paper - Institute for Systems Research
Acoustic measures for linguistic features distinguishing the semivowels/wj r 1/in American English Carol Y. Espy-Wilson Electrical, Computer andSystems Engineering Department, Boston University, 44Cummington Street, Boston, Massachusetts 02215andResearch Laboratory ofElectronics, Room36-545,MIT, Cambridge, Massachusetts 02139
(Received3 December1990;accepted for publication 23 April 1992)
Acousticproperties relatedto thelinguistic features whichcharacterize thesemivowels in AmericanEnglishwerequantified andanalyzedstatistically. Thefeatures canbedividedinto thosewhichseparate thesemivowels fromothersounds andthosewhichdistinguish amongthe semivowels. The featuresof interestaresonorant, syllabic,consonantal, high,back,front, and retroflex. Acousticcorrelates of thesefeatures wereinvestigated in thisstudyof the semivowels.The acousticcorrelates,which are basedon relative measures,were testedon a
corpusof 233polysyllabic words,eachof whichwasspoken onceby twomalesandtwo females.For themostpart,theappropriate distinctions aremadeby thechosenacoustic properties forfeatures. However,foreachproperty, therewassomeoverlapin theacoustic correlates of featuresfor thesounds beingdistinguished. An examination of thesounds in the overlapregions revealsthattheirsurface manifestation variessubstantially fromthecanonical form.In largepart,theobserved variabilitycanbeexplainedin termsof changes dueto feature spreadingandlenition.
PACS numbers:43.70.Fq,43.72.Ar,43.70.Hs
variabilityof someacousticproperties assumed to be associatedwith thesemivowels. For example,notall prevocalic An acousticstudyof the sounds/wj r l/was conducted andintervocalic/1/'sareassociated with spectraldiscontinas part of the development of a semivowelrecognitionsysuities.Finally,theacoustic properties of someof theother tem (Espy-Wilson,1987}. Recognitionof the semivowelsis sounds also change with context. In particular, someundera challengingtask since,of the consonants,the semivowels lyingly voiced obstruents surface as SOhorant consonants aremostlike the vowelsand,dueto phonotacticconstraints, and, in somecases,they resembleone or more of the theyalmostalwaysoccuradjacentto a vowel.Thus,acoustic semivowels.Some of these variations were also examined. changes betweensemivowels andvowelsareoftenquitesubobserved in theacoustic tle sothat therearenoclearlandmarksto guidethesampling We will arguethatthevariability manifestation of the semivowels and some of the other of acousticproperties. soundscanbecharacterizedin termsof changesin the phoMany studieshave examinedsomeof the acousticand perceptualproperties of oneor moreof the semivowels in nologicalfeatures. INTRODUCTION
English(Lisker, 1957;O'Connoret al., 1957;Lehiste,1962; Kameny, 1974; Daiston, 1975; Bladon and A1-Bamerni, 1976;Bond, 1976). Thesestudieshaveprimarily focusedon the acousticandperceptualcuesthat distinguishamongthe semivowels and
the
coarticulatory
effects between
semivowels and adjacentvowels.For the mostpart, these studieshavelookedat simplecontextsand a limited setof acousticproperties. While the resultsof pastwork were usedto guidethe presentexaminationof the semivowels, this studydiffers from previousresearchin that the acousticpropertiesinvestigatedwerechosento be closelyrelatedto the abstractlinguisticfeatureswhich comprisea phonologicaldescription of the semivowels.We examineacousticpropertiesfor featuresthat not only distinguishamongthe semivowels,but that alsoseparatethe semivowelsfrom other sounds.The acousticproperties wereanalyzedto quantifyhowthe surfacemanifestationof the semivowels changeswith context. The resultsobtainedsupportpreviousfindings,namelythat formantinformationcan be usedto distinguishamongthe semivowels.In addition,there are new findingsabout the 736
I. REVIEW OF THE ACOUSTIC PROPERTIES OF SEMIVOWELS
Spectrograms of the semivowels are shownin Figs. l and 2 wherethey occurin word-initialpositionbeforethe front vowel/i/and
the back vowel/u/. As can be seen,the
semivowels haveproperties that are similarto bothvowels and consonants.Like the vowels,the semivowelsare pro-
dueedorallywithoutcompleteclosureof thevocaltractand withoutanyfrieationnoise.Asisalsotrueforthevowels,the degreeof constrictionneededto producethe semivowels doesnot inhibitvoicing.Thus,asshownin thesefigures,the semivowels and vowels are both voiced with no evidence of
frication noise.In addition, the slowerrate of changeof the constriction size for the semivowels than other consonants
resultsin slowerspectrumchangesfor thesesoundscom-
paredto otherconsonants. Forexample, thespectrogram of theword"you"in Fig. 1 showsthattheformantsduringthe /j/stay relativelyconstantfor about130ms beforethey movetowardtheappropriate valuesforthefollowingvowel. Thus,asin the caseof vowels,a voicedsteadystateis often
J. Acoust.Soc.Am.92 (2), Pt. 1, August1992 0001-4966/92/080736-22500.80
@ 1992Acoustical Societyof America
736
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FIG. 1. Widebandspectrograms of the words"we" and "ye" (top), and
FIG. 2. Widebandspectrograms of the words"lee" and "re" (top), and
"woo" and "you" (bottom).
"rou" and "1ou" (bottom).
observedin spectrograms of the semivowels. frequencies than/u/, and/j/has a lowerF 1 frequency and Liketheotherconsonants, thesemivowels usuallyoccur usuallya higherF2 or F3 frequencythan/i/. Thesedifferat syllablemargins.That is, they generallydo not haveor encescanbe seenin the words"woo" and "ye" of Fig. 1. constitute apeakof sonority.{Sonority,in thiscase,isequatThe glidesoccurin prevocalicandintervocalicpositions edwithsomemeasure ofacoustic energy.)Asshownin Figs. within a word,suchasthe/j/in "you" and "yo-yo"andthe 1and2, oneor moreof theformantsduringthesemivowels is /w/in "we" and"away."In addition,theyoftenoccurphoconsiderably lowerin amplitudethanit isduringthefollow- netically{eventhoughthey are not phonologically speciing vowels.In the caseof/w/, it is F3 and the higherfor- fied) aspart of the transitionbetweentwo adjacentvowels. mantswhichareweaker.In thecaseof/j/, F 3 andthehigher An exampleof thismanifestation of a glideistheintervocalic formantsarefairly strong,butF2 isnot.For/1/, thereisless /j/sound oftenobserved between/i/and/o/in thepronunenergyin the high-frequency rangestartingaroundF4 for ciationof"radiology" ( [redijologi]vs [rediologi]). The semivowels/1/and/r/are often referred to as li"lee"andF3 for "1ou."Finally,F3 andthehigherformants arelowerin amplitudeduring/r/. The relativelylowampli- quids.Sproatand Fujimura {submitted)foundfrom articutudeofthesemivowels ascompared tothevowels isprobably latory and electromyographicdata obtainedfrom several due to a combinationof factors:a low-frequency first for- speakersthat the productionof all English/l/'s involves mant (Fant, 1960),a largeF 1bandwidthcausedbythe nar- both an apical and a dorsalgesturalcomponent.The key rowerconstriction(BickleyandStevens,1986),or interac- articulatorydistinctionbetweenthe two well established tion betweenthe vocal folds and the constriction(Bicklcy variants of/1/, light or clear/1/(as in "Lee") and dark/1/ andStevens,1986).At present,thisphenomenon isnotwell {as in "feel"), is that in dark/1/the tonguebody is more understood. retractedthan in light/1/, resultingin a much lower F2. The semivowels/w/and/j/are often referredto as Sproatand Fujimuraarguethat thisallophonicvariationis glidesor transitionalsounds.They are producedwith con- not categorical,but is the degreeto which the apicaland stantmotionof thearticulators. Consequently, theformants dorsalgestures arerealizedandthe timingbetweenthe two in the transitiontowardor awayfromadjacentvowelsexhib- gestures.Specifically,theyfoundthat in additionto a signifiit a smoothglidingmovement.The semivowels/w/and/j/ cantly greaterretractionof the tonguedorsumfor dark/1/ are producedwith vocal-tractconfigurations similar to comparedto light/1/, the maximumtonguedotsumposithoseof thevowels/u/and/i/, respectively, butwitha more tion for the dark/1/is achieved well in advance of the maxiextreme constriction.As a result,/w/has lower F 1 and F2 mum tonguetip position.On the otherhand,duringthe ges737
J. Acoust. Sec.Am.,VoL92, No.2, Pt.1, AugustI gg2
CarolY. Espy-Wilson: Acoustic measures forsemivowels
737
turefor thelight/I/, thetonguetip positionisreachedbefore the tonguedorsumpositionis achieved. In the caseof light/l/, the apicalgestureusuallyinvolyesthe placementof the centerof the tonguetip against the alveolarridge.The oftenrapidreleaseof the tonguetip fromtheroofof themouthresultsin a spectraldiscontinuity betweenthe/1/and the followingvowel (Daiston, 1975). JoGs(1948), reportedthat/1/is alwaysmarkedat itsbeginningand/or endby an abruptshiftin the formantpattern. Along this line, Fant (1960) observedthat the identification of an/1/reliesona suddenshiftupoff I fromthe/l/into the followingvowel. Finally, Daiston (1975) found that this abruptshiftin F 1isoftenaccompanied bya transientclickin the acousticspectrum.Someof thesepropertiescanbe ob-
that asthepharyngealconstrictionisnarrowed,theauditory impression of the/r/is enhanced. Regardlessof whether a bunchedor retroflexed/r/is produced,lip roundingmayoccurwhen/r/is eitherprevocalic or intervocalic and before a stressedvowel (Delattre
andFreeman,1968).The acousticconsequence of lip roundingisa loweringof all formants.Thiseffectmayaccountfor the lower F 1, F2, and F3 Lehiste (1962) observedfor initial
/r/ allophonesrelativeto final/r/allophones, for whichlip roundingdoesnot usuallyoccur. In summary,many of the acousticpropertieswhich characterizethesemivowels havebeenexaminedin previous studies.However,thiswork haslargelyfocusedon formant measurements. In this investigationof the semivowels, served at theboundary between the/1/and thefollowing acousticpropertiescalculatedfrom formantmeasurements vowelsin Fig. 2. andenergy-based parametersarequantifiedandanalyzedso In the caseof dark/1/, Sproatand Fujimura (submit- that we canstudyto a greaterextentsomeof the variability that occurs in the surface forms of the semivowels. Furtherted) reportthat apicalcontactis lessrobusteventhoughit was madeduring all of the/1/productions in their study. more, the acousticpropertiesare related to the linguistic GilesandMoll (1975) foundin anx-raystudyof English/1/ featureswhich providea frameworkfor understanding the that apicalcontactfor dark/l/'s wasnot alwaysachievedfor changesthat occur. all speakersand is dependentupon phoneticcontextand speakingrate. In addition,theyfoundthat the meanpeak II. MœTHOD velocityof thetongueapexmovementissignificantly slower A. Stimuli for dark/1/. Furthermore,they foundthat dark/1/shows A data base of 233 polysyllabicwords containing undershoot ofarticulatorypositions with increases in speaksemivowels in a varietyof phoneticenvironments wasselectingrate.Thisslowerandincomplete apicalgesturemayhelp ed from the 20 000-word Merriam-Webster Pocket dictioexplainwhy dark/1/productionsarenot associated with an nary.The semivowels occuradjacentto voicedandunvoiced abrupt spectralchange. consonants,as well as in word-initial, word-final, and interAlthough the distributionof dark and light/1/varies vocalicpositions.(Note that only/1/and/r/occur postvoacrossspeakers,canonicalsyllable-final/1/is dark and, in calieally.) The semivowels occuradjacentto vowelswhich manydialects,syllable-initial/1/is light. Sproatand Fujiare stressed and unstressed, high and low, and front and mura(submitted)foundin theirstudyof preboundaryl inback. In developing the database, wordswere chosenthat tervocalic/1/inthefallingstress context/i_ [/that thequalcontained several semivowels so that theysatisfymorethan ity of the/1/dependsuponthephoneticdurationof therime one category. The distribution of the semivowels in termsof whichcontainsit. (They alsoshowa correlationbetweenthe word position and stress is given in Table I. Examples of durationof the preboundaryrime and the strengthof the words in these categories are given in Table II. While most of phonologieal boundary.)Specifically theyfoundthat asthe the contextscontain severalwords, there are a few for which rimebecomes longer,thetonguebodyfor/1/becomeslower and more retracted.Therefore,intervocalic/1/occurring beforea major intonationboundaryis dark. On the other hand,they foundthat the preboundary/1/preceding the TABLE I. Distribution of semivowels in the test words. The number in the weakest boundariesare as light as initial /1/. These data parentheses specifiesthe numberof semivowelsoccurringnext to a vowel supportpreviousfindingsby Lehiste (1962) and Blandon with primarystress. and AI-Bamerni{ 1976) whichshowthat certainprebounCategory w j r i daryintervocalic/1/productions arelighterin qualitythan preboundary/1/in prepausalposition. Prevocalic 65 41 63 52 word-initial(prestressed) 11(9) 8(5) 10(5) 6(3) American/r/may beproducedwith eithera retroflexed or bunched articulation (Delattre and Freeman, 1968). If
the upperconstrictionis at the palate,it is madewith either
the tonguetip or the tongueblade.If, instead,the upper constrictionis further back near the velum, it is made with
thetonguebody.It isthepalatalor palato-velarconstriction which lowersF3 (Delattre and Freeman, 1968;Stevens,in preparation), whereasthe pharyngealconstrictionlowers F 2 and raisesF 1 (Delattre and Freeman, 1968). In termsof perception,Delattre and Freemanfoundwith the useof an electricmouth analogthat the palatal constrictionis primary in termsof producingthe/r/. However,they found 738
J. Acoust.Soc. Am., Vol. 92, No. 2, Pt. 1, August1992
stopcluster(prestressed)
18(8)
fricativecluster (prestressed) 19(13) stopfricativecluster(prestressed) 10(7) adjacentto SOhorantconsonant 7 Intervocalic
ll(4)
18(10) 10(4)
7(3) 8(3) 7
18(9) 10(7) 7
18(6) 10(5) 9
9
6
29
32
poststressed prestressed
2 5
I 4
13 10
16 8
unstressed
2
I
Postvocalic
word-final(poststressed)
6
8
25
26
13(10) 19(14)
obstruent cluster
6
4
adjacentto sonGrantconsonant
6
3
Carol Y. Espy-Wilson:Acousticmeasuresfor semivowels
738
TABLE II. Examplesof testwords.
Category
w
j
r
I
Prevocalic
word-initial:prestressed
walnut wa!loon aquarius quadruplet
yell euroiogist pule bucolic
requiem rhinoceros brilliant fibroid
swollen
view
frivolous
flourish
Swahili disqualify misquotation carwash
behavior spurious promiscuously brilliant
anthrax astrology widespread walrus
grizzly exclaim exploitation harlequin
poststressed prestressed
forward
Ghanaian
caloric
astrology
unaware
reunion
fluorescence
unilateral
unstressed
unctuous
diuretic
correlation
fraudulent
clear
dwell
other
stopcluster:prestressed other
fricativecluster:prestressed other
stopfricativecluster:prestressed other
adjacentto Sohorantconsonant
leapfrog linguistics bless chlorination
Intervocalic
Postvocalic
work-final:poststressed other
memoir
whippoorwill
obstruent cluster
cartwheel
oneself
adjacentto sorterantconsonant
forewarn
walnut
only a small number of words were availablein the dierio-
nary.For example,only the word "Ghanaian"had a poststressedintervocalie/j/. B. Speakers and recordings
For recording,the wordswereembeddedin the carrier
phrase" pa."The final"pa" wasaddedin orderto avoidglottalizationand other typesof utterance-finalvariability.Eachword wasspokenonceby two malesand two females. Giventhat thereareseveralwordsin mostcategories (see Table I), one repetitionof each word by each speakerprovidesat least 24 to 260 productionsof each semivowel in eachmajorcategory,e.g.,prevocalic/w/. In fact,asexplainedin Sec.II C, somecategories maycontain more instances of a semivowel than what is indicated in Ta-
bleI sincespeakers ofteninsertsemivowels between adjacent vowels.
The speakerswerestudentsandemployees at the Massachusetts Instituteof Technology.The femalespeakers werefromthenortheast andthemalespeakers werefromthe midwest.All werenativespeakers of Englishand reported havingnormalhearing.Thespeakers wererecorded in a quiet roomwith a pressure-gradient close-talking noisecanceling microphone(part of SennheiserHMD 224X microphone/headphone combination).They were instructedto saythe utterances at a naturalpace.
and measurementprocedureswill be indicatedin Sec.III. Segmentation and labelingof the waveformswas performedby theauthorwith the helpof playbackanddisplays of severalattributesincludingLPC and wide-bandspectra, thespeechsignalandvariousbandlimitedenergywaveforms (Cyphers,1985;Shipman,1982;Zue etal., 1986).The Merriam-WebsterPocketdictionaryprovideda baselinephonemic transcriptionof the words.However,modifications of someof thelabelsweremadebasedon thespeakers' pronunciations.In addition,whentranscribingthedatabase,we did not normallyconsiderthe/w/and/j/offglides of diphthongsas beingseparatefrom the vowel.In someinstances, however,the offglideof a diphthongwhichwasfollowedby anothervowel was articulatedwith a narrow enoughconstriction that a semivowel label was inserted. On the other
hand,someunderlyingpostvocalic liquids,particularly/1/, in words like "almost" were not alwaysclearly heard. In theseinstances,the liquid wasoften omittedfrom the transcription. D. Feature analysis
Distinctivefeaturetheorywasusedto providea guideto understanding what acousticpropertieswe shouldlook for to characterize the semivowels.In addition, distinctivefea-
ture theory providesa basisfor understandinghow the acousticpropertiesof the semivowels maychangeasa function of context.A featurespecificationof the semivowelsis C. Initial processing givenin TablesIII and IV. Table III containsfeaturesthat The utterances weredigitizedusinga 6.4-kHz low-pass separatethe semivowelsas a classfrom other soundsand filteranda 16-kHzsamplingrate.The speechsignalswere Table IV contains features that distinguishamong the alsopre-emphasized to compensate for the relativelyweak semivowels. spectralenergyat high frequencies (a particularissuefor The featureslistedare modifications of onesproposed *3norants). Finally, the test words were excisedand hand by Jakobsonet al. (1952) and later by Chomsky and Halle transcribed.This processresulted in 2378 vowels, 1689 (1968). For example,in Table IV, we list boththe features semivowels, 479 nasals,and 1894obstruents(stops,frica- backandfront. For practicalreasons,we choseto useboth tives, and affricates).Specificcharacteristics of measures featuresand classify/r/and prevocalic/1/as -back and 739
J. Acoust.Sec.Am.,Vol.92, No.2, Pt. 1, August1992
CarolY. Espy-Wilson: Acousticmeasuresforsemivowels
739
TABLE
III. Features which characterize various classes of consonants. A
"+ "indicatesthatthedesignated featureispresentin therepresentation of
TABLE IV. Featureswhichdiscriminateamongthe semivowels. A" +" indicatesthat the designatedfeatureis presentin the represention of the
the sound, and a" --" indicates the absenceof the feature.
sound,and a" --" indicatesthe absenceof the feature.
Sohorant
Syllabic
Nasal
Consonantal High
Back Front
Fricatives,stops,affricates
--
--
--
/w/
-
+
+
--
Semivowels
+
--
--
/j/
--
+
--
+
Nasals
+
-
+
/r/
....
Vowels
+
+
-
prevocalic/1/ postvocalic/1/
+ --
Retroflex --
-+
.... --
+
--
--
-front.2TheirF 2 values clearlyliebetween thoseoftheback androundedsemivowel/w/and the front semivowel/j/. We alsofoundit necessary to distinguish betweeninitial and final/1/allophones on the basisof the featuresconsonantal and back. As statedearlier, severalresearchershave observed a sharpspectraldiscontinuity betweena prevocalic /1/and a followingvoweldue to the rapid releaseof the tonguetip fromthealveolarridge,aswewouldexpectwith a changein the featureconsonantal. On the otherhand,in the productionof postvocalic/1/, alveolarcontactis often not realizedor is realizedonly gradually,so that the spectral changebetweenit anda preceding vowelis usuallygradual. In addition, a final/1/is more velarized than an initial/I/. Thus, F2 is much lower and closein value to that of the back and rounded/w/.
Finally, the featureretroflexis usedto distinguish/r/ fromall othersounds. Althoughtheterm"retroflex"isused, this featurerelatesto the acousticconsequence of either a bunchedor retroflexedtongueshape. TableV showstheacoustic properties for thefeaturesin Table III and IV (with the exceptionof the featurenasal) andthe parametersusedfor their extraction.To makethem insensitive to variationsin speaker, speaking rate,andspeakinglevel,all of the propertiesarebasedon relativemeasures insteadof absolutethresholds.That is, a measureeither ex-
semivowels and separatethem from othersounds.The results also indicate how the characteristics of the semivowels
andothersoundschangeasa functionof context. III. RESULTS A. Sonorant
trum.
In the followingsections, we will examinehow well the properties listed in Table V distinguish among the
measure
Like the vowels, the semivowelsare sonorant sounds.
That is, the main sourceof excitationis at the glottis,sothat all of the naturalfrequencies of the vocaltract are excited. Thus, unlike the obstruents,wherethe main sourceof excitation is furtherforwardin the vocaltract, thereis significant energyat low frequencies. The only other consonants that sharethesepropertiesare the nasals. The parameterusedto extractthe acousticcorrelateof the sonorant feature is the bandlimitedenergycomputed from 100to 400 Hz. More specifically,the valueof the parameter in each frame is the difference (in dB) betweenthe
maximumenergywithin the word and the energyin each frame.An exampleof this parameteris shownin the lower partof Fig. 3 for theword"chlorination."The energydifferenceis smallin the sonorantregions(vowels,semivowels, and nasals), and is large in the obstruentregions (stops, fricatives, and affricates).
aminesan attributein onespeech frame3 in relationto anotherframe,or, within a givenframe,examinesonepart of the spectrumin relationto anothernearbypart of the spec-
AND DISCUSSION
Figure4 showshowall of thesoundsdifferin sonority, as determined with this measure.For each sound, the mini-
mum energydifferenceoccurringwithin the hand-transcribedregionisused.Thereisconsiderable overlapbetween the distributionsof the vowels,semivowels,and nasals.If we set a threshold of - 20 dB to divide SOhorant and nonsonor-
TABLE V. Mappingof featuresinto acousticproperties.B0, B 1, B2, B3, andB4 are the bark transformations ofF0, FI, F2, F3, andF4, respectively. Feature
Acousticcorrelate
Parameter
Sonorant
No significantdeereacein energy
Nonsyllabic
at low frequencies Dip in midfrequency energy
Consonantal High
Abrupt amplitudechange Low F 1 frequency
Back
LowF2 frequency
B 2-B 1
low
Front
High F2 frequency
B 3-B 2
low
B 4--B 3
low
Retroflex
Low F3 frequency&
B 4-B 3
high
Close F2 and F3
B 3-B 2
low
Energy 100-400Hz Energy640-2800 Hz Energy2000-3000 Hz First differenceof adjacentspectra B I-B 0
Property
higha lowa low' high low
• Relative to a maximum value within the utterance.
740
J. Acoust. Sec. Am., Vol. 02, No. 2, Pt. 1, August 1992
Carol Y. Espy-Wilson:Acoustic measures for semivowels
740
"chlorination"
"everyday"
?
6 õ
kHz4
kHz
o.o
az
03
0.4 o.5
Time(seconds)
I
I
I FIG. 5. A spectrogramof the word "everyday"whichcontainsthe two obstruents /v/ and/d/ thathaveundergone lenition.
sonorontregions (b) FIG. 3. An illustration of theparameter usedtocapturethefeaturesorterant. (a) Widebandspectrogram of lhe word"chlorination."(b} The differ-
quencyenergytheyexhibitis presumably causedby vibrationsof the vocalcordswhichare transmittedthroughthe
encebetween themaximum valueofthelow-frequency energy(computed tissues around the neck. from 100 to 4•0 Hz) in the word and the value in each frame. B. Consonantal
measure
Consonantalsoundsare producedwith a narrow con-
ant sounds, thenonlyabout12.5%of thetypicallynonson- strictionat somepointalongthe midlineof the vocaltract. orantconsonants overlapwith the sohorantsounds.Of these consonants,72% are producedwith a weakenedconstriction (this processreferredto as lenitionis discussed in Catford, 1977) so that they are realizedas sonorants.Two ex-
amples areshown in Fig.5,whichcontains a spectrogram of theword"everyday."Boththe/v/and/d/surface assonorant consonants.
Theremainingsegments whichoverlapwiththesonorantsare the closedportionsof voicedstops.The low-fie-
Due to the narrow constriction, the releaseof the consonan-
tal soundinto the followingvowelinvolvesrapidmovement of some of the formants. The result of this formant move-
ment is an abruptchangein the spectrumoverat leastsome part of the frequencyrange(Stevensand Keyset, 1989).
The parameterusedto capturethe rate of spectral changebetweenconsonants and vowelsis basedon the outputsera bankof 40 linearcriticalbandfiltersto whichsome nonlinearities(designedto model the hair-cell/synapse transduction process in theinnerear) areappliedto enhance onsetsand offsets(Seneft,1986). An exampleis shownin part (b) of Fig. 6 for the word "correlation."The wave-
formsthat are spacedabouta half bark apart showsharp onsetsandoffsetsbetween/l/and thesurroundingvowelsin the frequencyregionbetween800 and 1200Hz and between 1800 and 2400 Hz.
Based on the first differences in time of waveforms like
the onesshownin part (b) of Fig. 6, we computedglobal onset and offset waveforms
vowels semivowels na•abobslruenls
for each consonant. The onset
waveformis computedby summing,in eachframe,all the negativefirst differencesin time. Similarly,the offsetwaveform is obtainedby summing,in eachframe,all the positive firstdifferences in time of the channeloutputs.The resulting onset and offset waveforms for the word "correlation"
Class
of
sound
FIG. 4. Averages andstandard deviations ofthechange(in dB} in thelowfrequency energycomputedfrom 100to 400 Hz withinSOhorant andnonsonorantsoundswith respectto themaximumenergywithintheword. 741
J. Acoust.Sec. Am.,Vol.92, No. 2, Pt. 1. August1992
are
shownin parts (c) and (d) of Fig. 6 wherethe sharpamplitude changesbetweenthe/1/and the surroundingvowels showup asa valleyanda peak,respectively. Note that since a 25-ms time window is used,there is a limit to the maximum CarolY. Espy-Wilson: Acousticmeasuresfor semivowels
741
40
"CORRELATION" 4.5
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10
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o o.o
30
0
0.85
w,j,r
I
nasals
obstruents
Sound
(b)
6400Hz.
40
200
Hz
30
0.0
0.85
Amplitude 0J•-'--'"'N/-•-'-"/"-• (c)
-Oj.o * ffset Onse%
I 0.85
2O
[] A
I
lO
nasal obstruent
o -25
0.0
0.85
TIME(seconds) FIG. 6. An illustrationof parameterswhich captureabrupt amplitude changes. (a) Widebandspectrogram of"correlation."(b) Channeloutputs of an auditorymodelwhichshowabruptspectralchanges in two frequency regions between the/1/and adjacentvowels.(c) Onsetwaveform(computed from the sumof the negativefirstdifferences of the channeloutputs) whichshowsa sharpvalleyat the onsetof the/l/. (d) Offsetwaveform (computedfromthesumof thepositivefirstdifferences of thechanneloutputs) whichshowsa sharppeakat theoffsetof the/1/.
-15
-10
-5
Intensity
o
(c) .5
-lO.
-t5.
rate of changethat canbe capturedby thismeasure. The onsetand offsetwaveformswereexaminedduring the time intervalbetweeneachconsonantand its neighboringvowel(s). We definedthe onsetof the consonantto be the maximum absolutevalue of the onsetwaveformoccurring betweenthe precedingvowel and the consonant.Likewise, we defined the offset of the consonant to be the maximum
value of the offsetwaveform occurringbetweenthe consonant and the followingvowel.The time at which thesevalues occurare indicatedby arrowsin parts (c) and (d) of Fig. 6. Figure 7 showsthe data on the onsetsand offsetsacross
.2o.
-25
I
nasals
obstruents
Sound
FIG. 7. Averagesand standarddeviationsof (a) the offsetsbetweenprevocalicconsonants and followingvowels,(b) the onsetsand offsetsbetween intervocalicconsonantsand adjacentvowels,and (c) the onsetbetween postvocalic consonants and precedingvowels.
all wordsand all speakers.The unitsof the onsetand offset valuesarelike dB sincethechanneloutputsafter nonlinearitieshavebeenappliedare approximatelylinear with ampli-
is oftena wide spreadin the distributionof onsetand offset
tude at low signal levels and logarithmic at higher signal levels (Seneft, 1986, p. 88). The data for/1/are separated from/w j r/since, of the semivowels,/1/is most associated
stresspattern of the words and the rate of spectral change betweenthe consonantsand adjacentvowels.That is, onsets
values.
We also observeda strong relationshipbetweenthe
with spectraldiscontinuities.Severalobservationscan be made from the data. First, in general,the spectralchanges betweenobstruentconsonants andadjacentvowelsaremore rapidthan thespectralchangesbetweensemivowelsand adjacentvowels.Second,the spectralchangebetween/1/and adjacentvowelstendsto be more abruptthan the spectral changebetweenthe othersemivowels and adjacentvowels.
and offsetsassociated with consonants that precedestressed vowelsare significantlystrongerthan thoseassociated with consonants that precedeunstressed vowels,presumablybecausetheconstrictionistighterandthe releaseis morerapid. For example,comparethe rate of spectralchangebetween the prevocalic/1/and adjacentvowelsin the words"blurt" and "linguistics,"and betweenthe intervocalic/1/and surroundingvowelsin "walloon" and "swollen"shownin Fig.
However, as can be seenfrom the standard deviations, there
8. The offsetassociatedwith the/1/in
742
J. Acoust. Soc. Am., Vol. 92, No. 2, Pt. 1, August 1992
"blurt" (at about 130
Carol Y. Espy-Wilson:Acoustic measures for semivowels
742
"blurt"
"linguistics"
dB
,'
o.o
oJ
oz
•s
o.,
I I,'
't
i
o.s
Time(seconds)
Time Iseconds)
Amplitude
s ?
FIG. 9. A schematicof an energywaveformfor a vowel-consonant-vowel (VCV) sequence. The extremawithin the waveformare usedto compute the energydifference betweenconsonants andvowels.In the caseof prevocalicconsonants(V, doesnot exist), pointsC and B are used.In the caseof postvocalic consonants (V 2doesnotexist),pointsA andB areused.Finally, in the caseof intervocalicconsonants,the smallerof the differencebetweenpointsC and B and betweenpointsA and B is takenasa measureof the energydip.
õ
kHz 4
quencyrange640--2800Hz because,relativeto the vowels, the lowerF 1 for the semivowels is expectedto causea deereasein theamplitudesof the formantsin thisregion.However, we found that severalintervocalic/r/'s have energy levelsin this rangewhich do not differ from thosefoundon surroundingvowels,presumablybecause of theproximityof F2 and F3. To avoid this problem,we also examinedthe bandlimitedenergyfrom 2000 to 3000 Hz. SinceF3 is nor-
FIG. 8. An illustration of therateof spectral change associated withthe /l/'s in "blurt," "linguistics,""wallach," and "swollen."(a) Wideband spectragrams.(b) Offsetwaveform.(c} Onsetwaveform.
ms) is muchmoreabruptthan the oneassociated with the /1/in "linguistics" (at about145ms). Similarly,the onset and offset associatedwith the intervocalie /l/ in "wallcon"
(at 190and 260 ms,respectively) are muchmoreabrupt than those associatedwith the intervocalic /i/ in "swollen"
(at 350 and410 ms, respectively). C. Syllabic measure
Becausetheyare moreconstrictedand hencehavea rel-
ativelylowF 1,thesemivowels usuallyhaveconsiderably less energyin the low- to midfrequencyrangethan the vowels. Likeotherconsonants, thesemivowels usuallyoccurasnonsyllabicsoundsadjacentto syllablenucleiat a syllable boundary.That is,theygenerallydo not haveor constitutea peakof sonority,whereweareequatingsonorityin thiscase with a mid-frequency acousticenergymeasure.An acoustic
manifestation of a syllableboundary appears to bea significantdip withinsomebandlimitedenergycontour. To accessthe differencein energybetweensemivowels and vowels,and, moregenerally,betweenconsonants and vowels,we usedto bandlimitedenergies in the frequency ranges640-2800 Hz and 2000-3000 Hz. We chosethe fre743
J. Acoust.Sec.Am.,VoL92, No.2, Pt. 1, August1992
mally between2000 and 3000 Hz for vowels,but fallsnearor below 2000 Hz for/r/, /r/ will usuallybe considerably weakerin the 2000-to 3000-Hzrangethananadjacentvowel(s).
Measurements of the midfrequency energy of semivowels are basedon energycontourslike theonein Fig. 9. All measures are relativeto energyin an adjacentvowel. The depthof theenergydip isconsidered to bethedifference (in dB) betweenthe minimumenergywithintheconsonant, pointB, andthe maximumenergywithin the adjacentvowel(s), pointA and/or pointC. In the caseof syllableswith prevocalicconsonants, the difference in energy between the prevocalic consonant (pointB) andthe followingvowel(pointC) wascomputed. For syllableswith postvocalie consonants, the differencein energybetweenthepastvocalic consonant(point B) andthe precedingvowel(point A) wascomputed.Finally, for intervocalicconsonants, both the differences in energyat points C and B and pointsA and B werecomputed.The depthof the energydip wastakento be the smallerof the two differences.
As a basisfor comparisonwith the semivowels,the depthsof severaltypesof intravowelenergydipswerecomputedas well. An illustrationof this procedureis shownin Fig. 10, which showsa schematicrepresentation of an energy contour of a vowel. First, an estimateof the natural risc in
energywithin word-initialvowelswascomputedby calculating the energydifferenceat pointsW and T. This vowel energyonsetiscomparedwiththeenergydifference between CarolY. Espy-Wilson: Acoustic measuresforsemivowels
743
(a)
55' 45' 35'
ß
vowel
0 [] A
semivowel nasal obstruent
25' 15'
dB
5' -5
I'"
V
•'ltime
' 15' ' 25' ' 35' '4'5'55' ' 65
(b)
55 45
FIG. 10.A schematic of anenergywaveformfor a vowel.The energydifferencebetweenpointsW and T is usedto determinethe energyrisewithin word-initialvowels.The energydifferencebetweenpointsW and Z is used to determinetheenergytaperwithinword-finalvowels.The smallerof the energydifferences betweenpointsW, X andbetweenpointsX andY isused in all vowelswith the appropriateenergywaveformto determinewithin vowelenergydips.
35 o o
25 15 5 -5
5' • '1'5'2'5'3'5'4'5'5'5'65
c
LIJ
prevocalic consonants andfollowingvowels.Second, anestimate of the natural energytaper within word-finalvowels wascomputedby calculatingtheenergydifference at points W and Z. This vowel energyoffsetis comparedwith the energydifference betweenpostvocalic consonants and precedingvowels.Finally,in caseswheretherewasan intravocalicdip, X, wecomputedthedifference betweentheenergy at pointsW andX andbetween pointsY andX. In thiscase,
(c)
55' 45 •
35' 25' 15'
5-' -5
the smaller of the two differenceswas recorded. Of course,
5' • '1'5'2'5'3'5'4'5'5'5'65
not all vowelswill havethistypeof energywaveformshape sothat there will not alwaysbe a point X and a point Y. In thesecases,the intravowelenergydip is simply0 dB. This energymeasure is comparedwith the energydifference betweenintervocalicconsonants and surroundingvowels. The resultsof thesemeasurement proceduresare plotted separatelyin Fig. 11 for prevocalic,intervocalic,and
Energy Dip 640-2800 Hz (dB)
FIG. l I. Averagesand standarddeviations of the midfrequency energy changes (in dB) between consonants andadjacent vowelsandwithinvowels. Data are shownseparatelyfor the (a) prevocalic,(b) intervocalic, and(c) postvocalic consonants.
postvocalic consonants. In eachplot,theconsonants aredividedinto obstruents,nasals,and semivowels.Also included
tweensemivowels and followingvowelsif the semivowelis
in thefigurearethedatafor theenergychanges withinvowels.The data showthat the differencein midfrequencyenergybetweentheconsonants andvowelsis, on average,much greaterthantheenergychangewithinvowels.Of theenergy changes betweenconsonants andadjacentvowels,the energy changeassociated with the semivowels is almostalways
not in a clusterwith another consonant,but is word-initial.
Furthermore,if the semivowelis in a clusterwith another consonant,thereis a greaterenergychangebetweenit and the followingvowelif the precedingconsonantis voiced, ensuringthat the semivowelis alsocompletelyvoiced.In additionto the contextualinfluenceof precedingconsonants,thedegreeof stressof thefollowingvowelalsomatters. smallest.In addition, as the standarddeviationsshow, there energychangebetweenthe is sometimes considerable overlapbetweenthe distributions There is a more pronounced of theenergychanges withinvowelsandtheenergychanges semivowel and vowel if the vowel is stressed. betweensemivowels and adjacentvowels.On closerexamiconsonants nationof thesedata, patternsin their distributionemerge 2. Intervocalic across consonantal contexts. In intervocalicpositions,most of the semivowels showedsubstantial differences in energycomparedto neighI. Prevocalic consonants boringvowels.The energydip computedfor theseVCV segOf In general,the differencein midfrequencyenergybe- mentswasgreaterthan2 dB for 90% of thesemivowels. the other semivowels which did not show a substantial diftween the prevocalicsemivowelsand followingvowelsis ferencein energyrelative to adjacentvowels,33% were greaterthanthemidfrequency energychangewithinthebeginningportionsof word-initialvowels.However,wordpo- /j/'s, 14% were/r/'s and 5% were /l/'s. Most of these semivowels follow a stressedvowel and precede an unsitionhasa strongeffecton the phoneticrealizationof the stressed vowel,suchasthe/1/in "astrology"andthe/r/in semivowels. There is a more significantenergychangebe744
J. Acoust.Soc. Am., Vol. 92, No. 2, Pt. 1, August1992
Carol Y. Espy-Wilson:Acousticmeasuresfor semivowels
744
"cartwheel"
"guarantee."This lack of an energydip for semivowels in thisenvironment maybea caseof phoneticlenition. Whilethemajorityof vowelsdonotnormallyhavesuch energydips,therewereseveralinstances of vowelswithenergy dipscomparableto thosebetweenintervocalicconson-
8
ants and adjacentvowels.An examinationof suchvowels
6
showedthat, in general,thosewith suchsignificant energy dips were either and/•,/, suchas the one in "plurality" wherean intervocalic /r/ wasnot includedin thetranscrip-
5
tion,or a diphthong, suchasthe/i •/ in "queer"andthe /u'
?
kHz 4
/ in "flour." 3
3. Postvocalic
consonants
The patternsin energychangewithin word-finalvowels and betweenvowelsand following semivowels(including only the liquids/r/and/1/) are very similar. Two factors contributeto the overlap.First, the/j/or/w/offglides of word-finaldiphthongsoftenresultin energychangesthat are comparable to the changesobserved betweena wordfinal liquid and the precedingvowel.Sucha largeenergy tapercanbeseenin Fig. 12for the word"view" whichhasa substantialenergychangein the frequencyrange640-2800 Hz. Second,postvocalicliquidsthat arefollowedby another consonant areoftenasstrongastheprecedingvowels,ascan be seenby comparingthe amplitudesof the formantsin the /ar/region (0.1 to 0.2 s) in the word "cartwheel"shownin Fig. 13.In manysucheases,thereissignificant assimilation betweenthepostvocalic consonant andtheprecedingvowel. The fairly constantformantamplitudesand the steadyF3 frequencyduring the/or/region of "cartwheel"suggest
2
0 0.0
0.[
0.2
0.3
0.4
0.5
Time (seconds) FIG. 13.A spectrogram withautomatically extracted formanttracksoverlaid on the word "cartwheel."
that the/u/and/r/are coarticulatedso that they are realized acousticallyas one segment.
This energycontinuitybetweenvowelsand following liquidsalsooccurs whenthepostvocalic liquidsarefollowed byanotherSohorant consonant whichisnotin thesamesyllable,suchas the/1/in "bellwether."In wordslike this, thereisa nonsyllabic regionbetween thevowelpreceding the postvocalic liquidandthevowelafterthesecond sonorant consonant(inthis case,the/e/before
the/1/and the after the/w/). However,there is little energychangebetweenthe postvocalic liquidand the precedingvowel.Instead,the energyoffset(referredto asconsonant onsetin See.III B) betweenthenonsyllabic regionandthepreceding voweloccursafterthe liquidandbeforethefollowingsonorant consonant. On theotherhand,whennasalsoccupythis
"view" 8
postvocalic position, thereis substantial energychange betweenthemandthepreceding vowelsothattheenergyoffset occursbeforethe postvocalicnasalconsonant. This differencein wheretheenergyoffsetoccursis illus-
kHzI-
(to
o.[
o.2
o.3 o.4
o.5
Tim e (se conds) '90
-90
tratedin Fig. 14 whichcontainsinformationrelatingto the intersonorant sequences/rm/and/nr/in the words"harmonize"and "unreality,"respectively. In the caseof "harmonize"(shownontheleft), thenonsyllabic dipoccursduringthe/m/and, asindictedby thearrows,theenergyoffset betweenthe/rm/cluster and the previousvowel occurs after the/r/, at the beginningof the/m/. In contrast,the energyoffsetbetweenthe/nr/cluster and the preceding vowel in "unreality" (shownon the right) occursat the pointof implosionfor the/n/at about 175msasindicated by the arrow.
(b! FIG. 12. Illustrationof a largeenergytaperin word-finaldiphthongs.The energytowardstheendof thevowelis 30 dB or morelessthanthemaximum valuewithinthevowel.(a) Widebandspectrogram of theword"view." (b) Energy640 to 2800 Hz.
745
J. Acoust.Soc.Am.,Vol.92, No. 2, Pt. 1, August1992
To capturethis differencein the temporalpropertiesof the energy offset for nasal-sonorantconsonant sequences
andliquid-sonorant consonants sequences, wecomputedthe durationof the intersonorantnonsyllabicregion.The durationof thisenergydip regionwastakento bethedifferencein CarolY. Espy-Wilson: Acousticmeasuresfor semivowe•s
745
"harmonize"
"unreality"
8 7
6 5
kHz 4
I
0.3
o.4 o.s
oo
o.?
oo
0.4
Time(s•nds)
o.s o.o oz
Time(second)
Amp,ilud[m Amplitude•[L• • (c) (b)
(c)
FIG. 14.(a) Wideband spectrograms ofthewords"harmonize" and"unreality." (b) Onsetwaveforms thatshowa valleyattheenergy offset indicating the beginning of thenonsyllabic region.(c) Offsetwaveforms whichshowa peakat theenergy onsetindicting theendof thenonsyllabic region.
timebetweentheenergyoffsetandtheenergyonsetimmediately surroundingthe energydip. In the caseof "harmonize," the energyonsetoccursafter the/r/and beforethe followingvowelat about295 ms.Thus the energydip region includesonly the/m/and is 75 msin duration.In the caseof "unreality," the energy onset also occursafter the second sonorantconsonantand beforethe followingvowel. However,in thiscase,theenergydip regionincludesbothconsonants and is 120 ms in duration.
Data acrossall wordscontainingintersonorant clusters are shownin Fig. 15. For comparison, we alsoincludedthe durationof the energydip regionswhen there is only one sonorantconsonantoccurringbetweentwo vowels,an intervocalicnasalor semivowel.In thiscase,theenergyoffsetand energyonsetwill correspond to the consonant onsetandoffset, respectively.Although there is no normalizationfor variabilityin speakingrates,the resultsin Fig. 15 showa distinctpattern.The distributionsof the durationof energy dip regionsassociated with only onesonorantconsonantand those associated with two sonorant consonants where the
firstconsonant isa liquidareessentially thesame.However, the averagedurationof the energydip regionsassociated
syllablenucleus.Thusit maybemoreappropriateto thinkof the liquid and precedingvowelas a diphthongwherethe liquid,like theglidesin thiscontext,isconsidered to bea part of the vowel.
The assertionthat postvocalic/1/acts as the second elementof a diphthonghasalsobeenmadeby GilesandMoll (1975). Basedon x-ray data of prevocalicand postvocalic
/1/, they foundthat postvocalic/1/showsrelativelyslow movementcharacteristicsand undershootof articulatory position.On the otherhand,prevocalic/1/hada relatively highrateof articulatorymovementandno undershoot char-
•'
1704
•'
150 -
E o
'•
130 -
;•
110-
m
with two sonorant consonants where the first is a nasal is
LU
considerably longerthan thoseof the other cases.We can infer from this patternthat the energyoffsetin the cluster wherethe first memberis a postvocalicliquid occursafter the postvocalic liquidsothat onlyoneof the sonorantconsonantsis containedin the energydip region.On the other hand,the energyoffsetin the clusterwherethe firstmember is a postvocalic nasaloccursbeforethe postvocalic nasalso that both sonorantconsonants are part of the energydip region. Theseresultsshowthat postvocalicliquidswhich are followedby anothersonorantconsonant arenot a part of the energydip region.Instead,theyappearto be a part of the
'•
74(}
J.Acoust. Soc.Am.,Vol.92,No.2, Pt.1,August 1992
-
e,-
90-
70-
o
•
5o
Q
30 SC Intersonorant
liquid+SC
nasal+SC
Consonants
FIG. 15.A comparison oftheaverage durations of thenonsyllabic regions ofwordscontaining oneintervocalic sonorant consonant (SC), wordscontaininganintervocalic liquid-sonorant consonant sequence (liquid+ SC) and wordscontainingan intervoealienasal-sonorantconsonantsequence (nasal + SC).
CarolY. Espy-Wilson: Acoustic measures forsemivowels
746
acteristics. Thus theyconcludethe prevocalic/I/ functions as a consonantwhile postvocalie/1/is vocalicin nature. Alongthisline,SproatandFujimura(submitted)postulate that a gestureinvolvinga nonperipheral articulator(suchas tonguedotsumretraction) is attractedto the syllablenucleuswhereasa gestureinvolvinga peripheralarticulator (suchas the tonguetip) is attractedto syllablemargins. With this assumption, they too concludethat postvocalic /l/, whichhasa moresignificanttonguedorsalretraction thanprevocalic/1/,shouldbeconsidered morevocalic.
(F 1, F2, and F31. Given minimal-pairwords,it has been shown(Lisker, 1957;O'Connoret al., 19571that F l separatesthe glides/w/and/j/from the liquids/1/and/r/, F2 separates/w/from/1 r/from/j/, and F3 separatesthe liquids/1/and/r/. The data in thisstudyconcurwith these observations.
D. Formant frequency measures
Important informationfor distinguishingamong the semivowelsare the frequenciesof the first three formants
A formant tracker (Espy-Wilson,19871was usedto automaticallyextractthe first four formantsduringthe sonorant regionsof the wordsin the database.The frequencies off 1,F 2,F 3, andF4 wereestimatedbyaveragingthevalue at the time of a minimum or maximumin a particularformant track and the samplesin the precedingand following frameswithin the hand-transcribedsemivowelregion.In the caseof/w/and/l/, the valuesof the formantswereaveraged
TABLE VI. Formantfrequencies (in Hertz) andformantdifferences (in Hertz andin bark) of semivowels averaged acrossall speakers. Prevocalic
(Hzl FI
F2
F3
F4
w I
381 399
848 1074
2320 2553
3525 3767
r
419
1285
1779
3350
j
317
2142
2827
3661
(Hz) FI-FO
F2-FI
F3-F2
(bark) F4-F3
F4-F2
B I-B0
B2-B
I
B3-B
2
B4-B3
B4-B2
w
241
467
1472
1204
2676
2.4
3.6
6.4
2.5
8.9
I
258
675
1479
1214
2693
2.6
4.9
5.5
2.3
7.8
r
242
866
493
1571
2064
2.8
6.0
2.1
3.9
5.9
j
174
1825
684
1518
1.7
1.7
1.6
3.2
834
10.2
Intervocalic
(Hz) FI w
F2
F3
F4
349
771
2340
3508
445 460
1060 1240
2640 1720
3762 3433
361
2270
2920
3824
(Hz) FI-FO
F2-FI
(bark)
F3-F2
F4-F3
F4-F2
B I-B0
B2-B
1
B3-B2
B4-B3
B4-B2
211
422
1570
1169
2737
2.1
3.4
7.0
2.4
9.4
305 317
610 783
1580 473
1123 1717
2707 2190
3.0 3.1
4.5 5.4
5.8 2.1
2.1 4.2
7.9 6.2
213
1910
648
906
1554
2.1
10.1
1.5
1.6
3.1
Postvocalic
(Hz)
I r
F!
F2
465 503
898 1300
F3
F4
2630 1830
3650 3391
(Hz) FI-FO
I r
747
323 363
F2-FI
433 799
F3-F2
1740 531
(bark) F4,-F3
1015 1554
J. Acoust.Soc. Am., VoL92, No. 2, Pt. 1, August1992
F4-F2
2752 2088
B I-B0
3.2 3.5
B2-B
3.2 5.4
I
/13 //2
6.9 2.1
B4•3'3
1.9 3.7
Carol Y. Espy-Wilson:Acousticmeasuresfor semivowels
D4--B2
8.8 5.8
747
aroundthe time of the F2 minimum.For/j/, the formant valueswereaveragedaroundthe time of the œ2 maximum, and for/r/the formant valueswere averagedaroundthe
effects andspeaker differences andtobettercapture someof theacoustic properties. Chistovich andLublinskaya (1979) havepostulated thatwhentwoformants arewithina critical
time of the F 3 minimum. Thus the formants were measured
distanceof 3.0 to 3.5bark of eachother,theyareinterpreted
duringthe timewhenthe vocaltract couldbeexpectedto be
bytheauditorysystem asonespectral peakwhose frequency is at the centerof gravityof the prominence. Syrdaland Gopal(1986),in an acoustic studyusingthePeterson and Barney(1952) voweldata,investigated whetherthiscon-
most constricted.
We normalizedthe formantsby computingbark differencesto reduce the acousticvariability due to contextual
Prevocali½ Semivowels
(a) 1
1
2
3
4
5
6
B1-B0 (Bark)
Prevocalic Semivowels (b) l
r-.. r
r
/' r ryrrr r
2
4
6
8
10
12
B3-B2 (Bark)
FIG. 16.Scatter plots oftheprevocalic semivowels spoken bytwomales andtwofemales according tothebarktransformed (a) F 2-F 1vsF I-F0 and(b) F 4F3 vsF3-F2. A two-dimensional 90% confidence regionisdrawnaroundthedatafor eachsemivowel.
748
J. Acoust. Soc.Am.,Vol.92, No.2, Pt.1, August1992
CarolY. Espy-Wilson: Acoustic measures forsemivowels
748
stantin auditoryunitsheld betweenseveralformantsand
greatlythe acousticvariabilitybetweenvowelsspokenby
between thefirstformantandthefundamental frequency
different talkers.
(F0). Theyfoundthat witha criticaldistanceof 3 bark,the difference between F 1 andF0 provideda reasonable repre-
The formant frequencies obtainedin this studyare in agreementwith previouslyreporteddata.The resultsacross sentation of thehigh-nonhigh voweldistinctions indepen- speakers areshownin TableVI for prevocalic, intervocalic, dentofspeaker, andthedifference between F 3andF2 repre- and postvocalic semivowels. Also includedin the tableare
sentedthefront-backvoweldistinctions. In addition,they
normalized tbrmant values (F1-F0, F2-F1, F3-F2, and F4-F 3 ) and bark differences(B 1-B 0, B 2-B 1,B 3-B 2, and
found that the bark difference transformations reduced
Intervocalic
Semivowels
(o) 1
w
I
0
1
2
3
4
1
5
B1-B0 (Bark)
Intervocalic Semivowels
lb) I
0
2
4
6
8
10
12
B3oB2(Bark)
FIG. 17.Scatterplotsof the intervocalicsemivowels spokenby two malesand two femalesaccordingto the bark transformed(a) F2-F 1 vsF I-F0 and (b) F4-F3 vsF3-F2. A two-dimensional 90% confidence regionisdrawnaroundthedata for eachsemivowel. 749
J. Acoust. Soc. Am_.Vol. 92. No. 2. Pt. 1. August 1992
Carol Y. Espy-Wilson:Acousticmeasures for semivowels
749
B 4-B 3). (F0 wasobtainedautomatically with thepitchde-
marizesthe classification of the semivowels accordingto a
tector describedin Gold and Rabiner, 1969. ) The distribu-
3.5-bark critical distance criterion and the five bark-differ-
tionsof the bark differences are shownin Figs. 16-18 for the prevocalic,intervocalic,andpostvocalic semivowels, respectively. A two-dimensional 90% confidenceregionis drawn aroundthe datafor eachsemivowel.Finally, Table VII sum-
encedimensions.A +- indicatesthat a majority of the semivowels are within 3.5 bark in the bark-difference dimen-
sion. Conversely,a -- indicatesthat a majority of the semivowels exceedsthe 3.5 bark in the bark-difference di-
Postvocalic Semivowels (o) 1
....... rf
,
rr
r
.................
? r [ rrr•. r•rrS rrrrrrrr ................. i., r r_ _'"-......
•r ',,. r•t •
• r•.•
(Received3 December1990;accepted for publication 23 April 1992)
Acousticproperties relatedto thelinguistic features whichcharacterize thesemivowels in AmericanEnglishwerequantified andanalyzedstatistically. Thefeatures canbedividedinto thosewhichseparate thesemivowels fromothersounds andthosewhichdistinguish amongthe semivowels. The featuresof interestaresonorant, syllabic,consonantal, high,back,front, and retroflex. Acousticcorrelates of thesefeatures wereinvestigated in thisstudyof the semivowels.The acousticcorrelates,which are basedon relative measures,were testedon a
corpusof 233polysyllabic words,eachof whichwasspoken onceby twomalesandtwo females.For themostpart,theappropriate distinctions aremadeby thechosenacoustic properties forfeatures. However,foreachproperty, therewassomeoverlapin theacoustic correlates of featuresfor thesounds beingdistinguished. An examination of thesounds in the overlapregions revealsthattheirsurface manifestation variessubstantially fromthecanonical form.In largepart,theobserved variabilitycanbeexplainedin termsof changes dueto feature spreadingandlenition.
PACS numbers:43.70.Fq,43.72.Ar,43.70.Hs
variabilityof someacousticproperties assumed to be associatedwith thesemivowels. For example,notall prevocalic An acousticstudyof the sounds/wj r l/was conducted andintervocalic/1/'sareassociated with spectraldiscontinas part of the development of a semivowelrecognitionsysuities.Finally,theacoustic properties of someof theother tem (Espy-Wilson,1987}. Recognitionof the semivowelsis sounds also change with context. In particular, someundera challengingtask since,of the consonants,the semivowels lyingly voiced obstruents surface as SOhorant consonants aremostlike the vowelsand,dueto phonotacticconstraints, and, in somecases,they resembleone or more of the theyalmostalwaysoccuradjacentto a vowel.Thus,acoustic semivowels.Some of these variations were also examined. changes betweensemivowels andvowelsareoftenquitesubobserved in theacoustic tle sothat therearenoclearlandmarksto guidethesampling We will arguethatthevariability manifestation of the semivowels and some of the other of acousticproperties. soundscanbecharacterizedin termsof changesin the phoMany studieshave examinedsomeof the acousticand perceptualproperties of oneor moreof the semivowels in nologicalfeatures. INTRODUCTION
English(Lisker, 1957;O'Connoret al., 1957;Lehiste,1962; Kameny, 1974; Daiston, 1975; Bladon and A1-Bamerni, 1976;Bond, 1976). Thesestudieshaveprimarily focusedon the acousticandperceptualcuesthat distinguishamongthe semivowels and
the
coarticulatory
effects between
semivowels and adjacentvowels.For the mostpart, these studieshavelookedat simplecontextsand a limited setof acousticproperties. While the resultsof pastwork were usedto guidethe presentexaminationof the semivowels, this studydiffers from previousresearchin that the acousticpropertiesinvestigatedwerechosento be closelyrelatedto the abstractlinguisticfeatureswhich comprisea phonologicaldescription of the semivowels.We examineacousticpropertiesfor featuresthat not only distinguishamongthe semivowels,but that alsoseparatethe semivowelsfrom other sounds.The acousticproperties wereanalyzedto quantifyhowthe surfacemanifestationof the semivowels changeswith context. The resultsobtainedsupportpreviousfindings,namelythat formantinformationcan be usedto distinguishamongthe semivowels.In addition,there are new findingsabout the 736
I. REVIEW OF THE ACOUSTIC PROPERTIES OF SEMIVOWELS
Spectrograms of the semivowels are shownin Figs. l and 2 wherethey occurin word-initialpositionbeforethe front vowel/i/and
the back vowel/u/. As can be seen,the
semivowels haveproperties that are similarto bothvowels and consonants.Like the vowels,the semivowelsare pro-
dueedorallywithoutcompleteclosureof thevocaltractand withoutanyfrieationnoise.Asisalsotrueforthevowels,the degreeof constrictionneededto producethe semivowels doesnot inhibitvoicing.Thus,asshownin thesefigures,the semivowels and vowels are both voiced with no evidence of
frication noise.In addition, the slowerrate of changeof the constriction size for the semivowels than other consonants
resultsin slowerspectrumchangesfor thesesoundscom-
paredto otherconsonants. Forexample, thespectrogram of theword"you"in Fig. 1 showsthattheformantsduringthe /j/stay relativelyconstantfor about130ms beforethey movetowardtheappropriate valuesforthefollowingvowel. Thus,asin the caseof vowels,a voicedsteadystateis often
J. Acoust.Soc.Am.92 (2), Pt. 1, August1992 0001-4966/92/080736-22500.80
@ 1992Acoustical Societyof America
736
"lee"
we sooo
500O
5000
4000
4000
4000
"re"
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,I •
5000-
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4000-
3000 -
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3ooo-
Hz
Hz
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2000
8000-
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o
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5000
4000
4000
3000-
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0.25
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3000-
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0.556
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Hz
1000
o
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5000-
2000
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0 F 0.428
0
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0.50G
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TIME (seconds)
0.582
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TIME (seconds)
FIG. 1. Widebandspectrograms of the words"we" and "ye" (top), and
FIG. 2. Widebandspectrograms of the words"lee" and "re" (top), and
"woo" and "you" (bottom).
"rou" and "1ou" (bottom).
observedin spectrograms of the semivowels. frequencies than/u/, and/j/has a lowerF 1 frequency and Liketheotherconsonants, thesemivowels usuallyoccur usuallya higherF2 or F3 frequencythan/i/. Thesedifferat syllablemargins.That is, they generallydo not haveor encescanbe seenin the words"woo" and "ye" of Fig. 1. constitute apeakof sonority.{Sonority,in thiscase,isequatThe glidesoccurin prevocalicandintervocalicpositions edwithsomemeasure ofacoustic energy.)Asshownin Figs. within a word,suchasthe/j/in "you" and "yo-yo"andthe 1and2, oneor moreof theformantsduringthesemivowels is /w/in "we" and"away."In addition,theyoftenoccurphoconsiderably lowerin amplitudethanit isduringthefollow- netically{eventhoughthey are not phonologically speciing vowels.In the caseof/w/, it is F3 and the higherfor- fied) aspart of the transitionbetweentwo adjacentvowels. mantswhichareweaker.In thecaseof/j/, F 3 andthehigher An exampleof thismanifestation of a glideistheintervocalic formantsarefairly strong,butF2 isnot.For/1/, thereisless /j/sound oftenobserved between/i/and/o/in thepronunenergyin the high-frequency rangestartingaroundF4 for ciationof"radiology" ( [redijologi]vs [rediologi]). The semivowels/1/and/r/are often referred to as li"lee"andF3 for "1ou."Finally,F3 andthehigherformants arelowerin amplitudeduring/r/. The relativelylowampli- quids.Sproatand Fujimura {submitted)foundfrom articutudeofthesemivowels ascompared tothevowels isprobably latory and electromyographicdata obtainedfrom several due to a combinationof factors:a low-frequency first for- speakersthat the productionof all English/l/'s involves mant (Fant, 1960),a largeF 1bandwidthcausedbythe nar- both an apical and a dorsalgesturalcomponent.The key rowerconstriction(BickleyandStevens,1986),or interac- articulatorydistinctionbetweenthe two well established tion betweenthe vocal folds and the constriction(Bicklcy variants of/1/, light or clear/1/(as in "Lee") and dark/1/ andStevens,1986).At present,thisphenomenon isnotwell {as in "feel"), is that in dark/1/the tonguebody is more understood. retractedthan in light/1/, resultingin a much lower F2. The semivowels/w/and/j/are often referredto as Sproatand Fujimuraarguethat thisallophonicvariationis glidesor transitionalsounds.They are producedwith con- not categorical,but is the degreeto which the apicaland stantmotionof thearticulators. Consequently, theformants dorsalgestures arerealizedandthe timingbetweenthe two in the transitiontowardor awayfromadjacentvowelsexhib- gestures.Specifically,theyfoundthat in additionto a signifiit a smoothglidingmovement.The semivowels/w/and/j/ cantly greaterretractionof the tonguedorsumfor dark/1/ are producedwith vocal-tractconfigurations similar to comparedto light/1/, the maximumtonguedotsumposithoseof thevowels/u/and/i/, respectively, butwitha more tion for the dark/1/is achieved well in advance of the maxiextreme constriction.As a result,/w/has lower F 1 and F2 mum tonguetip position.On the otherhand,duringthe ges737
J. Acoust. Sec.Am.,VoL92, No.2, Pt.1, AugustI gg2
CarolY. Espy-Wilson: Acoustic measures forsemivowels
737
turefor thelight/I/, thetonguetip positionisreachedbefore the tonguedorsumpositionis achieved. In the caseof light/l/, the apicalgestureusuallyinvolyesthe placementof the centerof the tonguetip against the alveolarridge.The oftenrapidreleaseof the tonguetip fromtheroofof themouthresultsin a spectraldiscontinuity betweenthe/1/and the followingvowel (Daiston, 1975). JoGs(1948), reportedthat/1/is alwaysmarkedat itsbeginningand/or endby an abruptshiftin the formantpattern. Along this line, Fant (1960) observedthat the identification of an/1/reliesona suddenshiftupoff I fromthe/l/into the followingvowel. Finally, Daiston (1975) found that this abruptshiftin F 1isoftenaccompanied bya transientclickin the acousticspectrum.Someof thesepropertiescanbe ob-
that asthepharyngealconstrictionisnarrowed,theauditory impression of the/r/is enhanced. Regardlessof whether a bunchedor retroflexed/r/is produced,lip roundingmayoccurwhen/r/is eitherprevocalic or intervocalic and before a stressedvowel (Delattre
andFreeman,1968).The acousticconsequence of lip roundingisa loweringof all formants.Thiseffectmayaccountfor the lower F 1, F2, and F3 Lehiste (1962) observedfor initial
/r/ allophonesrelativeto final/r/allophones, for whichlip roundingdoesnot usuallyoccur. In summary,many of the acousticpropertieswhich characterizethesemivowels havebeenexaminedin previous studies.However,thiswork haslargelyfocusedon formant measurements. In this investigationof the semivowels, served at theboundary between the/1/and thefollowing acousticpropertiescalculatedfrom formantmeasurements vowelsin Fig. 2. andenergy-based parametersarequantifiedandanalyzedso In the caseof dark/1/, Sproatand Fujimura (submit- that we canstudyto a greaterextentsomeof the variability that occurs in the surface forms of the semivowels. Furtherted) reportthat apicalcontactis lessrobusteventhoughit was madeduring all of the/1/productions in their study. more, the acousticpropertiesare related to the linguistic GilesandMoll (1975) foundin anx-raystudyof English/1/ featureswhich providea frameworkfor understanding the that apicalcontactfor dark/l/'s wasnot alwaysachievedfor changesthat occur. all speakersand is dependentupon phoneticcontextand speakingrate. In addition,theyfoundthat the meanpeak II. MœTHOD velocityof thetongueapexmovementissignificantly slower A. Stimuli for dark/1/. Furthermore,they foundthat dark/1/shows A data base of 233 polysyllabicwords containing undershoot ofarticulatorypositions with increases in speaksemivowels in a varietyof phoneticenvironments wasselectingrate.Thisslowerandincomplete apicalgesturemayhelp ed from the 20 000-word Merriam-Webster Pocket dictioexplainwhy dark/1/productionsarenot associated with an nary.The semivowels occuradjacentto voicedandunvoiced abrupt spectralchange. consonants,as well as in word-initial, word-final, and interAlthough the distributionof dark and light/1/varies vocalicpositions.(Note that only/1/and/r/occur postvoacrossspeakers,canonicalsyllable-final/1/is dark and, in calieally.) The semivowels occuradjacentto vowelswhich manydialects,syllable-initial/1/is light. Sproatand Fujiare stressed and unstressed, high and low, and front and mura(submitted)foundin theirstudyof preboundaryl inback. In developing the database, wordswere chosenthat tervocalic/1/inthefallingstress context/i_ [/that thequalcontained several semivowels so that theysatisfymorethan ity of the/1/dependsuponthephoneticdurationof therime one category. The distribution of the semivowels in termsof whichcontainsit. (They alsoshowa correlationbetweenthe word position and stress is given in Table I. Examples of durationof the preboundaryrime and the strengthof the words in these categories are given in Table II. While most of phonologieal boundary.)Specifically theyfoundthat asthe the contextscontain severalwords, there are a few for which rimebecomes longer,thetonguebodyfor/1/becomeslower and more retracted.Therefore,intervocalic/1/occurring beforea major intonationboundaryis dark. On the other hand,they foundthat the preboundary/1/preceding the TABLE I. Distribution of semivowels in the test words. The number in the weakest boundariesare as light as initial /1/. These data parentheses specifiesthe numberof semivowelsoccurringnext to a vowel supportpreviousfindingsby Lehiste (1962) and Blandon with primarystress. and AI-Bamerni{ 1976) whichshowthat certainprebounCategory w j r i daryintervocalic/1/productions arelighterin qualitythan preboundary/1/in prepausalposition. Prevocalic 65 41 63 52 word-initial(prestressed) 11(9) 8(5) 10(5) 6(3) American/r/may beproducedwith eithera retroflexed or bunched articulation (Delattre and Freeman, 1968). If
the upperconstrictionis at the palate,it is madewith either
the tonguetip or the tongueblade.If, instead,the upper constrictionis further back near the velum, it is made with
thetonguebody.It isthepalatalor palato-velarconstriction which lowersF3 (Delattre and Freeman, 1968;Stevens,in preparation), whereasthe pharyngealconstrictionlowers F 2 and raisesF 1 (Delattre and Freeman, 1968). In termsof perception,Delattre and Freemanfoundwith the useof an electricmouth analogthat the palatal constrictionis primary in termsof producingthe/r/. However,they found 738
J. Acoust.Soc. Am., Vol. 92, No. 2, Pt. 1, August1992
stopcluster(prestressed)
18(8)
fricativecluster (prestressed) 19(13) stopfricativecluster(prestressed) 10(7) adjacentto SOhorantconsonant 7 Intervocalic
ll(4)
18(10) 10(4)
7(3) 8(3) 7
18(9) 10(7) 7
18(6) 10(5) 9
9
6
29
32
poststressed prestressed
2 5
I 4
13 10
16 8
unstressed
2
I
Postvocalic
word-final(poststressed)
6
8
25
26
13(10) 19(14)
obstruent cluster
6
4
adjacentto sonGrantconsonant
6
3
Carol Y. Espy-Wilson:Acousticmeasuresfor semivowels
738
TABLE II. Examplesof testwords.
Category
w
j
r
I
Prevocalic
word-initial:prestressed
walnut wa!loon aquarius quadruplet
yell euroiogist pule bucolic
requiem rhinoceros brilliant fibroid
swollen
view
frivolous
flourish
Swahili disqualify misquotation carwash
behavior spurious promiscuously brilliant
anthrax astrology widespread walrus
grizzly exclaim exploitation harlequin
poststressed prestressed
forward
Ghanaian
caloric
astrology
unaware
reunion
fluorescence
unilateral
unstressed
unctuous
diuretic
correlation
fraudulent
clear
dwell
other
stopcluster:prestressed other
fricativecluster:prestressed other
stopfricativecluster:prestressed other
adjacentto Sohorantconsonant
leapfrog linguistics bless chlorination
Intervocalic
Postvocalic
work-final:poststressed other
memoir
whippoorwill
obstruent cluster
cartwheel
oneself
adjacentto sorterantconsonant
forewarn
walnut
only a small number of words were availablein the dierio-
nary.For example,only the word "Ghanaian"had a poststressedintervocalie/j/. B. Speakers and recordings
For recording,the wordswereembeddedin the carrier
phrase" pa."The final"pa" wasaddedin orderto avoidglottalizationand other typesof utterance-finalvariability.Eachword wasspokenonceby two malesand two females. Giventhat thereareseveralwordsin mostcategories (see Table I), one repetitionof each word by each speakerprovidesat least 24 to 260 productionsof each semivowel in eachmajorcategory,e.g.,prevocalic/w/. In fact,asexplainedin Sec.II C, somecategories maycontain more instances of a semivowel than what is indicated in Ta-
bleI sincespeakers ofteninsertsemivowels between adjacent vowels.
The speakerswerestudentsandemployees at the Massachusetts Instituteof Technology.The femalespeakers werefromthenortheast andthemalespeakers werefromthe midwest.All werenativespeakers of Englishand reported havingnormalhearing.Thespeakers wererecorded in a quiet roomwith a pressure-gradient close-talking noisecanceling microphone(part of SennheiserHMD 224X microphone/headphone combination).They were instructedto saythe utterances at a naturalpace.
and measurementprocedureswill be indicatedin Sec.III. Segmentation and labelingof the waveformswas performedby theauthorwith the helpof playbackanddisplays of severalattributesincludingLPC and wide-bandspectra, thespeechsignalandvariousbandlimitedenergywaveforms (Cyphers,1985;Shipman,1982;Zue etal., 1986).The Merriam-WebsterPocketdictionaryprovideda baselinephonemic transcriptionof the words.However,modifications of someof thelabelsweremadebasedon thespeakers' pronunciations.In addition,whentranscribingthedatabase,we did not normallyconsiderthe/w/and/j/offglides of diphthongsas beingseparatefrom the vowel.In someinstances, however,the offglideof a diphthongwhichwasfollowedby anothervowel was articulatedwith a narrow enoughconstriction that a semivowel label was inserted. On the other
hand,someunderlyingpostvocalic liquids,particularly/1/, in words like "almost" were not alwaysclearly heard. In theseinstances,the liquid wasoften omittedfrom the transcription. D. Feature analysis
Distinctivefeaturetheorywasusedto providea guideto understanding what acousticpropertieswe shouldlook for to characterize the semivowels.In addition, distinctivefea-
ture theory providesa basisfor understandinghow the acousticpropertiesof the semivowels maychangeasa function of context.A featurespecificationof the semivowelsis C. Initial processing givenin TablesIII and IV. Table III containsfeaturesthat The utterances weredigitizedusinga 6.4-kHz low-pass separatethe semivowelsas a classfrom other soundsand filteranda 16-kHzsamplingrate.The speechsignalswere Table IV contains features that distinguishamong the alsopre-emphasized to compensate for the relativelyweak semivowels. spectralenergyat high frequencies (a particularissuefor The featureslistedare modifications of onesproposed *3norants). Finally, the test words were excisedand hand by Jakobsonet al. (1952) and later by Chomsky and Halle transcribed.This processresulted in 2378 vowels, 1689 (1968). For example,in Table IV, we list boththe features semivowels, 479 nasals,and 1894obstruents(stops,frica- backandfront. For practicalreasons,we choseto useboth tives, and affricates).Specificcharacteristics of measures featuresand classify/r/and prevocalic/1/as -back and 739
J. Acoust.Sec.Am.,Vol.92, No.2, Pt. 1, August1992
CarolY. Espy-Wilson: Acousticmeasuresforsemivowels
739
TABLE
III. Features which characterize various classes of consonants. A
"+ "indicatesthatthedesignated featureispresentin therepresentation of
TABLE IV. Featureswhichdiscriminateamongthe semivowels. A" +" indicatesthat the designatedfeatureis presentin the represention of the
the sound, and a" --" indicates the absenceof the feature.
sound,and a" --" indicatesthe absenceof the feature.
Sohorant
Syllabic
Nasal
Consonantal High
Back Front
Fricatives,stops,affricates
--
--
--
/w/
-
+
+
--
Semivowels
+
--
--
/j/
--
+
--
+
Nasals
+
-
+
/r/
....
Vowels
+
+
-
prevocalic/1/ postvocalic/1/
+ --
Retroflex --
-+
.... --
+
--
--
-front.2TheirF 2 values clearlyliebetween thoseoftheback androundedsemivowel/w/and the front semivowel/j/. We alsofoundit necessary to distinguish betweeninitial and final/1/allophones on the basisof the featuresconsonantal and back. As statedearlier, severalresearchershave observed a sharpspectraldiscontinuity betweena prevocalic /1/and a followingvoweldue to the rapid releaseof the tonguetip fromthealveolarridge,aswewouldexpectwith a changein the featureconsonantal. On the otherhand,in the productionof postvocalic/1/, alveolarcontactis often not realizedor is realizedonly gradually,so that the spectral changebetweenit anda preceding vowelis usuallygradual. In addition, a final/1/is more velarized than an initial/I/. Thus, F2 is much lower and closein value to that of the back and rounded/w/.
Finally, the featureretroflexis usedto distinguish/r/ fromall othersounds. Althoughtheterm"retroflex"isused, this featurerelatesto the acousticconsequence of either a bunchedor retroflexedtongueshape. TableV showstheacoustic properties for thefeaturesin Table III and IV (with the exceptionof the featurenasal) andthe parametersusedfor their extraction.To makethem insensitive to variationsin speaker, speaking rate,andspeakinglevel,all of the propertiesarebasedon relativemeasures insteadof absolutethresholds.That is, a measureeither ex-
semivowels and separatethem from othersounds.The results also indicate how the characteristics of the semivowels
andothersoundschangeasa functionof context. III. RESULTS A. Sonorant
trum.
In the followingsections, we will examinehow well the properties listed in Table V distinguish among the
measure
Like the vowels, the semivowelsare sonorant sounds.
That is, the main sourceof excitationis at the glottis,sothat all of the naturalfrequencies of the vocaltract are excited. Thus, unlike the obstruents,wherethe main sourceof excitation is furtherforwardin the vocaltract, thereis significant energyat low frequencies. The only other consonants that sharethesepropertiesare the nasals. The parameterusedto extractthe acousticcorrelateof the sonorant feature is the bandlimitedenergycomputed from 100to 400 Hz. More specifically,the valueof the parameter in each frame is the difference (in dB) betweenthe
maximumenergywithin the word and the energyin each frame.An exampleof this parameteris shownin the lower partof Fig. 3 for theword"chlorination."The energydifferenceis smallin the sonorantregions(vowels,semivowels, and nasals), and is large in the obstruentregions (stops, fricatives, and affricates).
aminesan attributein onespeech frame3 in relationto anotherframe,or, within a givenframe,examinesonepart of the spectrumin relationto anothernearbypart of the spec-
AND DISCUSSION
Figure4 showshowall of thesoundsdifferin sonority, as determined with this measure.For each sound, the mini-
mum energydifferenceoccurringwithin the hand-transcribedregionisused.Thereisconsiderable overlapbetween the distributionsof the vowels,semivowels,and nasals.If we set a threshold of - 20 dB to divide SOhorant and nonsonor-
TABLE V. Mappingof featuresinto acousticproperties.B0, B 1, B2, B3, andB4 are the bark transformations ofF0, FI, F2, F3, andF4, respectively. Feature
Acousticcorrelate
Parameter
Sonorant
No significantdeereacein energy
Nonsyllabic
at low frequencies Dip in midfrequency energy
Consonantal High
Abrupt amplitudechange Low F 1 frequency
Back
LowF2 frequency
B 2-B 1
low
Front
High F2 frequency
B 3-B 2
low
B 4--B 3
low
Retroflex
Low F3 frequency&
B 4-B 3
high
Close F2 and F3
B 3-B 2
low
Energy 100-400Hz Energy640-2800 Hz Energy2000-3000 Hz First differenceof adjacentspectra B I-B 0
Property
higha lowa low' high low
• Relative to a maximum value within the utterance.
740
J. Acoust. Sec. Am., Vol. 02, No. 2, Pt. 1, August 1992
Carol Y. Espy-Wilson:Acoustic measures for semivowels
740
"chlorination"
"everyday"
?
6 õ
kHz4
kHz
o.o
az
03
0.4 o.5
Time(seconds)
I
I
I FIG. 5. A spectrogramof the word "everyday"whichcontainsthe two obstruents /v/ and/d/ thathaveundergone lenition.
sonorontregions (b) FIG. 3. An illustration of theparameter usedtocapturethefeaturesorterant. (a) Widebandspectrogram of lhe word"chlorination."(b} The differ-
quencyenergytheyexhibitis presumably causedby vibrationsof the vocalcordswhichare transmittedthroughthe
encebetween themaximum valueofthelow-frequency energy(computed tissues around the neck. from 100 to 4•0 Hz) in the word and the value in each frame. B. Consonantal
measure
Consonantalsoundsare producedwith a narrow con-
ant sounds, thenonlyabout12.5%of thetypicallynonson- strictionat somepointalongthe midlineof the vocaltract. orantconsonants overlapwith the sohorantsounds.Of these consonants,72% are producedwith a weakenedconstriction (this processreferredto as lenitionis discussed in Catford, 1977) so that they are realizedas sonorants.Two ex-
amples areshown in Fig.5,whichcontains a spectrogram of theword"everyday."Boththe/v/and/d/surface assonorant consonants.
Theremainingsegments whichoverlapwiththesonorantsare the closedportionsof voicedstops.The low-fie-
Due to the narrow constriction, the releaseof the consonan-
tal soundinto the followingvowelinvolvesrapidmovement of some of the formants. The result of this formant move-
ment is an abruptchangein the spectrumoverat leastsome part of the frequencyrange(Stevensand Keyset, 1989).
The parameterusedto capturethe rate of spectral changebetweenconsonants and vowelsis basedon the outputsera bankof 40 linearcriticalbandfiltersto whichsome nonlinearities(designedto model the hair-cell/synapse transduction process in theinnerear) areappliedto enhance onsetsand offsets(Seneft,1986). An exampleis shownin part (b) of Fig. 6 for the word "correlation."The wave-
formsthat are spacedabouta half bark apart showsharp onsetsandoffsetsbetween/l/and thesurroundingvowelsin the frequencyregionbetween800 and 1200Hz and between 1800 and 2400 Hz.
Based on the first differences in time of waveforms like
the onesshownin part (b) of Fig. 6, we computedglobal onset and offset waveforms
vowels semivowels na•abobslruenls
for each consonant. The onset
waveformis computedby summing,in eachframe,all the negativefirst differencesin time. Similarly,the offsetwaveform is obtainedby summing,in eachframe,all the positive firstdifferences in time of the channeloutputs.The resulting onset and offset waveforms for the word "correlation"
Class
of
sound
FIG. 4. Averages andstandard deviations ofthechange(in dB} in thelowfrequency energycomputedfrom 100to 400 Hz withinSOhorant andnonsonorantsoundswith respectto themaximumenergywithintheword. 741
J. Acoust.Sec. Am.,Vol.92, No. 2, Pt. 1. August1992
are
shownin parts (c) and (d) of Fig. 6 wherethe sharpamplitude changesbetweenthe/1/and the surroundingvowels showup asa valleyanda peak,respectively. Note that since a 25-ms time window is used,there is a limit to the maximum CarolY. Espy-Wilson: Acousticmeasuresfor semivowels
741
40
"CORRELATION" 4.5
•
.
'r•1
I•!!
%', c,', • ?' .
'
kHz ,•
,
"
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•I?
(a)
20
.. ' . :•
10
'.•.
o o.o
30
0
0.85
w,j,r
I
nasals
obstruents
Sound
(b)
6400Hz.
40
200
Hz
30
0.0
0.85
Amplitude 0J•-'--'"'N/-•-'-"/"-• (c)
-Oj.o * ffset Onse%
I 0.85
2O
[] A
I
lO
nasal obstruent
o -25
0.0
0.85
TIME(seconds) FIG. 6. An illustrationof parameterswhich captureabrupt amplitude changes. (a) Widebandspectrogram of"correlation."(b) Channeloutputs of an auditorymodelwhichshowabruptspectralchanges in two frequency regions between the/1/and adjacentvowels.(c) Onsetwaveform(computed from the sumof the negativefirstdifferences of the channeloutputs) whichshowsa sharpvalleyat the onsetof the/l/. (d) Offsetwaveform (computedfromthesumof thepositivefirstdifferences of thechanneloutputs) whichshowsa sharppeakat theoffsetof the/1/.
-15
-10
-5
Intensity
o
(c) .5
-lO.
-t5.
rate of changethat canbe capturedby thismeasure. The onsetand offsetwaveformswereexaminedduring the time intervalbetweeneachconsonantand its neighboringvowel(s). We definedthe onsetof the consonantto be the maximum absolutevalue of the onsetwaveformoccurring betweenthe precedingvowel and the consonant.Likewise, we defined the offset of the consonant to be the maximum
value of the offsetwaveform occurringbetweenthe consonant and the followingvowel.The time at which thesevalues occurare indicatedby arrowsin parts (c) and (d) of Fig. 6. Figure 7 showsthe data on the onsetsand offsetsacross
.2o.
-25
I
nasals
obstruents
Sound
FIG. 7. Averagesand standarddeviationsof (a) the offsetsbetweenprevocalicconsonants and followingvowels,(b) the onsetsand offsetsbetween intervocalicconsonantsand adjacentvowels,and (c) the onsetbetween postvocalic consonants and precedingvowels.
all wordsand all speakers.The unitsof the onsetand offset valuesarelike dB sincethechanneloutputsafter nonlinearitieshavebeenappliedare approximatelylinear with ampli-
is oftena wide spreadin the distributionof onsetand offset
tude at low signal levels and logarithmic at higher signal levels (Seneft, 1986, p. 88). The data for/1/are separated from/w j r/since, of the semivowels,/1/is most associated
stresspattern of the words and the rate of spectral change betweenthe consonantsand adjacentvowels.That is, onsets
values.
We also observeda strong relationshipbetweenthe
with spectraldiscontinuities.Severalobservationscan be made from the data. First, in general,the spectralchanges betweenobstruentconsonants andadjacentvowelsaremore rapidthan thespectralchangesbetweensemivowelsand adjacentvowels.Second,the spectralchangebetween/1/and adjacentvowelstendsto be more abruptthan the spectral changebetweenthe othersemivowels and adjacentvowels.
and offsetsassociated with consonants that precedestressed vowelsare significantlystrongerthan thoseassociated with consonants that precedeunstressed vowels,presumablybecausetheconstrictionistighterandthe releaseis morerapid. For example,comparethe rate of spectralchangebetween the prevocalic/1/and adjacentvowelsin the words"blurt" and "linguistics,"and betweenthe intervocalic/1/and surroundingvowelsin "walloon" and "swollen"shownin Fig.
However, as can be seenfrom the standard deviations, there
8. The offsetassociatedwith the/1/in
742
J. Acoust. Soc. Am., Vol. 92, No. 2, Pt. 1, August 1992
"blurt" (at about 130
Carol Y. Espy-Wilson:Acoustic measures for semivowels
742
"blurt"
"linguistics"
dB
,'
o.o
oJ
oz
•s
o.,
I I,'
't
i
o.s
Time(seconds)
Time Iseconds)
Amplitude
s ?
FIG. 9. A schematicof an energywaveformfor a vowel-consonant-vowel (VCV) sequence. The extremawithin the waveformare usedto compute the energydifference betweenconsonants andvowels.In the caseof prevocalicconsonants(V, doesnot exist), pointsC and B are used.In the caseof postvocalic consonants (V 2doesnotexist),pointsA andB areused.Finally, in the caseof intervocalicconsonants,the smallerof the differencebetweenpointsC and B and betweenpointsA and B is takenasa measureof the energydip.
õ
kHz 4
quencyrange640--2800Hz because,relativeto the vowels, the lowerF 1 for the semivowels is expectedto causea deereasein theamplitudesof the formantsin thisregion.However, we found that severalintervocalic/r/'s have energy levelsin this rangewhich do not differ from thosefoundon surroundingvowels,presumablybecause of theproximityof F2 and F3. To avoid this problem,we also examinedthe bandlimitedenergyfrom 2000 to 3000 Hz. SinceF3 is nor-
FIG. 8. An illustration of therateof spectral change associated withthe /l/'s in "blurt," "linguistics,""wallach," and "swollen."(a) Wideband spectragrams.(b) Offsetwaveform.(c} Onsetwaveform.
ms) is muchmoreabruptthan the oneassociated with the /1/in "linguistics" (at about145ms). Similarly,the onset and offset associatedwith the intervocalie /l/ in "wallcon"
(at 190and 260 ms,respectively) are muchmoreabrupt than those associatedwith the intervocalic /i/ in "swollen"
(at 350 and410 ms, respectively). C. Syllabic measure
Becausetheyare moreconstrictedand hencehavea rel-
ativelylowF 1,thesemivowels usuallyhaveconsiderably less energyin the low- to midfrequencyrangethan the vowels. Likeotherconsonants, thesemivowels usuallyoccurasnonsyllabicsoundsadjacentto syllablenucleiat a syllable boundary.That is,theygenerallydo not haveor constitutea peakof sonority,whereweareequatingsonorityin thiscase with a mid-frequency acousticenergymeasure.An acoustic
manifestation of a syllableboundary appears to bea significantdip withinsomebandlimitedenergycontour. To accessthe differencein energybetweensemivowels and vowels,and, moregenerally,betweenconsonants and vowels,we usedto bandlimitedenergies in the frequency ranges640-2800 Hz and 2000-3000 Hz. We chosethe fre743
J. Acoust.Sec.Am.,VoL92, No.2, Pt. 1, August1992
mally between2000 and 3000 Hz for vowels,but fallsnearor below 2000 Hz for/r/, /r/ will usuallybe considerably weakerin the 2000-to 3000-Hzrangethananadjacentvowel(s).
Measurements of the midfrequency energy of semivowels are basedon energycontourslike theonein Fig. 9. All measures are relativeto energyin an adjacentvowel. The depthof theenergydip isconsidered to bethedifference (in dB) betweenthe minimumenergywithintheconsonant, pointB, andthe maximumenergywithin the adjacentvowel(s), pointA and/or pointC. In the caseof syllableswith prevocalicconsonants, the difference in energy between the prevocalic consonant (pointB) andthe followingvowel(pointC) wascomputed. For syllableswith postvocalie consonants, the differencein energybetweenthepastvocalic consonant(point B) andthe precedingvowel(point A) wascomputed.Finally, for intervocalicconsonants, both the differences in energyat points C and B and pointsA and B werecomputed.The depthof the energydip wastakento be the smallerof the two differences.
As a basisfor comparisonwith the semivowels,the depthsof severaltypesof intravowelenergydipswerecomputedas well. An illustrationof this procedureis shownin Fig. 10, which showsa schematicrepresentation of an energy contour of a vowel. First, an estimateof the natural risc in
energywithin word-initialvowelswascomputedby calculating the energydifferenceat pointsW and T. This vowel energyonsetiscomparedwiththeenergydifference between CarolY. Espy-Wilson: Acoustic measuresforsemivowels
743
(a)
55' 45' 35'
ß
vowel
0 [] A
semivowel nasal obstruent
25' 15'
dB
5' -5
I'"
V
•'ltime
' 15' ' 25' ' 35' '4'5'55' ' 65
(b)
55 45
FIG. 10.A schematic of anenergywaveformfor a vowel.The energydifferencebetweenpointsW and T is usedto determinethe energyrisewithin word-initialvowels.The energydifferencebetweenpointsW and Z is used to determinetheenergytaperwithinword-finalvowels.The smallerof the energydifferences betweenpointsW, X andbetweenpointsX andY isused in all vowelswith the appropriateenergywaveformto determinewithin vowelenergydips.
35 o o
25 15 5 -5
5' • '1'5'2'5'3'5'4'5'5'5'65
c
LIJ
prevocalic consonants andfollowingvowels.Second, anestimate of the natural energytaper within word-finalvowels wascomputedby calculatingtheenergydifference at points W and Z. This vowel energyoffsetis comparedwith the energydifference betweenpostvocalic consonants and precedingvowels.Finally,in caseswheretherewasan intravocalicdip, X, wecomputedthedifference betweentheenergy at pointsW andX andbetween pointsY andX. In thiscase,
(c)
55' 45 •
35' 25' 15'
5-' -5
the smaller of the two differenceswas recorded. Of course,
5' • '1'5'2'5'3'5'4'5'5'5'65
not all vowelswill havethistypeof energywaveformshape sothat there will not alwaysbe a point X and a point Y. In thesecases,the intravowelenergydip is simply0 dB. This energymeasure is comparedwith the energydifference betweenintervocalicconsonants and surroundingvowels. The resultsof thesemeasurement proceduresare plotted separatelyin Fig. 11 for prevocalic,intervocalic,and
Energy Dip 640-2800 Hz (dB)
FIG. l I. Averagesand standarddeviations of the midfrequency energy changes (in dB) between consonants andadjacent vowelsandwithinvowels. Data are shownseparatelyfor the (a) prevocalic,(b) intervocalic, and(c) postvocalic consonants.
postvocalic consonants. In eachplot,theconsonants aredividedinto obstruents,nasals,and semivowels.Also included
tweensemivowels and followingvowelsif the semivowelis
in thefigurearethedatafor theenergychanges withinvowels.The data showthat the differencein midfrequencyenergybetweentheconsonants andvowelsis, on average,much greaterthantheenergychangewithinvowels.Of theenergy changes betweenconsonants andadjacentvowels,the energy changeassociated with the semivowels is almostalways
not in a clusterwith another consonant,but is word-initial.
Furthermore,if the semivowelis in a clusterwith another consonant,thereis a greaterenergychangebetweenit and the followingvowelif the precedingconsonantis voiced, ensuringthat the semivowelis alsocompletelyvoiced.In additionto the contextualinfluenceof precedingconsonants,thedegreeof stressof thefollowingvowelalsomatters. smallest.In addition, as the standarddeviationsshow, there energychangebetweenthe is sometimes considerable overlapbetweenthe distributions There is a more pronounced of theenergychanges withinvowelsandtheenergychanges semivowel and vowel if the vowel is stressed. betweensemivowels and adjacentvowels.On closerexamiconsonants nationof thesedata, patternsin their distributionemerge 2. Intervocalic across consonantal contexts. In intervocalicpositions,most of the semivowels showedsubstantial differences in energycomparedto neighI. Prevocalic consonants boringvowels.The energydip computedfor theseVCV segOf In general,the differencein midfrequencyenergybe- mentswasgreaterthan2 dB for 90% of thesemivowels. the other semivowels which did not show a substantial diftween the prevocalicsemivowelsand followingvowelsis ferencein energyrelative to adjacentvowels,33% were greaterthanthemidfrequency energychangewithinthebeginningportionsof word-initialvowels.However,wordpo- /j/'s, 14% were/r/'s and 5% were /l/'s. Most of these semivowels follow a stressedvowel and precede an unsitionhasa strongeffecton the phoneticrealizationof the stressed vowel,suchasthe/1/in "astrology"andthe/r/in semivowels. There is a more significantenergychangebe744
J. Acoust.Soc. Am., Vol. 92, No. 2, Pt. 1, August1992
Carol Y. Espy-Wilson:Acousticmeasuresfor semivowels
744
"cartwheel"
"guarantee."This lack of an energydip for semivowels in thisenvironment maybea caseof phoneticlenition. Whilethemajorityof vowelsdonotnormallyhavesuch energydips,therewereseveralinstances of vowelswithenergy dipscomparableto thosebetweenintervocalicconson-
8
ants and adjacentvowels.An examinationof suchvowels
6
showedthat, in general,thosewith suchsignificant energy dips were either and/•,/, suchas the one in "plurality" wherean intervocalic /r/ wasnot includedin thetranscrip-
5
tion,or a diphthong, suchasthe/i •/ in "queer"andthe /u'
?
kHz 4
/ in "flour." 3
3. Postvocalic
consonants
The patternsin energychangewithin word-finalvowels and betweenvowelsand following semivowels(including only the liquids/r/and/1/) are very similar. Two factors contributeto the overlap.First, the/j/or/w/offglides of word-finaldiphthongsoftenresultin energychangesthat are comparable to the changesobserved betweena wordfinal liquid and the precedingvowel.Sucha largeenergy tapercanbeseenin Fig. 12for the word"view" whichhasa substantialenergychangein the frequencyrange640-2800 Hz. Second,postvocalicliquidsthat arefollowedby another consonant areoftenasstrongastheprecedingvowels,ascan be seenby comparingthe amplitudesof the formantsin the /ar/region (0.1 to 0.2 s) in the word "cartwheel"shownin Fig. 13.In manysucheases,thereissignificant assimilation betweenthepostvocalic consonant andtheprecedingvowel. The fairly constantformantamplitudesand the steadyF3 frequencyduring the/or/region of "cartwheel"suggest
2
0 0.0
0.[
0.2
0.3
0.4
0.5
Time (seconds) FIG. 13.A spectrogram withautomatically extracted formanttracksoverlaid on the word "cartwheel."
that the/u/and/r/are coarticulatedso that they are realized acousticallyas one segment.
This energycontinuitybetweenvowelsand following liquidsalsooccurs whenthepostvocalic liquidsarefollowed byanotherSohorant consonant whichisnotin thesamesyllable,suchas the/1/in "bellwether."In wordslike this, thereisa nonsyllabic regionbetween thevowelpreceding the postvocalic liquidandthevowelafterthesecond sonorant consonant(inthis case,the/e/before
the/1/and the after the/w/). However,there is little energychangebetweenthe postvocalic liquidand the precedingvowel.Instead,the energyoffset(referredto asconsonant onsetin See.III B) betweenthenonsyllabic regionandthepreceding voweloccursafterthe liquidandbeforethefollowingsonorant consonant. On theotherhand,whennasalsoccupythis
"view" 8
postvocalic position, thereis substantial energychange betweenthemandthepreceding vowelsothattheenergyoffset occursbeforethe postvocalicnasalconsonant. This differencein wheretheenergyoffsetoccursis illus-
kHzI-
(to
o.[
o.2
o.3 o.4
o.5
Tim e (se conds) '90
-90
tratedin Fig. 14 whichcontainsinformationrelatingto the intersonorant sequences/rm/and/nr/in the words"harmonize"and "unreality,"respectively. In the caseof "harmonize"(shownontheleft), thenonsyllabic dipoccursduringthe/m/and, asindictedby thearrows,theenergyoffset betweenthe/rm/cluster and the previousvowel occurs after the/r/, at the beginningof the/m/. In contrast,the energyoffsetbetweenthe/nr/cluster and the preceding vowel in "unreality" (shownon the right) occursat the pointof implosionfor the/n/at about 175msasindicated by the arrow.
(b! FIG. 12. Illustrationof a largeenergytaperin word-finaldiphthongs.The energytowardstheendof thevowelis 30 dB or morelessthanthemaximum valuewithinthevowel.(a) Widebandspectrogram of theword"view." (b) Energy640 to 2800 Hz.
745
J. Acoust.Soc.Am.,Vol.92, No. 2, Pt. 1, August1992
To capturethis differencein the temporalpropertiesof the energy offset for nasal-sonorantconsonant sequences
andliquid-sonorant consonants sequences, wecomputedthe durationof the intersonorantnonsyllabicregion.The durationof thisenergydip regionwastakento bethedifferencein CarolY. Espy-Wilson: Acousticmeasuresfor semivowe•s
745
"harmonize"
"unreality"
8 7
6 5
kHz 4
I
0.3
o.4 o.s
oo
o.?
oo
0.4
Time(s•nds)
o.s o.o oz
Time(second)
Amp,ilud[m Amplitude•[L• • (c) (b)
(c)
FIG. 14.(a) Wideband spectrograms ofthewords"harmonize" and"unreality." (b) Onsetwaveforms thatshowa valleyattheenergy offset indicating the beginning of thenonsyllabic region.(c) Offsetwaveforms whichshowa peakat theenergy onsetindicting theendof thenonsyllabic region.
timebetweentheenergyoffsetandtheenergyonsetimmediately surroundingthe energydip. In the caseof "harmonize," the energyonsetoccursafter the/r/and beforethe followingvowelat about295 ms.Thus the energydip region includesonly the/m/and is 75 msin duration.In the caseof "unreality," the energy onset also occursafter the second sonorantconsonantand beforethe followingvowel. However,in thiscase,theenergydip regionincludesbothconsonants and is 120 ms in duration.
Data acrossall wordscontainingintersonorant clusters are shownin Fig. 15. For comparison, we alsoincludedthe durationof the energydip regionswhen there is only one sonorantconsonantoccurringbetweentwo vowels,an intervocalicnasalor semivowel.In thiscase,theenergyoffsetand energyonsetwill correspond to the consonant onsetandoffset, respectively.Although there is no normalizationfor variabilityin speakingrates,the resultsin Fig. 15 showa distinctpattern.The distributionsof the durationof energy dip regionsassociated with only onesonorantconsonantand those associated with two sonorant consonants where the
firstconsonant isa liquidareessentially thesame.However, the averagedurationof the energydip regionsassociated
syllablenucleus.Thusit maybemoreappropriateto thinkof the liquid and precedingvowelas a diphthongwherethe liquid,like theglidesin thiscontext,isconsidered to bea part of the vowel.
The assertionthat postvocalic/1/acts as the second elementof a diphthonghasalsobeenmadeby GilesandMoll (1975). Basedon x-ray data of prevocalicand postvocalic
/1/, they foundthat postvocalic/1/showsrelativelyslow movementcharacteristicsand undershootof articulatory position.On the otherhand,prevocalic/1/hada relatively highrateof articulatorymovementandno undershoot char-
•'
1704
•'
150 -
E o
'•
130 -
;•
110-
m
with two sonorant consonants where the first is a nasal is
LU
considerably longerthan thoseof the other cases.We can infer from this patternthat the energyoffsetin the cluster wherethe first memberis a postvocalicliquid occursafter the postvocalic liquidsothat onlyoneof the sonorantconsonantsis containedin the energydip region.On the other hand,the energyoffsetin the clusterwherethe firstmember is a postvocalic nasaloccursbeforethe postvocalic nasalso that both sonorantconsonants are part of the energydip region. Theseresultsshowthat postvocalicliquidswhich are followedby anothersonorantconsonant arenot a part of the energydip region.Instead,theyappearto be a part of the
'•
74(}
J.Acoust. Soc.Am.,Vol.92,No.2, Pt.1,August 1992
-
e,-
90-
70-
o
•
5o
Q
30 SC Intersonorant
liquid+SC
nasal+SC
Consonants
FIG. 15.A comparison oftheaverage durations of thenonsyllabic regions ofwordscontaining oneintervocalic sonorant consonant (SC), wordscontaininganintervocalic liquid-sonorant consonant sequence (liquid+ SC) and wordscontainingan intervoealienasal-sonorantconsonantsequence (nasal + SC).
CarolY. Espy-Wilson: Acoustic measures forsemivowels
746
acteristics. Thus theyconcludethe prevocalic/I/ functions as a consonantwhile postvocalie/1/is vocalicin nature. Alongthisline,SproatandFujimura(submitted)postulate that a gestureinvolvinga nonperipheral articulator(suchas tonguedotsumretraction) is attractedto the syllablenucleuswhereasa gestureinvolvinga peripheralarticulator (suchas the tonguetip) is attractedto syllablemargins. With this assumption, they too concludethat postvocalic /l/, whichhasa moresignificanttonguedorsalretraction thanprevocalic/1/,shouldbeconsidered morevocalic.
(F 1, F2, and F31. Given minimal-pairwords,it has been shown(Lisker, 1957;O'Connoret al., 19571that F l separatesthe glides/w/and/j/from the liquids/1/and/r/, F2 separates/w/from/1 r/from/j/, and F3 separatesthe liquids/1/and/r/. The data in thisstudyconcurwith these observations.
D. Formant frequency measures
Important informationfor distinguishingamong the semivowelsare the frequenciesof the first three formants
A formant tracker (Espy-Wilson,19871was usedto automaticallyextractthe first four formantsduringthe sonorant regionsof the wordsin the database.The frequencies off 1,F 2,F 3, andF4 wereestimatedbyaveragingthevalue at the time of a minimum or maximumin a particularformant track and the samplesin the precedingand following frameswithin the hand-transcribedsemivowelregion.In the caseof/w/and/l/, the valuesof the formantswereaveraged
TABLE VI. Formantfrequencies (in Hertz) andformantdifferences (in Hertz andin bark) of semivowels averaged acrossall speakers. Prevocalic
(Hzl FI
F2
F3
F4
w I
381 399
848 1074
2320 2553
3525 3767
r
419
1285
1779
3350
j
317
2142
2827
3661
(Hz) FI-FO
F2-FI
F3-F2
(bark) F4-F3
F4-F2
B I-B0
B2-B
I
B3-B
2
B4-B3
B4-B2
w
241
467
1472
1204
2676
2.4
3.6
6.4
2.5
8.9
I
258
675
1479
1214
2693
2.6
4.9
5.5
2.3
7.8
r
242
866
493
1571
2064
2.8
6.0
2.1
3.9
5.9
j
174
1825
684
1518
1.7
1.7
1.6
3.2
834
10.2
Intervocalic
(Hz) FI w
F2
F3
F4
349
771
2340
3508
445 460
1060 1240
2640 1720
3762 3433
361
2270
2920
3824
(Hz) FI-FO
F2-FI
(bark)
F3-F2
F4-F3
F4-F2
B I-B0
B2-B
1
B3-B2
B4-B3
B4-B2
211
422
1570
1169
2737
2.1
3.4
7.0
2.4
9.4
305 317
610 783
1580 473
1123 1717
2707 2190
3.0 3.1
4.5 5.4
5.8 2.1
2.1 4.2
7.9 6.2
213
1910
648
906
1554
2.1
10.1
1.5
1.6
3.1
Postvocalic
(Hz)
I r
F!
F2
465 503
898 1300
F3
F4
2630 1830
3650 3391
(Hz) FI-FO
I r
747
323 363
F2-FI
433 799
F3-F2
1740 531
(bark) F4,-F3
1015 1554
J. Acoust.Soc. Am., VoL92, No. 2, Pt. 1, August1992
F4-F2
2752 2088
B I-B0
3.2 3.5
B2-B
3.2 5.4
I
/13 //2
6.9 2.1
B4•3'3
1.9 3.7
Carol Y. Espy-Wilson:Acousticmeasuresfor semivowels
D4--B2
8.8 5.8
747
aroundthe time of the F2 minimum.For/j/, the formant valueswereaveragedaroundthe time of the œ2 maximum, and for/r/the formant valueswere averagedaroundthe
effects andspeaker differences andtobettercapture someof theacoustic properties. Chistovich andLublinskaya (1979) havepostulated thatwhentwoformants arewithina critical
time of the F 3 minimum. Thus the formants were measured
distanceof 3.0 to 3.5bark of eachother,theyareinterpreted
duringthe timewhenthe vocaltract couldbeexpectedto be
bytheauditorysystem asonespectral peakwhose frequency is at the centerof gravityof the prominence. Syrdaland Gopal(1986),in an acoustic studyusingthePeterson and Barney(1952) voweldata,investigated whetherthiscon-
most constricted.
We normalizedthe formantsby computingbark differencesto reduce the acousticvariability due to contextual
Prevocali½ Semivowels
(a) 1
1
2
3
4
5
6
B1-B0 (Bark)
Prevocalic Semivowels (b) l
r-.. r
r
/' r ryrrr r
2
4
6
8
10
12
B3-B2 (Bark)
FIG. 16.Scatter plots oftheprevocalic semivowels spoken bytwomales andtwofemales according tothebarktransformed (a) F 2-F 1vsF I-F0 and(b) F 4F3 vsF3-F2. A two-dimensional 90% confidence regionisdrawnaroundthedatafor eachsemivowel.
748
J. Acoust. Soc.Am.,Vol.92, No.2, Pt.1, August1992
CarolY. Espy-Wilson: Acoustic measures forsemivowels
748
stantin auditoryunitsheld betweenseveralformantsand
greatlythe acousticvariabilitybetweenvowelsspokenby
between thefirstformantandthefundamental frequency
different talkers.
(F0). Theyfoundthat witha criticaldistanceof 3 bark,the difference between F 1 andF0 provideda reasonable repre-
The formant frequencies obtainedin this studyare in agreementwith previouslyreporteddata.The resultsacross sentation of thehigh-nonhigh voweldistinctions indepen- speakers areshownin TableVI for prevocalic, intervocalic, dentofspeaker, andthedifference between F 3andF2 repre- and postvocalic semivowels. Also includedin the tableare
sentedthefront-backvoweldistinctions. In addition,they
normalized tbrmant values (F1-F0, F2-F1, F3-F2, and F4-F 3 ) and bark differences(B 1-B 0, B 2-B 1,B 3-B 2, and
found that the bark difference transformations reduced
Intervocalic
Semivowels
(o) 1
w
I
0
1
2
3
4
1
5
B1-B0 (Bark)
Intervocalic Semivowels
lb) I
0
2
4
6
8
10
12
B3oB2(Bark)
FIG. 17.Scatterplotsof the intervocalicsemivowels spokenby two malesand two femalesaccordingto the bark transformed(a) F2-F 1 vsF I-F0 and (b) F4-F3 vsF3-F2. A two-dimensional 90% confidence regionisdrawnaroundthedata for eachsemivowel. 749
J. Acoust. Soc. Am_.Vol. 92. No. 2. Pt. 1. August 1992
Carol Y. Espy-Wilson:Acousticmeasures for semivowels
749
B 4-B 3). (F0 wasobtainedautomatically with thepitchde-
marizesthe classification of the semivowels accordingto a
tector describedin Gold and Rabiner, 1969. ) The distribu-
3.5-bark critical distance criterion and the five bark-differ-
tionsof the bark differences are shownin Figs. 16-18 for the prevocalic,intervocalic,andpostvocalic semivowels, respectively. A two-dimensional 90% confidenceregionis drawn aroundthe datafor eachsemivowel.Finally, Table VII sum-
encedimensions.A +- indicatesthat a majority of the semivowels are within 3.5 bark in the bark-difference dimen-
sion. Conversely,a -- indicatesthat a majority of the semivowels exceedsthe 3.5 bark in the bark-difference di-
Postvocalic Semivowels (o) 1
....... rf
,
rr
r
.................
? r [ rrr•. r•rrS rrrrrrrr ................. i., r r_ _'"-......
•r ',,. r•t •
• r•.•
