The acoustic sensitivity of vocal tract constrictions: a preliminary report

Abstract

A basic premise underlying most recent vocal tract modeling is that the area functions at and near vocal tract constrictions are the ones most critical to the acoustic output; it is also the central point of many hypotheses concerning speech production, that speech targets are coded neurophysiologically with respect to acoustically significant area function features. To determine the relationship between constriction parameters and the acoustic output, we performed a number of vocal tract simulations that perturbed the locations and cross-sectional areas of the points of constriction for the three cardinal vowels, [i], [a] and [u]. The vocal tract area functions of the steady state vowels were approximated by first obtaining midsagittal X-rays of a single speaker’s production of the three vowels. These approximations were then used to construct a set of articulatory-derived acoustic continua that represented systematic perturbations of the cross-dimensions and locations of the constrictions for each sound. Empirically derived coefficients were used to convert the measured cross-dimensions to area functions. A third program calculated the transfer function and formants for each sample. Results of the perturbation analysis showed that formants for each of the vowels were relatively sensitive to minimal changes in crosssectional area but relatively insensitive to larger changes in constriction location. Vowel perception, however, was highly resistant to these step changes, suggesting a many-to-one relationship between articulatory/acoustic and perceptual targets.