Arthur S. Abramson

Some effects of context on voice onset time in English stops.

Recent work has led us to the conclusion that the English stop categories /bdg/ and /ptk/ are distinguished by the timing of changes in glottal aperture relative to supra-glottal ariculation. In word-initial position, the environment of current interest to us, this is manifested acoustically by voice onset time, that is, the time interval between the burst that marks release of the stop closure and the onset of quasi-periodicity which reflects laryngeal vibration. For citation forms of words this measure of voice onset time completely separates the two phonemic categories. In running speech, however, the separation is less sharp ; there is some overlap along the dimension of voice onset time. We have examined running speech in some detail to discover the extent to which certain contextual features are responsible for this overlap. It is clear that the presence of a voiceless stop in a stressed syllable makes for a greater lag in the onset of voicing. In unstressed syllables, an environment of high contextual redundancy as well as low functional yield for the phonemic contrast, there is considerable reduction of the distinction along the dimension. A further increase in voicing lag is noted in syllables bearing the final sentence stress. In addition, the farther such a syllable is from the end of the sentence, the less the effect is likely to be. The importance of voice onset time continues to be apparent, even in running speech, although other effects of context remain to be explored.

The Identification and Discrimination of Synthetic Vowels

A series of thirteen two-formant vowels was synthesized and used as the basis of labelling and discrimination tests with a group of English-speaking listeners. The sounds varied only in F1/F2 plot and the resulting vowel qualities were such that listeners found no difficulty in assigning each sound to one of three phonemic categories, those of the vowels in bid, bed and bad. The results of the tests were compared with those previously obtained in experiments involving the consonant phonemes /b, d, g/. It appears from the data that the phoneme boundaries in the case of the three vowel phonemes are less sharply defined than in the case of the stop consonants. The labelling functions for the vowels show a gradual slope and the discrimination functions do not show any marked increase in sensitivity to change in the region of the phoneme boundaries. It is clear also that the listeners were able to discriminate differences very much smaller than would need to be distinguished simply in order to place vowels in the appropriate category. The results show further that the effect of sequence or acoustic context in the perception of vowels is very considerable. In all the aspects examined in these experiments, the perception of synthetic vowels is found to be different from that of synthetic stop consonants. These differences lend some support to the hypothesis that the degree of articulatory discontinuity between sounds may be correlated with the sharpness of the phonemic boundaries that separate them.

Static and dynamic acoustic cues in distinctive tones.

It is conventional to classify phonemic tones into dynamic or contour tones and static or level tones. The perceptual relevance of this impressionistic dichotomy is considered here for Central Thai, which has two dynamic tones (falling and rising pitches) and three static tones (high, mid, and low). A fundamental-frequency range appropriate to an adult male voice was used to synthesize three series of tonal variants on a syllable type available for five tonally differentiated words: (1) 16 Fo levels at intervals of 4 Hz, (2) 16 Fo movements from a mid origin to end points ranging from top to bottom of the range in steps of 4 Hz, and (3) 17 variants rising from the bottom to end points from top to bottom in steps of 4 Hz. The stimuli were played to native speakers for identification. The results indicate that level variants contain sufficient cues for identification as static tones but with considerable overlap. Identification, however, is enhanced by slow Fo movement. Rapid Fo movement is required for dynamic tones. Although imprecise, the typological dichotomy is useful.

Transillumination of the Larynx in Running Speech

A fundamental distinction between speech sounds depends on whether the excitation is a noise source or a quasiperiodic one. The noisy or voiceless sounds are presumably produced with an opened and quiescent larynx, while for voiced sounds the larynx is closed down and in rapid oscillation. Direct evidence of this has come from motion pictures recorded through the open mouth, a method obviously limited to particular sounds. Running speech requires different methods, and is being studied by a transillumination technique. A fiber‐optics bundle introduced into the laryngeal vestibule through the nose is used to illuminate the glottis, while a photocell placed below the thyroid cartilage registers the variable light transmitted through the glottis and the tissues of the neck. The “glottograms” so obtained are compared with acoustic waveforms simultaneously recorded with air and throat microphones to determine how the voiced‐voiceless distinction correlates with closed versus open states of the larynx.

Voice register in Suai (Kuai): an analysis of perceptual and acoustic data.

Analyses of the perceptual and acoustic characteristics of the Register 1 (ʼclearʼ) versus Register 2 (ʼbreathyʼ) distinction have been carried out on the Kuai dialect of Suai, a Mon-Khmer language. The perception results were obtained from five-parameter synthesized stimuli. They showed that the primary parameter underlying the distinction is the frequency of onset of laryngeal excitation (F0). One other parameter making a significant contribution was the open quotient. The F0 result was confirmed by an acoustic analysis of eight pairs of natural utterances produced by native speakers. We conclude that the Suai language is in a state of flux with respect to the voice registers, although the distinction has not disappeared. The perceptual data reveal mixed levels of sensitivity, and the production data indicate that some speakers maintain a fairly good distinction, while others do not. The language seems to be replacing the register distinction with a prosodic one of pitch accent, possibly as a stage leading to tonogenesis.

The perception of word-initial consonant length: Pattani Malay *

The most salient physical manifestation of phonemically distinctive consonant length is the duration of the closure or constriction of the short consonant relative to that of its long counterpart. The contrast is rare in languages of the world in word-initial, and thus potentially utterance-initial, position. Perception in this position would seem to depend upon the audibility of closure excitation. This is plausible for nasals, laterals, and fricatives. The closures of voiced stops, however, may have only low-amplitude excitation, while voiceless stops have none. Pattani Malay was investigated to find out how robust the length feature is in perception. Listening tests yielded good differentiation of the two length classes for isolated words, with a lesser effect for voiceless stops. Experiments with incrementally lengthened short closures and shortened long closures confirmed the sufficiency of duration as a cue. For the voiceless stops, these experiments could be run only in intervocalic position.

The Perception of English Stops by Speakers of English, Spanish. Hungarian, and Thai: A Tape-Cutting Experiment

American English stops, including residual stops,i.e., stops in /s/-clusters after the removal of the /s/, were presented in front of stressed vowels for identification on the one hand to native speakers of American English, on the other, to native speakers of Puerto Rican Spanish, Hungarian, and Thai, languages with differences in the phonetic composition of their stop phonemes. Speakers of American English identified the residual stops with the voiced (lenis) stop; the others, with the voiceless stop. The results suggest that there is a hierarchic organization among the features of these stops: the lack of aspiration tends to force the evaluation of stops in the direction of /b, d, g/ in American English, whereas in the languages where other distinctions exist, the evaluation is different.

Thai Final Stops: Cross-Language Perception

As part of a project seeking a better understanding of the links between speech production and perception, we have conducted experiments on the perception of Thai word-final stop consonants by native speakers of Thai and native speakers of American English. The final stops of Thai are never released audibly. In English, however, final stops occur with and without audible releases. Previous work has shown that released stops are likely to be more intelligible than unreleased ones. That is, identification is better when information as to place of articulation in the formant transitions of the closing articulatory gesture is supplemented by acoustic information in a release burst. Both languages have labial, alveolar, and velar places of articulation, but Thai also has glottal closure. Two native speakers of Thai recorded sets of monosyllabic words minimally distinguished by the four final stops . Randomized lists of the isolated words were presented to 19 native speakers of English who responded with the labels ‘p’, ‘t’, ‘k’, and ‘other’. They were also presented to 30 native speakers of Thai for identification in Thai script as words. The highly significant differences between the two groups imply the possibility that the closing gestures of the Thai stops include a component that compensates somewhat for the absence of release, a component to which the English speakers are perhaps less well attuned. Such an interference will be tested in future work by tracking of the gestures in both languages with the Haskins Laboratories magnetometer.

Vowel height and the perception of consonantal nasality

By means of an articulatory synthesizer, the perception of the oral-nasal distinction in consonants was explored experimentally. This distinction was chosen because it is achieved by a very simple articulatory maneuver and because it is phonologically relevant in virtually every language. Lowering the velum in equal increments provided continua of CV syllables varying in size of velopharyngeal port which were divided perceptually into /d/ and /n/ categories by American English listeners. To test the hypothesis that the coarticulation of these nasal consonants with lower (more open) vowels requires a larger area of velopharyngeal coupling to give a nasal consonant precept, three oral-nasal continua incorporating the vowels /i/, /delta/, and /alpha/, respectively, were presented for identification. The results were compared with those of A. S. House and K. N. Stevens [J. Speech Hear. Disord. 21, 218-232 (1956)] and A. S. House [J. Speech Hear. Disord. 22, 190-204 (1957)] obtained with steady-state vowels and consonantal murmurs and with those of M. H. L. Hecker [J. Acoust. Soc. Am. 34, 179-188 (1962)]. Three conclusions emerged. First, the relationship between vowel height and the amount of velopharyngeal coupling needed for a nasal precept occurs in conditions where subjects are required to make linguistically relevant judgments. Second, the relationship can arise in conditions where vocalic coarticulation is present. Third, the relationship is not confined to vowels but can also be observed in the case of dynamically articulated consonants. One of the continua was also used for discrimination experiments, which yielded the classical pattern of high discriminability at the category boundary.

Gradient Effects of Fundamental Frequency on Stop Consonant Voicing Judgments

The post-stop-release rise or fall of fundamental frequency (F0) is known to affect voicing judgments of syllables with ambiguous voice onset times (VOTs). In 1986, Silverman claimed that the critical factor was not direction of F0 change but rather its direction relative to the intonational contour. He further claimed that only F0s that start above and fall to the contour have an effect proportional to the size of the frequency change; F0s that rise to the contour by different amounts were claimed to be equivalent. In our first experiment, we examined the effect on voicing judgments of five onset F0s preceding a single, flat contour. Only falling F0s were differentiated in the first set of judgments, but after increased exposure to the syllables, even F0s below the contour differentially affected the voicing judgment. In a second experiment, the contour of the final part of the syllable was flat, rising or falling. F0 contour affected the judgments, as did onset F0s, but the two factors did not interact, indicating that the onset values were not being judged by reference to the contours. However, the contour which was predicted to result in more voiceless judgments also ended at a higher F0 in the vowel, and another effect of voicing is that the F0 is higher throughout the vowel after voiceless stops. In a third experiment, F0 contours were created to contrast contour and mean F0. The effect of the F0 during the vocalic segment appeared to be attributable to the average F0 rather than the contour. In all three experiments, the F0 onset values contributed to the voicing judgment whether they were above or below the putative intonation contour. The contribution of the lower F0s, while significant, was not as great as that of the higher F0s, which argues for a noncategorical contribution of intonation.