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
"lee"
we
sooo
500O
5000
4000
4000
4000
"re"
.•
,I
•
5000-
I I
%#
4000-
3000 -
@ooo
3ooo-
Hz
Hz
Hz
2000
8000-
1ooo
I000-
o
Hz
POO0
1000-
o
0.454
o
0
o
0.25
5000
4000
4000
3000-
3000 F:•
h
'1
1000
'"'•'
''
0.2
0.48
--
5000-
I 'p
4000•oood
Hz
2000-
•000]
! 000-
logo
•%'!
'rqis
F• . iI•111111m,
.,. ,
0
5000-
0.25
I'rou"
HZ
1
0
"Jou"
3000-
2000 F2
0 FI .ram
0.556
4000-
Hz
1000
o
mmyoum'
5000-
2000
0.48
0 F
0.428
0
0.2g
0.50G
o
TIME (seconds)
0.582
0.556
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
,•
,
"
'"
•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•.•<,•l,r
/'.................
ß
rrf
r
I
I
2
3
4
B1-B0 (Bark)
Postvocalic Semivowels
(b)
1
r
r
/
[
r rrr
r..................
rr .........
/.. 1:,
rr r,•r•
r rrr ',..,...
r •:1['•'[
.......
rtrrrr •rY rr .....
'. r r
i r r r•r•jr•i•?frrr'!
',.
1[ lr¾
r*lr
I 11
:
..... r •g•re
.....
rrrr
rrr •-•r•-•rlrr/.',/}p
.'"f-•• ' Ill• , 1•""7'-...
.......
5 err
I,l • rr,, L ,I'll •..
•, •
'r"-...r..5
....[.......•
..............
•,,,,1'
ll..'
1:1'17!
f!lfl'a,i•11,
I ]I 11111•lll•
2
4
6
8
10
12
B3-B2 (Bark)
FIG. 18.Scatter
plotsofthepostvocalic
semivowels
spoken
bytwomales
andtwofemales
according
tothebarktransformed
(a) F 2-F 1vsF 1-F0 and(b) F 4F 3 vs F 3-F2. A two-dimensional
90% confidence
regionis drawnaroundthedatafor eachsemivowel.
750
d. Acoust.Soc. Am., Vol. 92, No. 2, Pt. 1, August1992
Carol Y. Espy-Wilson:Acousticmeasuresfor semivowels
750
TABLE VII. Semivowd classificationbased on critical distance features in
five bark-difference dimensions.
Dimensions
B I-B0
Semivowels <3.5 bark
B2-B
1
<3.5 bark
B3-B2
<3.5 bark
B4-B3
<3.5 bark
B4-B2
• bark
it produces
an F 3 whichis closeto F4. The "spectralcenter
ofgravity"ofthebroadprominence
formedbyF2, F 3andF4
is in the regionofF3 or higher (Carsonet al., 1970).
SyrdalandGopal (1986) foundthat theB 3-B 2 dimensiondistinguished
betweenfrontandbackvowels.As shown
in Table VII, the B 3-B 2 dimensionclassifies
/r/ and/j/ as
front,and/w/and/1/as back.In thecaseof/w/, thelarge
Prevocalic
w
+
+
--
+
I
+
--
--
+
r
+
--
+
--
j
+
--
+
+
w
+
+
--
+
I
+
--
--
+
r
+
--
+
--
j
+
-
+
+
-
+
-
+
Intervocalic
Postvocalic
I
distancebetweenF 3 andF 2 impliesa closespacingbetween
F 1andF2. As statedabove,the featurebackisstrengthened
in /w/ by the features round and labial (Stevenset al.,
1986). This enhancementis evidencedby the B 2-B I difference in Table VI. If we exclude those semivowels that are in
clusterswith unvoicedconsonants
wheretheyarelikelyto be
at leastpartiallydevoiced,themeanB 2-B 1differencefor the
prevocalic/w/ is reducedfrom 3.6 to 3.1 bark andit remains
substantiallyhigher than 3.5 bark for the other prevocalic
semivowels.
In termsof theirdistribution,69% of/w/productionshave a differenceB 2-B l lessthan 3.5 bark, whereas
only 13% of/1/and 4% of/r/productions fall into this
category. Similarly, in the case of the intervocalic
semivowels,61% of/w/'s have a B 2-B 1 differencelessthan
mension.Below,we discuss
separately
the bark-difference
dimensions
usedto quantifythe acoustic
propertyfor the 3.5 bark whereasonly 26% of/l/'s and 3% of/r/'s fall into
thiscategory.Thus,thespacingbetweenF2 andF 1for most
features
high,back,front,andretroflex.
/w/'s is closeenoughthat they may be perceivedby the
auditorysystemasoneformantaccordingto the conclusions
L High measure
of Chistovichand Lublinskaya(1979). Most of the/w/'s
Segments
designated
as [ + high] are produced
with
with a largerspacingbetweenF 1andF2 areeitheradjacent
the tonguebodyraisedabovethelevelit holdsin the neutral
to a nonbackvowel(s) and/or theyarein an unstressed
conposition.This configuration
of the tonguebodyresultsin a
text suchas thosein the words"withhold" and "periwig."
lowered
F 1.Themeasure
usedtocapture
theacoustic
propAs comparedto the/1/in prevocalicor intervocalicpoertyforthefeaturehighisthedifference
B I-B 0. Syrdaland
sitions,the postvocalic/1/has a much closerspacingbeGopal(1986) foundthat the B 1-B 0 dimension
separated tweenF I andF2. In fact,thedifferenceB 2-B 1for a postvothehighvowels/iI u u/from thenonhighvowels.
calic/1/is comparableto the valuesobtainedfor the/w/'s.
The datain TableVI showthat thedifference
B I-B 0 is,
That is, 84% of postvocalic/l/'s havea differenceB 2-B 1
onaverage,lessthan3.5 barkfor theprevocalic
andintervo- lessthan3.5 bark (lessthan8% of thepostvocalic/r/'shave
calicsemivowels
and,in thissense,
thismeasure
putsthemin
sucha closespacingbetweenF 1 andF2). This differencein
the sameclassas the highvowels.In addition,B I-B 0 is
smallerfortheglides/w/and/j/than fortheliquids/1/and
/r/. Finally,postvocalic
liquidshavethe highestmeandifference.
Thisisnotsurprising
sincetheytendto belessconstrictedthantheprevocalic
allophones.
2. Back-front
measures
In sounds
thatare [ + back], thebodyof thetongueis
retractedfromtheneutralposition,resultingin a loweredF 2
thatiscloser
toF 1thanF 3.Of thesemivowels,
thislowering
ofF2 isespeciallysalientfor/w/since F2 isloweredfurther
by roundingof the lips (introductionof the featureround)
andby a greaternarrowingof thelip opening(introduction
of the featurelabial). For front sounds,on the otherhand,
thetonguebodyisdisplaced
forwardin themouthrelativeto
theneutralposition.Consequently,
F2 is raisedsothatit is
closer
toF 3 thantoF !. Of thesemivowels,
thisF2 raising
is
especially
markedfor/j/. In additionto frontingof the
the valuesof B 2-B 1 for a postvocalic/1/compared
to the
prevocalicand intervocalic/1/supportspreviousfindings
with regardto itsallophonicvariation.That is,a postvocalic
/i/is more velarized,with lessof a constrictionformed by
the tongueblade,resultingin a muchlowerF2, a higherF 1
and,therelbre,a smallerF2-F 1 difference.Thosepostvocalic/l/'s that fall outsidethecriticalrangearenot word-final,
but they are adjacentto the semivowel/j/, a frontedsonorant, in words like "brilliant" and "cellular." Thus the back
articulationof the/1/is influencedby the front articulation
of the following/j/.
Theclassification
of/r/as a frontsonorant
mayseem
a
bit unusual.[SyrdalandGopal(1986) alsofoundthat the
B 3-B 2 dimensionclassifiedthe retroflexedvowel/•/as be-
ingfront.] However,Peterson
andBarney(1952)foundina
perceptualexperimentusingreal speechthat 80% of the
/•/'s that were not heard as such were confusedwith the
front vowels/e/and/•e/. As can be seenfrom the distribu-
tongue,
theproduction
of/j/involvesa raising
ofthetongue tionsinFigs.16-18,therearea few/r/'s ineachplotwitha
bladetowardthe roof of the mouth (in troductionof the fea-
ture coronal),creatinga narrowchannelbetweenthe front
of thetongueandthehardpalate.Thisconfiguration
results
in a further reduction in the distancebetweenF 2 and F 3 and
751
J. Acoust.Sec.Am.,Vol.92, No.2, Pt. 1, August1992
B 3-B 2 differencegreaterthan 3.5 bark. In the caseof the
prevocalic/r/, this largedifference
may be due to "additional" roundingwhich would lower all of the formants.
Thatis,asDelattre
andFreeman
(1968)found,
liprounding
CarolY. Espy-Wilson:
Acoustic
measures
forsemivowels
751
accompanies
all prestressed
prevocalicAmerican /r/'s.
However,from listeningto thesetokensand judging from
their formant frequencies,
it appearsthat speakers,two in
particular,produceda soundthatisa crossbetween/w/and
/r/. For example,in onerepetitionof the word "rule," F2
withinthe/r/is aslowas620 Hz, whichisin theexpected
F2
rangeof/w/. However,F3 is 1240Hz whichis toolow for a
/w/, but in the F3 rangeof/r/. In this case,B 3-B 2 is 4.3
While/r/and/j/both havea closespacingbetweenF3
and F2, the frequencyrangeof this spectralprominenceis
quitedifferent.For/r/, thisprominenceoccursin a midfrequencyrangebetween1000and2000Hz. On theotherhand,
"forewarn" and "harlequin," where they are followedby a
for/j/, thisspectralprominence
occursin a high-frequency
rangebetween2000and 3000Hz. Thus,asstatedabove,the
featurefront for/j/is enhancedby the featurecoronal(Stevenset al., 1986), resultingin a broadspectralprominence
whichincludesF2, F3, andF4. Figures16and 17showthat
/j/always hasa B 3-B 2 difference
lessthan3 bark.In fact,as
canbe seenfrom Table VI, the averagespacingbetweenF2
andF4 isalsowithin thecriticalseparationof 3.0 to 3.5 bark.
/w/or an/1/. In this environment, two effectscan occur.
First, F2 in the/r/is usuallyreducedby the velarizationor
3. Retroflex
bark.
The postvocalic/r/'s with a B 3-B 2 differencegreater
than 3.5 bark are not word-final, but occur in words like
roundingoccurringwithinthesebacksonorants.
Second,the
/r/is often mergedwith the precedingvowelso that the
surfacemanifestationof the voweland following/r/is an
/r/-colored vowel.An examplewherebothphenomenaoccur is shownin Fig. 19, which is a spectrogramof the word
"Norwegian."Both the word-initial/n/and the vowel/•/
are retroflexed.The visibleportionof F 3 at the beginning
andendof thefirstsonorantregion(60 to 300ms) aswell as
the automaticallyextractedF3 track showthat the lowest
pointofF3 occursat thebeginningof the/n/around 60 ms.
F2 duringthe regionthat is transcribedas/r/becomes as
low as 612 Hz due to the/w/.
measure
The majoracousticconsequence
of the featureretroflex
appearsto be a low third formant. For a typical male
speaker,the frequencyofF3 for an/r/is usuallyat or below
2000 Hz. As a result,F3 and F4 are usuallywell separated
whereas the difference between F3 and F2 is small. The data
in Table VI showthat/r/can normallybe separatedfrom
the other semivowelsbasedon the differenceB 4-B 3, which
isusuallygreaterthan 3.5bark for/r/and lessthan3.5bark
for the other semivowels.However, there are some/r/'s
with a B 4-B 3 difference that is less than 3 bark.
The exceptions
occurfor severalreasons.First, in addition to loweringF3 within/r/, somespeakerslower F4 as
Finally, the situationfor the intervocalic/r/ for which
B 3-B 2 fallsoutsidethe3.5-barkrangeissimilarto thatof the
postvocalic
allophones.
That is,eitherthe/r/is mergedwith
a precedingvowelor it occursin wordslike "already"and
"bulrush"wherean underlyingand preceding/1/was not
transcribed,but the precedingsonorantregionis produced
ablyaffectedby the articulationof a followingsoundsothat
F3 ishigherthannormal.Finally,for someword-final/r/'s
F3 doesnotgetverylow,suggesting
thatthedegreeof palatal constrictionmay be lessened.
with a back articulation which results in a lowered F 2 within
E. Formant
the/r/.
The widespreadin thedistributionof averageformant
valuesgivenin Figs. 16-18 showsthat the formantfrequenciesof the semivowels
are affectedby adjacentsounds.Becauseof this contextualeffect,thereis someoverlapin the
"Norwegian"
well. Second,the articulation of/r/is
transition
sometimesconsider-
measures
distributions of the bark differences for the semivowels. For
example,there is substantialoverlap betweenthe formant
distributions
of/w/and/1/in Figs. 16and 17.However,the
figuresalsoshowthat the influenceof adjacentsoundscould
8
lead to confusions between/w/and/r/based
differences alone. Most of the/w/'s
and/r/'s
on the bark
which have
similar formant values are in clusters with unvoiced conson-
antsandtheyarepartiallydevoiced,sothat theinformation
whichdistinguishes
betweenthemisoftenoutsideof thesonorant regions.In the few caseswhere voiced/w/and/r/
productions
are confused
on the basisof their formantval-
kHz
i
u•g, they are digtinguighableif the formant transitionsare
takeninto account.Thus,in additionto thespacingbetween
the formants, the formant transitions are sometimesneeded
to helpmakecertaindistinctions.
To determine the direction and extent of these formant
o.o
o.i
o.2
0.3
o.4
o.5
o.6
Time(seconds)
FIG. 19.Shownisa widebandspectrogram
of theword"Norwegian"with
automaticallyextractedformanttracksoverlaid.The phoneticallytranscribed/r/shows a largeseparationbetweenF3 and F2 which resultsin a
differencegreaterthan 3.5 bark.
752
J. Acoust.Sec. Am., Vol. 92, No. 2, Pt. 1, August1992
movements,the average semivowel formant values were
subtractedfrom the averageformant valuesof the adjacent
vowel(s). The targetvowelformantfrequencies
werecomputedfromthevalueat thetimeof themaximumF 1frequency within the hand-transcribed
vowelregionandthe values
in the previousandfollowingframes.
Carol Y. Espy-Wilson:Acousticmeasuresfor semivowels
752
L FI transitions
The averageand standarddeviationof the F 1 differ-
ences
between
vowels
andadjacent
semivowels
areplotted
in
Fig.20. ThedatashowthatF 1 normallyincreases
froma
prevocalic
semivowel
into the followingvowel.F 1 is also
consistently
lowerin anintervocalic
/w/ and/j/ relativeto
itsvalueintheadjacent
vowels.
Thisisalsothecaseformany
intervocalic/r/'s
and/l/'s. However,
whentheliquidsare
adjacentto both a high voweland a low vowel,their F 1
values will sometimes lie somewhere between the F 1 fre-
quencies
of theneighboring
sounds.
In thefewcases
wherea
postvocalie
liquidhada higherF 1thanthatofthepreceding
vowel,the vowelis characterizedas highand, therefore,it
normallyhasquitea low F 1 frequency.
Examplesof such
words are "cartwheel"
and "clear."
2. F2 transitions
The averageand standarddeviationof the F2 differencesbetweenvowelsandadjacentsemivowels
areplottedin
Fig. 21. The datashowthatF2 generallyincreases
betweena
/w/ or /1/ and an adjacentvowel,and it usuallydecreases
betweena/j/and an adjacentvowel.However,betweenan
/r/and surrounding
vowels,F 2 mayincrease
or decrease.
In
1ooo
350'
(a)
(a)
300
500
250
200
N
~
150
o
100
0'
-50'
-lOO0
-100
w
j
I
w
r
j
I
r
semivowels
semivowels
1000'
350'
300'
250'
[]
200'
N
W
Oj
N
150'
A
ß
100'
I
r
50'
0'
-lOOO
..............
-lOOO :5'ob
-100
'50
0
50
100
Hz
150
200
250
5oo
io'oo
Hz
1ooo
(c)
(c)
250'
20O-'
150'
N
100'
50-'
0-'
-5o'
- 1 O0
I
r
semivowels
FIG. 20. Averagesand standarddeviationsof the differences(in Hz)
tweentheaverage
F 1frequency
of (a) prevocalic
semivowels
andfollowing
vowels,(b} intervocalic
semivowels
andsurrounding
vowels(the change
relativeto the precedingvowelis on the horizontalaxisand the change
relativeto thefollowingvowelison theverticalaxis},and(c) postvocalic
semivowels
and precedingvowels.
753
J. Acoust.Sec.Am.,Vol.92, No.2, Pt. 1, August1992
-lOOO
semivowels
F1G. 21. Averagesand standarddeviationsof the differences
(in Hz) betweentheaverage
F2 frequency
of (a } prevocalic
semivowels
andfollowing
vowels,(b) intervocalic
semivowels
andsurrounding
vowels(the change
relativeto the preceding
vowelis on the horizontalaxisand the change
relativeto thefollowingvowelis on theverticalaxis),and (c) postvocalic
semivowelsand precedingvowels.
CarolY. Espy-Wilson:
Acousticmeasuresfor semivowels
753
1 ooo
thecaseof prevocalic/r/, a decrease
in F 2 from/r/into the
followingvowelmainlyoccurredwhen/r/was in a cluster
witha preceding
coronalconsonant
suchasthe/d/in "with-
(a)
500
draw." In addition, there were a few caseswhere the F2
differencesbetweenthe vowel/u/and the precedingwordinitial/r/in the words"rule" and "roulette" werealsonegative. However,in all but onecase,therewasan initial risein
F2 from the/r/before
N
it fell into its lower value for the/u/.
-500
ThistypeofF2 trajectorywasalsonotedby Lehiste(1962).
As for intervocalic/r/, most have a lower F2 value than
-1000
that of adjacentvowels.However,if an intervocalic/r/ is
w
preceded
byabackvowel
a•nd
followed
byafrontvowel,
asin
j
I
"chlorination" ( [kbrlne ysan] ), then there may be a rise in
F2 from the back vowelthroughthe/r/and into the front
vowel.Likewise,if the/r/is precededby a front voweland
followedby a backvowel,asin "heroin" ( [here•' In] ), then
F2 may fall steadilyfrom the front vowel through the/r/
1 ooo
(b)
,+
500
and into the back vowel.
Finally, in the caseof postvocalic/r/ and preceding
vowels, F2 may increaseor decrease,depending upon
whether the vowel is front or back. That is, if the vowel is
back,F2 may risewhile F3 falls, narrowingthe difference
r
semivowels
"
0
o j
ß
r
-500
betweenF3 and F2. However, if the vowelis front, both F2
andF3 will fall into the appropriatevaluesfor an/r/.
-1000
-1000 .........
-500
3. F3 transitions
The averageand standarddeviationof the F 3 differencesbetweenvowelsandadjacentsemivowels
areplottedin
Fig.22.ThedatashowthatF 3 isalmostalwayssubstantially
higherin/j/than it isin an adjacentvowel.In thefewcases
(•.........
500
1000
Hz
1 ooo
(c)
500
when this is not true, another coronal consonantwas nearby,
suchasthe/1/in "uvula" ( [juvjula ] ). In wordslike thisF 3
steadilyrosefrom its valuein the/j/to a somewhathigher
value in the nearbyconsonant.
As expected,Fig. 22 showsthat F3 is almostalways
substantially
lower in/r/than it is in an adjacentvowel.
However, there are severalinstanceswhere F3 for/r/is
comparable
to or higherthanthat of the preceding
vowel.
With theexception
of theintervocalic
/r/ in onepronunciationof theword"guarani,"these/r/'s occurin postvocalic,
but not word-final,position.That is, they are alwaysfollowed by another consonant,such as the /r/'s in "cartwheel,""harlequin,"and "Norwegian."An exampleof this
typeoff 3 trajectoryisshownin thespectrogram
of theword
"cartwheel"in Fig. 13.As canbeseen,thelowestpointoff 3
within the/or/region (0.1 to 0.2 s) occursnearthe beginningof the/o/. Acoustically,
thevoweland/r/appear to be
completelyassimilatedin that no discernibleacousticcue
pointsto separate/o/and/r/segments. Thusit appearsasif
the/o/and following/r/are mergedintoan r-coloredvowel.
The F 3 transitionfor/w/and an adjacentvowelcanbe
positiveor negative.A negativeF 3 transitionfrom/w/into
an adjacentvowelmay seemsurprisingsince/w/is pro-
-500
-1000
semivowels
FIG. 22. Averagesand standarddeviationsof the differences(in Hz) betweentheaverageF3 frequencyof (a) prevocalicsemivowels
andfollowing
vowels,(b) intervocalicsemivowels
and surroundingvowels(the change
relativeto the precedingvowelis on the horizontalaxis and the change
relativeto the followingvowel is on the verticalaxis), and (c) postvocalic
semivowelsand precedingvowels.
precedingretroflexedvowelisabout300Hz, andthe average
decrease
in F 3 intoa followingretroflexedvowelisabout200
Hz. An exampleof this phenomenoncanbe seenin the spectrogramandformanttracksof theword"froward,"whichis
displayedin Fig. 23. AlthoughF3, dueto itslow amplitude,
is not alwaysvisiblewithin the/w/the directionof the F3
movement can be inferred from the visible transitions in the
adjacentvowels,andit isapparentin theaccompanying
for-
duced with a labial constriction. However, we found this to
mant tracks.
be the casemainly when/w/is adjacentto a retroflexed
vowel.The averagechangein F 3 betweenprevocalic/w/
and followingretroflexedvowelsis about -- 215 Hz. In the
caseof intervocalic/w/, the averageincreasein F 3 from a
If we excludethose/w/'s whichare eitheradjacentto a
retroflexedsoundor one segmentremovedfrom a retroflexedsound(e.g., "guarani"),the averageincreasein F 3
754
J. Acoust.Sec. Am., Vol. 92, No. 2, Pt. 1, August1992
froma prevocalic
/w/ intoa followingvowelis 105Hz. For
Carol Y. Espy-Wilson:Acousticmeasuresfor semivowels
754
"froward
siderableoverlap remainsin the distributionsfor/w/and
/1/.
Subsequent
work (Espy-Wilson,1989) suggeststhat
5
other featuressuchas coronaland labial maybeusefulin
distinguishing
betweenthesetwo sounds.Otherinformation
4
suchasphonotactic
constraints
(/1/can occurin postvocalie positionwhile/w/cannot) and the directionof the F3
3
transitionbetweenthe semivoweland adjacentvowel are
kHz
alsohelpfulin makingthe/w/-/1/distinction.
In additionto the overlapbetweenthe distributionsfor
/w/ and /1/, overlap between the distributionsfor the
semivowelsand other classesof soundsshowthat a recognition systembasedonlyonthe acousticpropertiesfor features
investigated
in thisstudywill notalwaysbeableto recognize
correctlythe semivowels.
0.0
OI
Q2
0.3
0.4
0.5
0.6
In light of our generalgoalof a semivowelrecognition
Time(seconds)
system,it is importantto understandwhy this overlapoccurred. From our analysis,we attribute the exceptionsto
FIG. 23.An illustration
of F3 movement
between/w/andnearbyretroseveralreasons.First, the resultsof thisstudysuggestrefineflexedsounds
in "froward."Shownisa wideband
spectrogram
withautomaticallyextractedformant tracksoverlaid.
mentsin the acousticproperties.For example,the feature
highgroupsall of the prevocalicsemivowels
togethereven
thougha divisionshouldbe madebetweenthe liquidsand
glides.This lack of discriminationsuggests
that other conintervocalic/w/'s, theaverageincreasein F 3 intothefollowsiderations
mayneedto betakenintoaccountin thedevelopingvowelis70 Hz andtheaverageincrease
intothepreced- ment of the appropriateacousticpropertiesfor features.In
ing vowelis 164Hz.
particular,it may be the casethat the acousticcorrelatesof
Finally,theF 3 differences
involving/I/showthatF 3 is someor all of thefeaturesdifferdependinguponwhetherthe
almost
always
substantially
higherin/1/relativetoapreced- soundisvocalicandthusthe vocaltractisrelativelyopen,or
ingvowel,andthat thereis usuallylittle changein F3 between/1/and a followingvowel.Thesedatasupportpre-
viousfindings(Lehiste, 1962) whichshowthat F3 for/1!
tendsto be equalto or higherthan that of adjacentvowels.However,as can be inferredfrom the standarddeviations,
thereareseveralinstances
wherea prevocalic
andintervocalic/1/had a substantially
lowerF 3frequency
thanthatofthe
adjacentvowel.This phenomenon,
which usuallyoccurs
when/1/is adjacentto a front vowel,wasobservedin words
suchas"leapfrog"and"Swahili,"whereF3 is alreadyat a
highfrequency,
closeto F4.
IV. GENERAL DISCUSSION
The resultsof this investigation
have shownthat the
acousticmeasuresusedin this studyfor the featuressonor-
ant,syllabic,
consonantal,
high,back,front,
andretroflex
are,
for the mostpart,extractingrelevantinformationfromthe
speech
signal.That is,thequantified
acoustic
properties
for
thesefeatures
generallyseparate
thesemivowels
fromother
nonvocalic and thus the vocal tract is more constricted.
Second,there is the segmentationproblem.As the F3
transitiondata for/r/of
Sec. III E show, speechsounds
often overlap, at least to someextent, so that someof the
strongest
acousticevidencefor a featurethat is distinctive
for a particularsoundmayoccuroutsideof the regiontranscribed for Ihat sound.
Finally,labelingof thesegments
isan issue.The labeling
of somesoundsis inherentlysubjectiveevenwith a phonemic transcriptionavailableasa reference.In somepronunciations of words like "flower," a clear/w/will
be heard and
in othersthe presenceofa/w/will bequestionable.
In addition, thereare featurechangeswhich occurin somecontexts,
but they are not anticipatedin the transcription.For in-
stance,even though the underlying/v/ in "everyday"
shownin Fig. 5 is realizedas a SOhorant
consonantas opposedto a fiicativeconsonant,
the labelassigned
to thisportion of the speechsignaldoesnot reflectthis largeacoustic
change.Thus, mismatchesbetweenassignedlabelsand the
expectedacousticpropertiesare to beexpected.By relating
the measuredacousticpropertiesto the articulartorycorre-
soundsanddistinguish
amongthesemivowels.
The acoustic
measures
aresummarized
in TableV. Theacoustic
property
for -- syllabicseparates
the semivowels
fromvowelsandthe
lates for features, we are able to understand these feature
acoustic
propertyfor + sonorant
separates
the semivowels modificationsaschangesin articulation.
from most consonants(the nasalsare also sonorantconsonAs the resultsofSecs.IIIA andC show,the semivowels
ants). In addition,as hasbeenshownin paststudies,the
canin generalbeseparated
fromothersounds
(exceptna-
acoustic
propertyfor + retroflex
separates/r/from/wj I/
andtheacoustic
propertyfor +front separates/r
j/from
/w 1/. However,nooneacoustic
measure
investigated
in this
study provides a clear distinction between /w/
sals)onthebasisof theacoustic
properties
for thefeatures
+ SOhorant
and -- syllabic.However,blurringof thisdistinction between semivowelsand most other soundsdoes
and /1/.
sometimes
occur (e.g., the exampleof "everyday"menWhile the acousticpropertyfor + consonantal
separates tionedabove).Thisapparent
neutralization
is caused
by a
prestressed/1/from the other semivowelsand the acoustic
reduction
in thedegree
oftheconstriction
normally
madein
propertyfor + backseparates
many/w/'s from/j r 1/, contheproduction
ofconsonants--usually
poststressed
consort755
J. Acoust.Soc. Am.,Vol.92, No. 2, Pt. 1, August1992
CarolY. Espy-Wilson:
Acousticmeasuresfor semivowels
755
"disreputable"
studyisfeaturespreading(cfi Henke, 1966;Moll and Daniloff, 1971;Kent et al., 1974). The F3 transitiondata of Sec.
III E 3 point to the mergingof postvocalic/r/ with preceding vowels,resultingin r-coloredvowels.Suchassimilation
is evidentin Fig. 13betweenthe/a/and/r/in
"cartwheel"
andin Fig. 19betweenthe/n/,/a/and/r/in
"Norwegian."
Espy-Wilson(1991) foundthat this anticipationof a postvocalic/r/ dependson severalfactorsincludingspeaking
rate, consonantal
context,and speakerdifferences.
In addition,we haveobservedthe backwardspreading
of retroflexionfrom a prevocalic/r/, acrossa labial consonant, to a precedingvowel.An exampleof this occurrence
can be seenin the word "everyday" of Fig. 5. The lowest
point of F 3 occursduringthe/v/which not only is retroflexed,but alsois lenited.From a speechproductionviewpoint, retroflex spreadingin this context is not surprising
(kHz)
Ibl
Ol
o
1oo
200
300
400
500
600
700
TIME (ms)
FIG. 24. A widebandspectrogram
of theword"disreputable"
whichcontains a lenited /b/ that resembles a/w/and
an/1/.
since the/v/is
a labial consonant and, therefore, does not
require a particular placementof the tongue. Thus the
tongueconfigurationfor the/r/can be anticipatedduring
the consonantandpossiblyearlier.
V. SUMMARY
ants.The term oftenusedto refer to this type of variability is
lenition (eft Catford, 1977). Data from Sec. III A show that
underlyingintersonorantvoicedstopsand voicedfricatives
are sometimessufficientlyweakenedthat they surfaceas
voicedapproximantswhich exhibitthe propertyof sonorancy.Most of the lenitedconsonantsobservedin this study
areprecededby a vowelwith a higherdegreeof stressthan
the voweloccurringeitherafterthemor afterthe following
sonorant consonant.In some cases,theseweakened consonants bear a close resemblance to one or more semivowels.
For example,the/b/in "disreputable"shownin Fig. 24 is
acousticallysimilar to a/w/and an/1/. In addition to being
SOhorant,
F 1,F 2, F 3, andF 4 at thelowestpointoff 2 during
the/b/(around 540 ms) are 348, 820, 2714, and 3672 Hz,
respectively.
Thesevaluesare closeto the averageformant
frequencies
listedin Table VI for intervocalic/w/and/1/.
In conclusion,we havestudiedseveralacousticproperties of the semivowelswhich generallyseparatethem as a
class from other speechsounds,and which distinguish
amongthem.In addition,wehaveobserved
howtheacoustic
propertiesof the semivowels
andof othersoundscanchange
asa functionof context.The observedvariabilitycanbe understoodin termsof productionand, at the more abstract
level of features,in termsof lenition and assimilation.These
featurealteringprocesses
can occurindependentlyand simultaneously.Understandingvariability in terms of how
andwhenfeaturesmay changeandwhichfeaturesareinvariant hasimplicationsfor the representationof lexicalitems.
Suchknowledgeshouldnot only help usunderstandthe humanspeechsystem,but it may alsocontributeto the developmentof feature-based
systems
for speechrecognitionand
to the synthesisof natural soundingspeech.
Furthermore, since/b/is a labial consonant,it has formant
transitions similar to those of/w/.
Data from Secs.III C and
C 3 show that poststressed
intervocalicand postvocalic
semivowels
arealsosometimes
weakenedsothat theyarenot
nonsyllabic.Instead,they appearto form a part of the syllable nucleus.
ACKNOWLEDGMENTS
Thisworkwassupported
in partby a XeroxFellowship
and NSF Grant BNS-8920470.It is basedin part on a Ph.D.
thesisby the authorsubmittedto the Departmentof Electri-
cal Engineering
andComputerScienceat MIT. The author
gratefullyacknowledges
the encouragement
andadviceof
lenitionistheseparation
of/l/from theothersemivowels
on
ProfessorKenneth N. Stevens.I alsowant to acknowledge
thebasisof theacoustic
propertyforthefeatureconsonantal. the assistanceof Marie Huffman, SuzanneBoyce, Sharon
The resultsof Sec.III B showthat not all prevocalic/l/'s are
Manuel, and Corine Bickley, whosecommentsgreatly imassociatedwith abrupt spectralchanges.As statedin Sec.I,
provedthe quality and clarity of this paper.The editorial
severalresearchers(Joos, 1948; Fant, 1960; Daiston, 1975)
comments
andsuggestions
of Jesse
G. KennedyIII, JohnH.
havenoticedsometype of sharpspectraldiscontinuitybeSaxman,and two anonymousreviewersare alsoverymuch
tween/1/and a following vowel. In agreementwith this
appreciated.
Specialthanksto JeffreyMarcusand David
observation,the data in Sec. III B show abrupt spectral Goodinefor their technicalassistance
in generatingthe forchangesbetween/1/and followingstressedvowels.Howmant plots.
Another distinction which is sometimesobscuredby
ever, the resultsalsoshowthat the spectralchangebetween
prevocalic/1/(as well asotherconsonants)
and following
unstressed
vowelscanbequitegradual.In thiscase,the prevocalic/1/does not appearto be consonantal.
Another categoryof variabilityobservedoften in this
756
J. Acoust.Soc. Am., Vol. 92, No. 2, Pt. 1, August1992
tThepreboundary/l/'soccurred
beforephonological
boundaries
which
variedin strengthsothat in somecases(e.g.,beforea majorintonation
break)the/1/is syllable
finalandin othercases(e.g.,between
twovowels)
it is unclearwhetherthe/1/is syllablefinalor syllableinitial.
Carol Y. Espy-Wilson:Acousticmeasuresfor semivowels
756
2Achange
in thephonological
representation
ofthese
sounds
isnotbeing
arguedfor. However,for the kindsof phonetic
characterizations
being
made,it is appropriateto separate/w/from/I r/from/j/in
the front-
back continuum.
3Frames
occurat 5-msintervals
andtheyareobtained
bywindowing
the
speech
signalwith a 25.6-msHammingwindow.
Bickley,C., and Stevens,K. N. (1986). "Effectsof a Vocal Tract Constriction on the Glottal Source:Experimentaland Modeling Studies,"J.
Phon. 14, 373-382.
Bladon,R. A. W., and AI-Bamerni, A. (1976). "CoarticulationResistance
Allophones
of English/1/,"Phonetica
31,206-227.
Henke,W. (1966). "DynamicArticulatoryModelof SpeechProduction
UsingComputerSimulation,"
Ph.D. dissertation,
MIT, Cambridge,
MA.
Jakobson,
R., Fant,G., and Halle, M. (1952). "Preliminaries
to Speech
Analysis,"MIT Acoustics
Lab.Tech.Rep.No. 13.
Joos,M. (1948). "AcousticPhonetics,"Lang.Suppl.24, 1-136.
Kameny,I. (1974}. "Comparisonof the Formant Spacesof Retroflexed
andNon-retroflexed
Vowels,"IEEE Symp.SpeechRecog.80-T3-g4-T3.
Kent, R., Carney, P., and Severcid,L. (1974). "Velar Movementand Tim-
ing:Evalua,,ion
ofa Modelfor BinaryControl,"J.Speech
Hear.Res.17,
470-488.
in English/1/," J. Phon.4, 137-150.
Bond,Z. S. (1976), "Identificationof VowelsExcerptedfrom/i/and/r/
Lehiste,I. (I962). "Acoustical
Characteristics
ofSelected
EnglishConsonants,"Reix,rt No. 9, Universityof Michigan,CommunicationSciences
contexts," J. Acoust. Soc. Am. 60, 906-910.
Carlson,R., Oranstrfm, B., and Fant, G. (1970). "SomeStudiesConcern-
Laboratory, Ann Arbor, MI.
Lisker, L. (1957). "Minimal Cuesfor Separating/wj,r,I/in Intervocalic
Position," Word 13, 256-267.
Moil, K., and Daniloff, R. (1971). "Investigationof the Timing of Velar
MovementsDuring Speech,"J. Acoust.Soc.Am. 50, 678-694.
ing Perceptionof IsolatedVowels,"SpeechTransmiss.Lab.:Q. Pros.
Stat. Rep. 2-3, 19-35.
Catford, J. C. (1977). Fundamental Problemsin Phonetics(Indiana U. P.,
Bloomington,IN).
Chistovich,L. A., and Lublinskaya,V. V. (1979). "The 'Centerof Gravity'
Effect in Vowel Spectraand Critical Distancebetweenthe Formants:
Psychoacoustical
Studyof the Perceptionof Vowel-likeStimuli,"Hear.
Res. 1, 185-195.
Chomsky,N., andHalle,M. (1968). TheSoundPatternofEnglish(Harper
and Row, New York).
Cyphers,D. S. (1985), "Spire:A Research
Tool," S.M. thesis,MIT, Cambridge,MA.
Daiston,R. M. (1975). "AcousticCharacteristics
of English/w,r,I/Spoken Correctlyby YoungChildrenand Adults," J. Acoust.Soc.Am. 57,
462-469.
Delattre, P., and Freeman, D. (1968). "A Dialect Study of American R's
by X-ray Motion Picture," Language44, 29-68.
Espy-Wilson,
C. Y. (1987). "An Acoustic-Phonetic
Approachto Speech
Recognition:
Applicationto theSemivowels,"
TechnicalReportNo. 531,
ResearchLaboratory of Electronics,MIT.
Espy-Wilson,C. Y. (1989). "The Influenceof SelectedAcousticCueson
thePerception
of/I/and/w/," J. Acoust.SOc.Am. Suppl.1 86, S49.
Espy-Wilson,
C. Y. (1991). "Consistency
in/r/Trajectories in American
English,"Proc. 12thInt. Cong.Phon.Sei.4, 370-373.
Fant, G. (1960). deousticTheoryof SpeechProduction(Mouton, The
Netherlands).
Gold,B.,andRabiner,L. (1969). "ParallelProcessing
Techniques
for EstimatingPitchPeriodsof Speechin the Time Domain,"J. Acoust.SOc.
Am. 2, 442-448.
O'Connor, J. D., Gertsmart, L. J., Liberman, A.M.,
Delattre, P. C., and
Cooper, F. S. (1957). "AcousticCues for the Perceptionof Initial
/wd,r,l/in English,"Word 13, 24-43.
Peterson,G. E., and Barney,H. (1952). "Control MethodsUsed in a Study
of the Vowels," J. Aeoust. Soc.Am. 24, 175-184.
Scheft,S. (1986). "A ComputationalModel for the PeripheralAuditory
System:
Applicationto SpeechRecognition
Research,"
IEEE Int. Conf.
Acoust.SpeechSig.Process.
3, 1938-1986.
Shipman,D. W. (1982). "Developmentof speechresearch
softwareon the
MIT lispmachine,"J. Acoust.Soe.Am. Suppl.1 71, S103.
Sproat,R., and Fujimura,O. (submitted)."On the Natureof Allophonic
Variation and the Phonology/Phonetics
Mapping:the Caseof/I/in
English,"J. Phon.
Stevens,K. N., Keyset, S. J., and Kawasaki, H. (1986). "Toward a Phonetic and PhonologicalTheory of RedundantFeatures,"lnoarianceand
Variabilit?in SpeechProcesses,
editedby J. Perkelland D. Klatt (Erlbaum,Hillsdale, NJ ), pp. 426-449.
Stevens,K. N., and Keyset, S. J. (1989). "Primary Featuresand their Enhancementin Consonants,"
Language65, 81-106.
Stevens,K. N.. "AcousticPhonetics"(in preparation).
Syrdal,A. K., and Gopal, H. S. (1986). "A PerceptualModel of Vowel
Recognition
Basedon theAuditoryRepresentation
of AmericanEnglish
Vowels," J. Acoust. Soc. Am. 79, 1086-1100.
Zue,V., Cyphers,D., Kassel,R., Kaufman,D., Leung,H., Randolph,M.,
Scheft,S., Uaverferth,J., and Wilson,T. (1986). "The Developmentof
the MIT LISP-MachineBasedSpeechResearchWorkstation,"IEEE
Giles,S. B., and Moll, K. L. (1975). "Cinefluorographic
Studyof Selected
Int. Conf. Acoust.SpeechProcess.,7.6.1-7.6.4.
757
Carol Y. Espy-Wilson:Acousticmeasuresfor semivowels
J. Acoust.Soc. Am., Vol. 92, No. 2, Pt. 1, August1992
757