Richard S. McGowan

An aeroacoustic approach to phonation

A fluid mechanical, or aeroacoustic, point of view is followed to study possible sources of sound during phonation. Concentration is on two features of the vocal tract during phonation: abrupt area change from the glottis to the vocal tract and the finite length of the vocal tract. With these features, a source of sound distinct from the volume velocity source can be identified and a preliminary account of its effect on the acoustic field given. This source of sound is an oscillating force resulting from an interaction of rotational fluid motion with itself. Because of the schematic nature of the geometry of the model used here, this source may be considerably modified in actual phonation. It is concluded that specification of volume velocity is not enough to specify the source during phonation, even neglecting source-tract interaction.

Tongue-tip trills and vocal-tract wall compliance

The effect of vocal-tract wall compliance on tongue-tip trills is to create a favorable pressure-flow relation at the tongue tip for sustained vibration. The governing equations are derived for a model based on this mechanism, and data on unvoiced trills are used to help set parameters for a numerical simulation of the model.

Recovering articulatory movement from formant frequency trajectories using task dynamics and a genetic algorithm: preliminary model tests

Abstract Articulatory trajectories of an articulatory model were recovered by means of a genetic algorithm from acoustic information using a task-dynamic model of speech articulation. Tests on simulated utterances / əbae/ and / ədae/ show that the method can recover most of parts of an original trajectory, but it has trouble in obtaining precise timing. For the recovery of articulation, formant frequency trajectories should be supplemented by additional acoustic information, such as RMS amplitude.

Differences in fricative production between children and adults: Evidence from an acoustic analysis of /∫/ and /s/

Speech samples of 12 speakers (8 children and 4 adults) producing the fricatives /s/ and/sh/ followed by the vowels /i/ and /u/ were analyzed to locate the major spectral prominences. Results showed that the fricative low-frequency prominences for childrens samples differed from those of adults in three important ways: (1) They were generally higher in frequency; (2) they were greater in amplitude relative to higher frequency regions; and (3) they showed greater effects of vowel context. The first finding can be explained by a simple scaling of adult models of fricative production to accommodate childrens smaller vocal tracts. The other two findings suggest, however, that there are other anatomical and articulatory differences between children and adults affecting fricative production. The data presented here suggest that one important difference may be the relative sizes of the fricative constriction and the glottal opening.

Vocal tract aerodynamics in /aCa/ utterances: measurements

Abstract This paper examines air flow patterns at vowel-consonant and consonant-vowel transitions. Oral air flow was recorded in six speakers of American English producing reiterant speech. The air flow signal was inverse filtered to obtain an estimate of the glottal pulse. Measurements were made of peak and minimum flow, open quotient, pulse area and fundamental frequency. The results show that at the transitions between vowels and voiceless consonants the pulse properties show large variations. In particular, the source is characterized by a breathy mode of phonation. Breathiness was indexed by large values of peak and minimum flow, and an open quotient close to 1. The observed variations can be accounted for by the laryngeal adjustments that are made for voiceless consonants, in particular the glottal opening movement and its phasing with the oral articulatory events. Individual differences suggest that speakers vary in their use of the longitudinal tension of the vocal folds in controlling voicelessness.

Development of [ɹ] in young, Midwestern, American children

Beginning at the age of about 14 months, eight children who lived in a rhotic dialect region of the United States were recorded approximately every 2 months interacting with their parents. All were recorded until at least the age of 26 months, and some until the age of 31 months. Acoustic analyses of speech samples indicated that these young children acquired [inverted r] production ability at different ages for [inverted r]s in different syllable positions. The children, as a group, had started to produce postvocalic and syllabic [inverted r] in an adult-like manner by the end of the recording sessions, but were not yet showing evidence of having acquired prevocalic [inverted r]. Articulatory limitations of young children are posited as a cause for the difference in development of [inverted r] according to syllable position. Specifically, it is speculated that adult-like prevocalic [inverted r] production requires two lingual constrictions: one in the mouth, and the other in the pharynx, while postvocalic and syllabic [inverted r] requires only one oral constriction. Two lingual constrictions could be difficult for young children to produce.

Vocal tract aerodynamics in /aCa/ utterances: simulations

Abstract Aerodynamic simulations of /aCa/ utterances were made using a low-frequency model for upper vocal tract airflow and a two-mass model for the voice source. These simulations helped increase insight into the results of an empirical study of flow during running speech. The various sources of flow, including wall compliance, were examined for their contributions to total flow from the mouth. The two-mass model was modified to allow for more natural glottal flow during abduction and adduction. Even with modifications the two-mass model was not sufficient to model source variations during running speech.

The quasisteady approximation in speech production

Because boundary‐layer separation is important in determining force between flowing air and solid bodies and separation can be sensitive to unsteadiness, the quasisteady approximation needs to be examined for the flow‐induced oscillations of speech (e.g., phonation and trills). A review of the literature shows that vibratory phenomena, such as phonation and tongue‐tip trills, may need to be modeled without the quasisteady approximation.

Compact Green's functions extend the acoustic theory of speech production

Abstract A brief tutorial discussion of the method of compact Greens functions applied to sources of sound in the vocal tract is given. A vocal tract transfer function may be regarded as a specialized Greens function. However, a Greens function enables detailed analysis of the transfer of energy from the hydrodynamic mode of fluid motion into sound. Source regions within the vocal tract often are small compared to the acoustic wavelength, which leads to a simplified model of the acoustics of the source region. This permits calculation of a ‘compact’ Greens function. In this approach much of the classical acoustic theory of speech production remains unchanged. The method is illustrated by the calculation of the sound source when a vortex is swept past an obstruction in the vocal tract. This is discussed in terms of the differences between the pressure sources of sibilant fricative production and of voicing and aspiration.

Introduction to papers on speech recognition and perception from an articulatory point of view

The following group of papers resulted from a special session entitled Speech Recognition and Perception from an Articulatory Point of View that was held during the spring 1994 meeting of the Acoustical Society of America in Cambridge, Massachusetts. Organization of the session began when Richard McGowan, Terry Nearey, and Juergen Schroeter invited speakers to give papers and critiques on the role of articulation in human perception and machine recognition. Presentations were invited from three speakers or groups of speakers who were representative of the three speech areas in the Society: production, perception, and processing. One talk was given by Bjorn Lindblom, another was given by John Ohala, and the third by Rick Rose, Juergen Schroeter, Mohan Sondhi, and Oded Ghitza. The invited critiquers for the Lindblom paper were Ken Stevens, Robert Remez, and Bishnu Atal; for the Rose et al. paper, Roger Moore, Joe Perkell, and Terry Nearey; and for the Ohala paper, Mary Beckman, Douglas O’Shaughnessy, and Ca...