Contains report on one research project.U. S. Air Force (Electronics Systems Division) under Contract AF 19(628)-5661National Institutes of Health (Grant 5 RO1 NB-04332-05)National Institutes of Health (Grant 1 PO1 GM-14940-01)Joint Services Electronics Programs (U. S. Army, U. S. Navy, and U. S. Air Force) under Contract DA 28-043-AMC-02536(E

Stevens, Kenneth N.

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XVI. SPEECH COMMUNICATIONAcademic and Research StaffProf. K. N. StevensProf. M. HalleProf. W. L. HenkeM. E. BarronA. J. GoldbergProf. D. H. KlattProf. A. V. OppenheimDr. H. IshidaGraduate StudentsA. R. KentM. F. MedressR. M. SachsDr. Margaret BullowaDr. Paula MenyukDr. J. S. PerkellG. L. WickelgrenJ. J. WolfA. ACOUSTIC CORRELATES OF PLACE OF ARTICULATIONFOR STOP AND FRICATIVE CONSONANTSStop and fricative consonants are generated by forming a narrow constriction orclosure at some point along the vocal tract. In the case of fricative consonants, theconstriction is sufficiently small that air flow through the narrow opening creates tur-bulence, thereby resulting in a random noise source that then forms acoustic excitationfor the cavities that are anterior and posterior to the constriction. For stop consonants,complete closure is effected for a short interval. Immediately upon release of a stopconsonant there is a brief period of turbulence noise generation near the constriction,followed by a transition into the following vowel.A rough indication of the acoustic properties that one would expect when stop or1 b lef '.Fig. XVI-1. Idealized model of constrictedvocal-tract configuration cor-responding to a consonant.The constriction is adjustedto different positions to rep-resent different places ofarticulation.fricative consonants are generated at different points within the vocal tract can beobtained by examining the natural frequencies of a constricted acoustic tube having theconfiguration shown in Fig. XVI-1. This idealized model of the vocal tract has a*This work was supported principally by the U. S. Air Force (Electronics SystemsDivision) under Contract AF 19(628)-5661; and in part by the National Institutes ofHealth (Grants 5 RO1 NB-04332-05 and 1 PO1 GM-14940-01), the Joint Services Elec-tronics Programs (U. S. Army, U. S. Navy, and U. S. Air Force) under ContractDA 28-043-AMC-02536(E).QPR No. 89 199(XVI. SPEECH COMMUNICATION)uniform cross-section area except at the constriction, which is formed by inserting inthe tube a plug in which a narrow hole simulates the opening through which turbulent airflow can occur.The frication noise that is generated at the constriction acts as a source of excitation,and resonance peaks will be obtained in the output sound at the natural frequencies of theacoustic tube. Some resonances will be excited more strongly than others, particularlythe resonances of the portion of the tube anterior to the constriction. It is of interest,therefore, to examine how the natural frequencies of the tube change as the constrictionis moved to different points along the tube. If the constriction is sufficiently small, itcan be assumed that the coupling between the resonances of the sections of the tube ante-rior and posterior to the constriction can be neglected, except when the resonant fre-quencies of the two sections are close together.The resonances of the anterior section, of length if, occur at frequencies c 43c5c f f4P ..... where c is the velocity of sound. We assume for purposes of this rough cal-fculation that the radiation impedance is negligible (compared with the characteristicimpedance of the tube) and that there are no losses. The posterior section (of length fb)is terminated at both ends in an infinite impedance, and its resonances occur, therefore,at frequencies c c ..catb 2 b b 2-bLet us consider a tube of total length 16 cm (roughly the length of the vocal tractfor a male talker), with a constriction of length 3 cm. For this configuration, ff + b =13 cm. The first two resonances of the front cavity, plotted as a function of b,' thelength of the back cavity, are shown as dashed lines in Fig. XVI-2. The solid lines inthe figure represent the resonant frequencies of the back cavity. Points of intersectionof dashed and solid lines correspond to constriction positions for which computed valuesof front and back resonances are equal. Since the constriction provides some couplingbetween the two sections of the tube, the resonant frequencies cannot exactly coincide,but are split apart as shown by the dotted lines. The amount of separation of the tworesonances at these points depends, of course, on the cross-section area of the con-striction. For frequencies above approximately 5000 Hz, the front-cavity resonanceswill generally be highly damped, since the resistive component of the radiation imped-ance becomes comparable to the characteristic impedance of the tube when the diameterof tube becomes comparable to a quarter-wavelength (-1 cm at 5000 Hz). In addition tothe resonances of the front and back portions of the tube, there are resonances of theconstricted portion. For a constriction of length 3 cm, the lowest resonance of theconstriction is at ~5700 Hz, under the assumption of negligible coupling to the front andback sections, and hence is above the frequency range shown in Fig. XVI-2.For a constriction of very small cross-section area, the lowest resonant frequency,or first formant, is essentially zero (except in the limiting case fb = 0). Consequently,QPR No. 89 200(XVI. SPEECH COMMUNICATION)z0z0©Fig. XVI-2./ - F403.03 / F F3032.0 -S2F 21.0 20 .5 I I I I I0 2 4 6 8 10 12 14LENGTH OF BACK CAVITYeb (cm)Relations between natural frequencies and the position of the con-striction for the configuration shown in Fig. XVI-1. The over- alllength of the tube is 16 cm and the length of the constriction is3 cm. Dashed lines represent the two lowest resonances of thefront cavity (anterior to the constriction); solid lines represent thethree lowest resonances of the back cavity. Dotted lines near thepoints of coincidence of two resonances represent the resonantfrequencies when there is a small amount of coupling betweenfront and back cavities. The resonances of a 16-cm tube with noconstriction are shown by the arrows at the right. The curves arelabeled with the appropriate formant numbers.for each value of fb the resonances can be labeled, in order of increasing frequency, asF 2 , F 3 , F 4 , etc., representing the second and higher formant frequencies. The cavitywith which a given formant number is associated depends upon the position of the con-striction. For fb = 9. 5 cm, for example, F 2 is a half-wave resonance of the posteriorcavity, while F 3 is a quarter-wave resonance of the anterior cavity. For fb = 7 cm,the cavity affiliations of F 2 and F 3 are interchanged.As we have noted, there are certain values of kb for which two formant frequenciesare very close together. For the particular dimensions of the model of Fig. XVI-1,F and F 3 coincide for fb = 8. 7 cm, F 3 and F 4 for fb = 10.4 cm (and also forb =5. 2 cm), and F 4 and F 5 for fb = 11. 1 cm. The acoustical behavior of a configuration forwhich front and back cavity resonances approximately coincide has rather special prop-erties. Since the formant frequencies are spread apart in the manner shown by thedotted lines, there is a small range of values of fb surrounding each of these pointswithin which there is no appreciable change in the formant frequencies. That is to say,QPR No. 89 201(XVI. SPEECH COMMUNICATION)the formant frequencies are rather insensitive to perturbations in 2b within theseregions. Furthermore, for such a configuration one would expect that one of the reso-nances would be rather strongly coupled to the velocity fluctuations in the turbulent flow.This would be the "antisymmetric" resonance (the higher of the two resonances of thecoupled system) for which sound-pressure fluctuations on either side of the constrictionare in opposite phases, and thus give rise to a relatively large velocity in theconstriction.Thus for values of 2b corresponding to two coincident resonances, a maximum isobtained in the sound output and the resonant peak is relatively insensitive to small per-turbations in the positions of the constriction. There exist, as it were, certain discreteor quantal constriction positions (places of articulation) that are optimal from the pointof view of sound generation.It is of some interest, then, to examine whether these are indeed the places of artic-ulation that are used in generating the fricative and stop consonants in various languages.The distances fb as read from the chart of Fig. XVI-2 cannot be expected to apply tothe actual vocal-tract dimensions, since the model of Fig. XVI-1 is highly idealized.We note, however, that there are three values of 2b for which pairs of formant frequen-cies coincide below 5000 Hz. The most posterior of these, 2b = 8. 7 cm, correspondingto coincidence of F2 and F 3 , is roughly in the place of articulation for the consonants/k g/. The next critical value of fb is at the point of coincidence of F 3 and F 4 , andwould correspond to the alveolar consonants /s z c j/ and to retroflex consonants suchas /d s/, which occur in many languages. The most anterior of the three points of con-striction is where F 4 and F 5 coincide, and would, perhaps, correspond to the point ofarticulation for /s z t d/.An indication as to whether or not these points of articulation correspond to the threequantal steps predicted by the theory can be obtained by examining acoustic data on thevarious consonants. One would expect to find that the spectrum of the frication noise fol-lowing the release of /k/ and /g/ should be characterized by a peak in energy, and thispeak should then split into the second and third formants during the transition into thefollowing vowel. The separation of F2 and F 3 would occur as the size of the constric-tion is gradually increased from the consonantal configuration. For the consonants/s z c J/, on the other hand, the frication interval should be characterized by a majorspectral peak at a somewhat higher frequency. This peak should split into F 3 and F 4during the transition to the following vowel. Likewise, for the postdental consonantsthe major spectral peak should be at a still higher frequency, and should split into F 4and F 5 during the transition to the following vowel.In order to verify these hypotheses, we have examined a number of consonant-vowelutterances by three speakers, drawn from an inventory of data that have been assembledpreviously.1 Spectral sections were examined (using a computer-generated display)QPR No. 89 202(XVI. SPEECH COMMUNICATION)every 8 msec throughout relevant portions of the utterances. Positions of spectralpeaks were obtained simply through visual examination of the spectra.Examples of some of the data are displayed in Fig. XVI-3. For each CV sequence,the first sample is taken near the beginning of the consonant (at the instant of release inthe case of stop consonants) and locations of major spectral peaks are plotted. Althoughthe data that have been examined are relatively limited and the procedures for collectingthe data are rather imprecise, it is evident that the predictions of the theory are sub-stantially correct. Postdentals (upper part of Fig. XVI-3) are articulated at a pointthat gives the major spectral peak in the range of F 4 and F 5 (apparently at F 5 in theexamples shown) during the frication interval. For alveolars (middle of Fig. XVI-3)the major concentration of energy is in the F3-F 4 region. The fact that both of theseresonances appear to be excited might suggest that the back cavity resonance is actuallythe resonance of the long constriction that is used in generating these sounds. Theoret-ical considerations would predict coupling of the turbulence noise source to both of thesemodes for this configuration. In the case of velars (lower section of Fig. XVI-3), themajor energy concentration for the frication interval coincides with F 2 when the conso-nant precedes a back vowel, and with F 3 when the consonant precedes a front vowel.Although there is a tendency for F 2 and F 3 to merge for the velars, the examples thathave been examined suggest that this effect is not always evident.The curves shown in Fig. XVI-2 not only demonstrate that there are certain regionsof coincidence of pairs of formants for consonants, but also provide an indication of thefrequencies of the formants that would be expected at the start of the transition betweena consonant and a following vowel. Thus, for example, for a value of fb correspondingto coincidence of F 3 and F 4 in the idealized model of Fig. XVI-1, the second-formantfrequency is ~1800 Hz, according to Fig. XVI-2. With no constriction in the tube, thesecond resonance is ~1600 Hz (as shown by the arrow at the right). Thus as the cross-section area of the constriction increases from zero to the size of the larger tube thesecond formant falls from 1800 Hz to 1600 Hz. The second-formant frequency corre-sponding to the constricted configuration, of course, can be manipulated by changing theconfiguration of the back cavity. In the case of the / / configuration, the tongue isadjusted to give a tapered vocal-tract shape posterior to the constriction, therebyyielding a second-formant frequency that is higher than the value indicated inFig. XVI-2.In general, it is evident that two kinds of acoustic variables can be manipulated inorder to generate consonants at different places in the vocal tract. First, selection ofthe primary place of articulation is made by specifying the pair of formants that isapproximately coincident. Once this selection is made, the absolute frequency at whichthe coincidence occurs and the initial frequencies of the formant transitions into thefollowing vowel can be manipulated by adjusting the detailed shape of the vocal tractQPR No. 89 203I 1 i[sE] 2-240- F5F4F3F2-100 0 100[E] 2-270FF3F-100 0 100[g]1-114F4F3o F2-100 0F2-100 0 1000 100TIME FROM ONSET OF VOICING (msec)Fig. XVI-3. Frequencies of spectral peaks for spectra sampled at a num-ber of instants of time preceding and following the onset ofvoicing for several consonant-vowel syllables. Spectra wereobtained from a 36-channel filter bank. Points on the curvesrepresent frequencies of spectral peaks derived from visualobservation of the spectra displayed on a computer oscillo-scope. After voicing onset, the data represent the second andhigher formant frequencies of the vowels.QPR No. 89 204(XVI. SPEECH COMMUNICATION)with the tongue and lips. Such manipulation would correspond to rounding, palatalization,velarization or pharyngealization of the consonant. These aspects of consonant produc-tion require further study, including examination of these classes of consonants as theyoccur in a variety of languages.Other consonant types that must be included in the scheme presented here are thelabials /f v p b/ and the dentals /0 ~/. For the labials there is no anterior cavity, andhence there is no possibility of a coincidence of anterior and posterior resonances.Labial or labiodental fricatives show minor spectral peaks at frequencies correspondingto resonances of the posterior cavity, but these fricatives are of much lower intensitythan sounds generated with a more posterior point of articulation. Labial stop conso-nants in initial position are also characterized by weak frication intervals with no pro-nounced spectral peaks.The dentals /0 6/ are generated with a constriction position anterior to that for/s z/, and hence with an anterior cavity that is shorter. Thus there will be no coinci-dence of a pair of resonances in the frequency range below 5000 Hz, and the conditionsfor appreciable noise energy will not be met. It might be speculated that a coincidencebetween a pair of resonances in the manner we have described is a correlate of thefeature stridency; lack of such a coincidence, therefore, would result in nonstridentconsonant. Such a categorization would then identify labiodentals as nonstridentconsonants.K. N. StevensReferencesA. S. House, "On Vowel Duration in English," J. Acoust. Soc. Am. 33, 1174-1178 (1961).QPR No. 89 205

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