Effects of complicated vocal tract shape on vocal tract transfer function

To investigate the effects of vocal tract shape on the formant frequency of speech sounds below 4000 Hz, in this study, we compare the estimated transfer functions and sound pressure distributions of three dimensional (3-D) vocal tract models with complicated and simple shapes. A 3-D vocal tract model (Model-O) with a complicated shape caused by oral lesions is designed by magnetic resonance imaging (MRI). Four deformed vocal tract models are constructed on the basis of Model-O. These deformed models have the same cross-sectional-area function and different cross-sectional shapes. The transfer functions of these models are estimated by means of the finite element method (FEM) and the equivalent circuit model using vocal tract transfer function. FEM can estimate the numver of peaks of the transfer functions of Model-O, the same as the number of speech formants. However, for a transfer function estimated by the equivalent circuit model, even the number of peaks is different from that of speech formants. These results suggest that FEM can estimate transfer functions accurately. Even in the low-frequency region, not only a cross-sectional-area function but also the shape of the vocal tract affects the vocal tract transfer function. The transfer functions of the deformed models replaced with the oral area by cylindrical tubes estimated by FEM are also different from speech spectra. This is because the sound pressure contour of such models shows that sound wave is planar in the oral cavity