the fact

that, for a two-stepchange, he change n the maximumam-

plitudedeviation n the spectrum, elative o that for a non-

nasalvowel,becomes malleras he pole-zeroseparation e-

comes arger. The Indian listeners, ut not the Americans,

alsoshowa decreasen accuracyof discrimination or stimu-

li at the non-nasal end of the continuum for these three vow-

els,with a peak n the middle ange.One cansurmise hat the

experience f the Indian listenerswith the non-nasal-nasal

distinction orients them to focus on the primary nasality

property.This conclusions supported y the fact that their

discrimination eaks end to be close o their identification

boundaries for these vowels. The American listeners,on the

other hand, tend not to show reduced discrimination for the

stimuli at the non-nasal end of the continuum for these three

vowels.The Americans,not possessingready way of cate-

gorizing hese owelsn their anguage,maybemoreanalyti-

cal in their judgmentsof the stimuli, and may focuson de-

tailed attributes such as a change n amplitude at high

frequencies, r shifts n the frequencies r amplitudes f par-

ticularspectral eaks. his strategy ould esult n improved

discrimination at the lower end of the continuum.

C. Implications and further questions

The hypothesishat there sa primarynasalityproperty

and accompanying econdary roperties aisessomeaddi-

tionalquestions. or example, he shift n vowelheightwith

nasalization, bservedor some owelsn thisexperiment,s

a secondary roperty hat is found n some anguages. his

change n vowel height occurs n only a minority of lan-

guages, ut when t doesoccur he tendencys for highvow-

els o lowerand ow vowels o raise Beddor, 983}.Shifts n

height or the midheight owels re ess onsistent, ut there

is a tendency or these owels o lowerwhennasalized.Why

is t that some anguageseem o introduce dditionalprop-

erties,suchas changesn vowel quality, to complement r

enhance he basicpropertyof nasality?We have noted n

Fig. 5 that the spectralmodifications ssociatedwith the

additionof a pole-zeropair n the vicinityof the first ormant

tend o produce low-frequency ole-zero-pole ombination

that varies ery ittle fromonevowel o another. he percep-

tual result s a lossof discriminabilitybetweennasalvowels

along he high-low dimension, situation avoring he de-

velopmentof additionalenhancing roperties. his lossof

discriminability or nasalvowelshas been verifiedexperi-

mentallyby Wright (1980}.

Languages ary in how they dealwith this reduction n

discriminability.Some anguages ave the samenumberof

nasalas non-nasal owels,with no reporteddifferencesn

quality between he two sets. n a substantialminority of

languageshat contrastnasaland non-nasal owels, here s

a reducednumber of nasalvowels Ferguson, 963, 1975;

Ruhlen, 1975}.Most commonly t is the midvowelshat are

missing n these mbalancedsystems Wright, 1980). The

problem of reduced discriminability s thereby avoided n

that only thosevowelswith the mostdistinctive aluesoff I

are retained.

A finalquestionsconcerned ith the physiologicalnd

psychophysicalasis or the perception f the primarynasal

property tself.On the basisof physiological ata, we have

speculatedhat there s a distinctive hangen the auditory

responseesulting rom modification f the spectralpromi-

nence n the vicinityof the first ormant.Furtherphysiologi-

caldataareneeded, owever, or stimuliwith spectral rom-

inences f the type found n nasalvowels.Of relevancen the

psychophysical omain is the work of Chistovichet al.,

(1979}. n a task n whichone-formant owelswerematched

against two-formant vowels with various spacings, hey

found center-of-gravityffect henhe ormantpacing

was3-3.5 Bark or less.Beddorand Hawkins 1984} ounda

similar effect or the pole-zero-pole ombinations f nasal

vowels,although the center of gravity of the prominence

couldnot be simplydescribedn termsof spectralmeasures.

PsYchophysicaltudiesre learlyeedednorderounder-

stand further the perceptionof soundscharacterizedby

spectralpeakswith variousdegrees nd shapesof promi-

nence.

ACKNOWLEDGMENTS

We would ike to thank our subjectsor their coopera-

tion, and Patrice Beddor, Gunnar Fant, Len Katz, Christo-

pher Darwin, Rena Krakow, and SuzanneBoyce or helpful

discussions.his work wassupportedn part by NIH grants

NS-04332 (to the Massachusettsnstitute of Technology}

and NS-07237and RR-05596 (to HaskinsLaboratories}, nd

by grant MCS-87-12899 rom the National Science ounda-

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acoustic and perceptual correlates of the non-nasal-nasal

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