Impact of Vocal Tract Resonance on the Perception of Voice Quality Changes Caused by Varying Vocal Fold Stiffness.

Experiments using animal and human larynx models are often conducted without a vocal tract. While it is often assumed that the absence of a vocal tract has only small effects on vocal fold vibration, it is not actually known how sound production and quality are affected. In this study, the validity of using data obtained in the absence of a vocal tract for voice perception studies was investigated. Using a two-layer self-oscillating physical model, three series of voice stimuli were created: one produced with conditions of left-right symmetric vocal fold stiffness, and two with left-right asymmetries in vocal fold body stiffness. Each series included a set of stimuli created with a physical vocal tract, and a second set created without a physical vocal tract. Stimuli were re-synthesized to equalize the mean F0 for each series and normalized for amplitude. Listeners were asked to evaluate the three series in a sort-and-rate task. Multidimensional scaling analysis was applied to examine the perceptual interaction between the voice source and the vocal tract resonances. The results showed that the presence or absence of a vocal tract can significantly affect perception of voice quality changes due to parametric changes in vocal fold properties, except when the parametric changes in vocal fold properties produced an abrupt shift in vocal fold vibratory pattern resulting in a salient quality change.