Contains research objectives and reports on two research objectives.U.S. Air Force (Air Force Cambridge Research Center, Air Research and Development Command) under Contract AF19(604)-6102National Science Foundatio

Fujimura, O.

Halle, Morris

House, A. S.

Stevens, Kenneth N.

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XVIII. SPEECH COMMUNICATION'Prof. M. Halle Jane B. Arnold H. FujisakiProf. K. N. Stevens C. G. Bell M. H. L. HeckerProf. J. B. Dennis P. T. Brady J. M. HeinzDr. A. S. House O. Fujimura C. I. MalmeDr. T. T. Sandel G. RosenRESEARCH OBJECTIVESAt present, work on the following topics is in progress: studies of the generationand perception of certain classes of consonant sounds, utilizing an electrical speechsynthesizer; analysis of speech by means of a digital computer, particularly the extrac-tion of low-information-rate signals from speech signals; studies of human perceptionof speech-like sounds, particularly the categorization and learning of members ofmultidimensional auditory displays; studies towards the characterization of cursivewriting, which has been shown to possess striking structural parallels with speech.The main problems that we are now pursuing in our research are: (a) What are thecapabilities of the human vocal apparatus as a source of speech signals? What limita-tions are imposed on the signal by the restricted control that man has over his vocalorgans? (b) How does man perceive sounds, in general, and speech-like sounds, inparticular? What are the limitations of the human organism in dealing with acousticstimuli that serve as vehicles for the identification of messages? (c) What are the phys-ical properties of utterances in various languages? In particular, what are the propertiesthat serve to distinguish utterances that are different from each other? (d) What is thenature of human language, in general, and of various national languages, in particular?How does the structure of the language determine the physical properties of utterances?M. Halle, K. N. StevensA. FORMANT BANDWIDTH AND VOWEL PREFERENCEThe extent to which the bandwidths of vowel formants play a role in the perception ofspeech was investigated by means of a response-oriented experiment. The stimuli wereproduced by a resonance type of electrical synthesizer and represented five commonvowels of American English. Seven variants of each vowel, differing only in values offormant bandwidth, were synthesized, paired in all intravowel combinations, and storedon magnetic tape for presentation to listeners. Eight listeners selected the "most repre-sentative" item in each stimulus pair. The vowel in question was always known beforepresentation, and appropriate methodological procedures were followed.The essential findings are displayed in Fig. XVIII-1, where for each vowel the aver-age number of times that a given formant condition was preferred is plotted in percent-ages of the maximum possible score. The horizontal axis displays the conditions, withthe narrowest bandwidths (A; Fl = 40 cps, F2 = 50 cps, F3 = 60 cps) at the extremeleft, and progressively wider values at the right (for example, G; Fl = 220 cps, F2 =260 cps, F3 = 300 cps). For convenience, the five sets of responses are displayedtogether with overlap along the abscissa; the average responses for each vowel areThis research was supported in part by the U.S. Air Force (Air ForceCambridge Research Center, Air Research and Development Command) underContract AF19(604)-6102; and in part by National Science Foundation.172(XVIII. SPEECH COMMUNICATION)AB C D E F GBANDWIDTH CONDITIONSFig. XVIII-1. Average preferences of 8 subjects for the seven bandwidth conditionsof 5 vowels. Bandwidth conditions (A, B, ... ) are displayed along theabscissa. To facilitate visual comparison, the response curves areplotted with overlapping abscissae, and abscissa values are shownonly for [i]; responses on each curve are arranged in the same order.The curves were smoothed by visual inspection.ordered A, B, ... , G, however. The curves through the response averages weresmoothed by visual inspection.The curves of Fig. XVIII-1 illustrate a general reduction in the subjects' preferencesfor vowels when formants are made sufficiently wide, and also suggest a systematicvowel-to-vowel variation of the most-preferred bandwidth condition. If the smoothingimposed upon the data is reasonable, the curves indicate that preference varied as afunction of tongue height in the classical vowel diagram.These data have been discussed at greater length in a meeting paper (1) and in areport (2) prepared for publication.A. S. HouseReferences1. A. S. House, Effect of formant bandwidth on vowel preference, a paper presentedat the Thirty-Fifth Annual Convention, American Speech and Hearing Association,Cleveland, Ohio, Nov. 11-14, 1959.2. A. S. House, Formant bandwidth and vowel preference (to be published inJ. Speech Hearing Res.).173(XVIII. SPEECH COMMUNICATION)B. HIGH-SPEED MOTION-PICTURE STUDY OF THE MOVEMENT OF THE LIPSThe movement of the lips when bilabial stops and nasals are pronouncedin various contexts has been studied by means of a stroboscopic technique. A rateof 240 frames/sec was used for the major part of this study. Examples of the picturestaken in this way are shown in Fig. XVIII-2.One subject pronounced each of the syllables listed in Table XVIII-1. Simultane-ously, the sound and a time-marking pulse train were recorded on a dual-track mag-netic tape. The pulse train, which was started and stopped by a manual control, wasused also to fire an argon lamp that marked dots on the photographic film. The filmran continuously while the subject was illuminated intermittently with an exposure dura-tion of several microseconds. The pictures thus obtained were inspected closely, andthe vertical distance across the lip opening was plotted as a function of time (framenumber). Examples of these plots are given in Fig. XVIII-3. For each of the wordsspoken, the time .(frame number) required for the vertical distance across the lipopening to reach 4 mm was determined, and is shown in Table XVIII-2. The arrowsin Fig. XVIII-3 indicate the onset times of glottal vibration, which were estimated onthe sonagrams of the simultaneously recorded sounds. A highpass filter was employedfor the time-marking pulse train when the time-marked sonagrams were taken bymixing the output of two recording tracks, with the result that the speech sounds on thesonagrams were not obscured by the pulses. The onset times of glottal vibration forall the words containing [p] are listed in Table XVIII-3. Some profile views also weretaken for a few of the words listed in Table XVIII-1. An example of these profile photo-graphs is given in Fig. XVIII-4.Some of the findings are:i. The speed at which the lips separate is quite high at the beginning of the lip-opening process, especially in the case of initial stops. The difference between [p] and[b] or [m] seems to be significant when they are in the initial position; for example, thelip movement was apparently faster for [pak] than for [bak] or [mak]. The rate for [p]in the initial position, on the other hand, was faster than that after an unaccented vowel[a]; [p] preceded by [s] showed a relatively low initial speed of separation of the lips;for [p] followed by [r] the movement showed a marked oscillatory tendency, as discussedlater, and the narrowed lip opening persisted for a longer time (see Table XVIII-2).ii. Intervocalic [p]'s have a distinctly late onset time of glottal vibration (that is, alonger aspiration period) compared with initial [p]'s (see Table XVIII-3). When pre-ceded by [s], [p] has shorter aspiration periods in general, with the exception of oneinstance of [spak], which requires further investigation.iii. There is clearly an oscillatory tendency in the motion of the lips, and this isespecially apparent when [p] is followed by [r] (see Fig. XVIII-3). The motion of lips174(a)(b)Fig. XVIII-2. Front views of the mouth of the subject (a) pronouncing [pak],and (b) pronouncing [mak]. Both represent the beginning phaseof the mouth-opening immediately after the plosions of [p] and[m], respectively. Time sequence is from right to left, and thetime interval between adjacent frames is approximately 4 msec.175- - L- -- --~ -- ------ --~---~-----aTable XVIII-1. List of words on which the front-view pictures of Fig. XVIII-2are based.pockpeatpopeprovebockbeatbopemockmeatmopea pocka peata popeapprovea bocka beata bopea mocka meata mopespockspeechspopespruceTable XVIII-2. Time units (frame numbers) taken to reach 4-mm distance betweenlips for bilabial consonants in various phonetic environments. Thevalues for 3 mm and 2 mm, as well as for 4 mm, are shown for wordscontaining [r]. One time unit is approximately 4 msec.dmsp1 O C1 2 12 33 3 26 6 52 5 22 4 28 9 2ru(4 mm)27Table XVIII-3. The length of the unvoiced portion (delay of the onset of glottal vibra-tion referred to the plosion) of [p] in various phonetic environments.Time is in msec.i o a rup 12 35 19 105ap 54 52 47 65sp 8 26 22 22176(XVIII. SPEECH COMMUNICATION)can be considered as being composed of two components. One is a smooth variation,apparently including the motion of the jaw, and the other, which is superposed on thefirst one, is a damped vibration of one or both lips. The movements for [m] were alwayssmoother and had less apparent vibratory components.15 -EE- 10LL0 t bit -"spi -a.0 5 10 20 30FRAME NUMBER (x4msec =TIME)Fig. XVIII-3. Plots of the vertical distance across the lip opening as a func-tion of time (frame number). Arrows indicate the onset timesof glottal vibration.iv. The jaw is already lowered considerably when the plosion occurs. The move-ment of the jaw after the plosion is considerably slower than that of the lips. The move-ment of the lower lip seems to be the most noticeable. For [a] and [o] the upper teethare usually completely covered by the upper lip all through the mouth-opening phase,whereas the lower teeth appear markedly in the wide-open phase of the vowel.In initial stops, approximately 1/2 of the maximum width (in the horizontal direction)and 1/3 to 1/4 of the maximum distance between the lips are attained within approxi-mately 5 msec. This means that approximately 1/6 to 1/8 of the opening area isobserved within this period. In this earlier stage, the teeth constriction is apparentlynot a very important factor in determining the formant frequencies, and, if we assumea uniform tube with a variable constriction at the open end, the resonant frequenciesof the system can be expressed approximately as a linear function of the square root ofthe opening area. One sixth of the maximum change in the area means that 40 per centof the total traverse of the formant transitions might well occur within approximately5 msec. If we take into account the fact that the effective opening in the final stage ismuch smaller than that made by lips alone, that is, the fact that the formant frequencydoes not quite reach the value predicted from the separation of the lips owing tothe narrower constriction of the teeth, the figure of 40 per cent should be increased1773 2 I 0 -1 -2Fig. XVIII-4Profiles of the subject pronouncing [pak].The frame interval is approximately 4 msec.Negative numbering of frames refers to theframes before the plosion of [p]. The frameat the extreme right represents a frame 330msec before the plosion.(XVIII. SPEECH COMMUNICATION)considerably. Thus, the change in the formant frequency in the initial phase should bevery rapid. If, for example, the frequency of the first formant increases to 500 cps forthe stationary portion of the vowel, the initial rate of the increasing frequency would beat least 200 cps/5 msec to 300 cps/5 msec, that is, the period of the resonance fre-quency is comparable to the time that was required to change from zero to 200-300 cps.The smearing of the formant on the sonagram caused by the rapid shift of the formantshould be dominant, and the sensation of pitch.cannot be clear, with the result that thiskind of rapid transient can better be described as a spike (short burst) rather than aformant transition. Therefore, it is reasonable to assume, at least for bilabial stops,that the resonance frequencies for the articulatory configuration of the consonant do notnecessarily represent the starting points of the "formant transition."The fact that initial bilabial stops, when not followed by [r], result in an appreciablelip distance within several milliseconds after the plosion suggests that the noise is notgenerated at the lip constriction, except at the very beginning. It is, therefore, rea-sonable to describe the acoustic structure of a /p/ + vocalic syllable as composed of aspike (with a duration of several milliseconds), an aspiration (with a duration of approx-imately 100 milliseconds), and a voiced period. The cues relating to formant transi-tions are contained, possibly, both in the aspiration period and in the voiced period.Synthesis experiments by use of a dynamic analog of the vocal tract suggest that thetransition can be completed by the time the voice excitation is initiated, without losingthe stop identification. As a matter of fact, in the sonagrams of naturally pronouncedinitial [p] or [b], we sometimes observe second or third formant transitions (includingthe aspiration of [p]) that are not negative at all. This can be explained only if weassume that the rising transition of the formant cannot be observed because it is toofast.In the profiles shown in Fig. XVIII-4, in which the subject is uttering [pak], we cansee that the lips are pushed forward before they are blown apart. As soon as plosionoccurs, the lips resume the relaxed shape. This fact can be observed also in the frontviews, particularly in the different appearance of the upper lip before and after theplosion (notice the apparent change in thickness). The front view of [m] does not showthis clear change of shape and, therefore, shows a smoother motion of lip separation,without any noticeable vibratory component. For [m], the overpressure behind theobstruction formed by the lips cannot be built up appreciably because of the open nasalby-pass. The vibratory motion of the lips can be considered as excited by the over-pressure that is released immediately after the plosion. The mechanism of this kindof motion is probably similar to that of vocal-cord vibration. The period of the dampedvibration of the lips is estimated at from 25 to 30 msec.We need more data before drawing quantitative conclusions, because some fluctua-tion in pronunciation can be expected among different speakers, and also among different179(XVIII. SPEECH COMMUNICATION)utterances of the same speaker. More accurate measurements of the jaw-opening proc-ess are needed, as well as simultaneous sound recordings with substantially bettersignal-to-noise ratio.The stroboscope technique that was used for this study was made possible byProfessor H. E. Edgerton and by the technical assistance of Mr. P. Yamin.O. Fujimura180

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