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
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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 '"-.-
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-,.-z::. .......,,- ;:;:.: .:...
: ....::'] :::: ?,---- ' [':[-.:'
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0.2 s
6o. ::t.................'....i.tTpi"........,
o I 2 3 4 5 kHz
2
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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
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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,
1566 J. Acoust.Soc. Am., Vol. 77, No. 4, April 1985 S. Hawkinsand K. N. Stevens:Correlatesof vowel nasality 1566
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"::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
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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
-
7/25/2019 Acoustic and Perceptual Correlates of the Non-nasal-nasal
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
-
7/25/2019 Acoustic and Perceptual Correlates of the Non-nasal-nasal
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
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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
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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
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7/25/2019 Acoustic and Perceptual Correlates of the Non-nasal-nasal
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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
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7/25/2019 Acoustic and Perceptual Correlates of the Non-nasal-nasal
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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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