Finite-element method analysis of acoustic characteristics of the vocal tract with the nasal cavity during phonation of Japanese /a/

Abstract

For this study, the transfer functions and active sound intensities of a vocal tract model with and without a nasal cavity were computed using a three-dimensional finite-element method (FEM). The models were based on vowel data obtained by magnetic resonance imaging (MRI) of the vocal tract with a nasal cavity during phonation of the Japanese /a/. The oral cavity was also coupled with the nasal cavity in a three-dimensional volume of radiation. Effects of wall impedance were also examined. Coupling of the nasal cavity to the oral cavity indicated the following aspects. Additional peaks appeared below 3 kHz for the lossless condition. However, they disappeared in the simulation for the soft wall condition. The sound energy circulation did not occur in the simulation for the soft wall condition. Regarding effects of the wall boundary condition on the spectral envelope, three-dimensional simulations confirmed the upward shift of lower formant frequencies. However, disagreement of the formant frequencies between simulated and real speech should be investigated further by adjusting the wall boundary condition to a more realistic one.