Determination of the Shape of the Human Vocal Tract from Acoustical Measurements

Articulatory events are imitated under rapid shadowing

We tested the hypothesis that rapid shadowers imitate the articulatory gestures that structure acoustic speech signals-not just acoustic patterns in the signals themselves-overcoming highly practiced motor routines and phonological conditioning in the process. In a first experiment, acoustic evidence indicated that participants reproduced allophonic differences between American English /l/ types (light and dark) in the absence of the positional variation cues more typically present with lateral allophony. However, imitative effects were small. In a second experiment, varieties of /l/ with exaggerated light/dark differences were presented by ear. Acoustic measures indicated that all participants reproduced differences between /l/ types; larger average imitative effects obtained. Finally, we examined evidence for imitation in articulation. Participants ranged in behavior from one who did not imitate to another who reproduced distinctions among light laterals, dark laterals and /w/, but displayed a slight but inconsistent tendency toward enhancing imitation of lingual gestures through a slight lip protrusion. Overall, results indicated that most rapid shadowers need not substitute familiar allophones as they imitate reorganized gestural constellations even in the absence of explicit instruction to imitate, but that the extent of the imitation is small. Implications for theories of speech perception are discussed.

Vowel constrictions are recoverable from formants

The area function of the vocal tract in all of its spatial detail is not directly computable from the speech signal. But is partial, yet phonetically distinctive, information about articulation recoverable from the acoustic signal that arrives at the listener's ear? The answer to this question is important for phonetics, because various theories of speech perception predict different answers. Some theories assume that recovery of articulatory information must be possible, while others assume that it is impossible. However, neither type of theory provides firm evidence showing that distinctive articulatory information is or is not extractable from the acoustic signal. The present study focuses on vowel gestures and examines whether linguistically significant information, such as the constriction location, constriction degree, and rounding, is contained in the speech signal, and whether such information is recoverable from formant parameters. Perturbation theory and linear prediction were combined, in a manner similar to that in Mokhtari (1998) [Mokhtari, P. (1998). An acoustic-phonetic and articulatory study of speech-speaker dichotomy. Doctoral dissertation, University of New South Wales], to assess the accuracy of recovery of information about vowel constrictions. Distinctive constriction information estimated from the speech signal for ten American English vowels were compared to the constriction information derived from simultaneously collected X-ray microbeam articulatory data for 39 speakers [Westbury (1994). Xray microbeam speech production database user's handbook. University of Wisconsin, Madison, WI]. The recovery of distinctive articulatory information relies on a novel technique that uses formant frequencies and amplitudes, and does not depend on a principal components analysis of the articulatory data, as do most other inversion techniques. These results provide evidence that distinctive articulatory information for vowels can be recovered from the acoustic signal.

The synergy between speech production and perception

Speech intelligibility is known to be relatively unaffected by certain deformations of the acoustic spectrum. These include translations, stretching or contracting dilations, and shearing of the spectrum (represented along the logarithmic frequency axis). It is argued here that such robustness reflects a synergy between vocal production and auditory perception. Thus, on the one hand, it is shown that these spectral distortions are produced by common and unavoidable variations among different speakers pertaining to the length, cross-sectional profile, and losses of their vocal tracts. On the other hand, it is argued that these spectral changes leave the auditory cortical representation of the spectrum largely unchanged except for translations along one of its representational axes. These assertions are supported by analyses of production and perception models. On the production side, a simplified sinusoidal model of the vocal tract is developed which analytically relates a few "articulatory" parameters, such as the extent and location of the vocal tract constriction, to the spectral peaks of the acoustic spectra synthesized from it. The model is evaluated by comparing the identification of synthesized sustained vowels to labeled natural vowels extracted from the TIMIT corpus. On the perception side a "multiscale" model of sound processing is utilized to elucidate the effects of the deformations on the representation of the acoustic spectrum in the primary auditory cortex. Finally, the implications of these results for the perception of generally identifiable classes of sound sources beyond the specific case of speech and the vocal tract are discussed.

Limitations of acoustic rhinometry determined by a simple model

To determine the accuracy of a commercially available transient signal acoustic rhinometer we constructed simple models from tubing of known dimensions. A series of models was constructed in which the main cylinder was constant whilst the aperture of an included constricted area was varied from 0.07 cm2-0.95 cm2. The area reconstruction for each model was then compared with the true area. Two parameters based on those used in clinical practice of acoustic rhinometry were employed to evaluate the error. The results demonstrate that the measurement of both the volume beyond a constriction and the area of the constriction may be associated with systematic errors. The reconstructed profile of each model is similarly erroneous. As the nose presents a proximal constriction, these findings are of considerable relevance to the clinical use of acoustic rhinometry.