Acoustic and perceptual effects of changes in body layer stiffness in symmetric and asymmetric vocal fold models.

At present, it is not well understood how changes in vocal fold biomechanics correspond to changes in voice quality. Understanding such cross-domain links from physiology to acoustics to perception in the "speech chain" is of both theoretical and clinical importance. This study investigates links between changes in body layer stiffness, which is regulated primarily by the thyroarytenoid muscle, and the consequent changes in acoustics and voice quality under left-right symmetric and asymmetric stiffness conditions. Voice samples were generated using three series of two-layer physical vocal fold models, which differed only in body stiffness. Differences in perceived voice quality in each series were then measured in a "sort and rate" listening experiment. The results showed that increasing body stiffness better maintained vocal fold adductory position, thereby exciting more high-order harmonics, differences that listeners readily perceived. Changes to the degree of left-right stiffness mismatch and the resulting left-right vibratory asymmetry did not produce perceptually significant differences in quality unless the stiffness mismatch was large enough to cause a change in vibratory mode. This suggests that a vibration pattern with left-right asymmetry does not necessarily result in a salient deviation in voice quality, and thus may not always be of clinical significance.