Variability of vowel formants due to the finite impedances of the glottis, the velopharyngeal port, and the vocal tract walls

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Vowel formant frequencies are often thought to be determined primarily by the cavity shape of the vocal tract, and their bandwidths by various loss mechanisms such as the soft walls, viscous friction, and sound radiation from the lips. However, several studies suggest that formant frequencies are also considerably affected by certain boundary conditions, in particular by the finite acoustic impedance of the glottis during phonation, by the soft walls of the vocal tract, and by incomplete velopharyngeal closure. The effect of all three mechanisms is similar: the first two formants are shifted to a higher frequency and their bandwidths become larger (compared to a vocal tract with rigid walls, a fully closed glottis, and a fully closed velopharyngeal port). So far, the three mechanisms have only been considered individually, and mostly using physical models of the speech apparatus or using computer simulations (rather than humans). The goal of the project proposed here is to study these mechanisms in more detail by combining experiments on subjects, with physical vocal tract models, and computer simulations. Of particular interest is how multiple concurrent mechanisms interact in shifting the formants, what their common physical cause is, to what extent the effects are perceptually relevant, and whether and how they are intentionally used in speech or singing. The results of the project would allow, among other things, a better interpretation of measured formant frequencies, improvement of methods for estimating vocal tract length or vocal tract shape from formant frequencies, improvement of inverse filtering methods, and more accurate simulation models of speech production.