On the ability of a physiologically constrained area function model of the vocal tract to produce normal formant patterns under perturbed conditions
An area function model of the vocal tract is tested for its ability to produce typical vowel formant frequencies with a perturbation at the lips. The model, which consists of a neutral shape and two weighted orthogonal shaping patterns (modes), has previously been shown to produce a nearly one-to-one mapping between formant frequencies and the weighting coefficients of the modes [Story and Titze, J. Phonetics, 26, 223-260 (1998)]. In this study, a perturbation experiment was simulated by imposing a constant area ``lip tube'' on the model. The mapping between the mode coefficients and formant frequencies was then recomputed with the lip tube in place and showed that formant frequencies (F1 and F2) representative of the vowels [capomega,oh,you] could no longer be produced with the model. However, when the mode coefficients were allowed to exceed their typical bounding values, the mapping between them and the formant frequencies was expanded such that the vowels [capomega,oh,you] were compensated. The area functions generated by these exaggerated coefficients were shown to be similar to vocal-tract shapes reported for real speakers under similar perturbed conditions [Savariaux, Perrier, and Orliaguet, J. Acoust. Soc. Am., 98, 2428-2442 (1995)]. This suggests that the structure of this particular model captures some of the human ability to configure the vocal-tract shape under both ordinary and extraordinary conditions.