Articulatory compensation: A study of ambiguities in the acoustic-articulatory mapping

Abstract

We present a method of constructing various vocal-tract shapes having identical acoustical characteristics. A 20-dimensional vocal tract model is used to compute the frequencies, amplitudes, and bandwidths of the first five formants. The dimensions consist of 20 uniformly spaced cross-sectional areas of the vocal tract. The model includes losses due to finite wall impedance, glottal leakage, friction, heat conduction, and radiation. Articulatory regions (fibers) are computed in which the frequencies and amplitudes of the first three formants remain constant. In general these fibers contain 14 dimensions, not all of which provide physiologically reasonable area profiles. A theory for selecting among the dimensions within a fiber according to maximum physical smoothness of the vocal tract is presented. Application of the theory to the 14-dimensional data indicate that 2 or 3 of the 14 dimensions correspond to physically realizable perturbations in the vocal tract and that along these dimensions changes in the vocal-tract shape may be made without changing the three formant frequencies and amplitudes. Almost no audible change in the sound is produced by these articulatory variations. Examples of synthetic speech illustrating these results will be played.