Effects of vocal tract tremor on vowel acoustics: a computational modeling study

Voice tremor in the acoustic speech signal may arise from multiple sources of modulation. The present study focused on vocal tract tremor, in which a localized region of the vocal tract is modulated sinusoidally in cross-sectional area, resulting in time-varying perturbations of the vocal tract resonance frequencies. The first aim was to quantify the effects of vocal tract tremor on the first three resonance frequencies as a function of tremor location and spatial extent. Using computational models of male and female vocal tracts configured for a neutral vowel, /a/, /i/, and /u/, sinusoidal tremor was imposed at locations spanning the length of the vocal tract. The resulting resonance-frequency modulation extents were quantified and visualized as modulation-extent functions and pseudo-midsagittal heat maps. The simulations showed vowel-dependent patterns of minima and maxima in resonance-frequency modulation extent, indicating that the acoustic consequences of vocal tract tremor depend on both tremor location and vocal tract configuration. The second aim was to examine how vocal tract tremor influences fundamental frequency through source-filter interaction in a coupled model of phonation and vocal tract acoustics. Simulations demonstrated that vocal tract tremor can induce measurable modulations of both fundamental frequency and radiated amplitude, even though the control parameters of the vocal fold model remained unchanged. The magnitude of these effects depended on vowel configuration and tremor location. The results suggest that vocal tract tremor may contribute to various types of fluctuations in the speech signal through modulation of resonance frequencies, fundamental frequency, and acoustic amplitude, and demonstrate the utility of computational modeling and heat-map visualization for identifying vocal tract regions that are most sensitive to tremor-induced modulations.