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

Abstract

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.

Figure 1

A sketch of the physical vocal fold model used in this study.

Figure 2

(Color online) The four-feature source spectral model with amplitudes of individual harmonics adjusted so that slope decreased smoothly within each frequency range. More details of the analysis-by-synthesis approach can be found in Kreiman et al. (2010).

Figure 3

(Color online) The visual sort-and-rate task as implemented in PowerPoint. Listeners played each stimulus by clicking on an icon, then placed it on the line by clicking and dragging.

Figure 4

Selected physical variables and acoustic measures as a function of the body stiffness of the symmetric physical models (series I). P_th = phonation threshold pressure; A_th = mean glottal opening area at phonation onset; F₀ = phonation onset frequency.

Figure 5

MDS stimulus coordinates as a function of the body stiffness of the symmetric physical models in series I.

Figure 6

Selected physical variables and acoustic measures as a function of the body stiffness of the asymmetric physical models in series II. P_th = phonation threshold pressure; A_th = mean glottal opening area at phonation onset; F₀ = phonation onset frequency. Note that a phase difference of 180° is the same as −180°.

Figure 7

MDS stimulus coordinates as a function of body stiffness of the asymmetric physical models in series II.

Figure 8

Selected physical variables and acoustic measures as a function of body stiffness in the asymmetric physical models in series III. P_th = phonation threshold pressure; A_th = mean glottal opening area at phonation onset; F₀ = phonation onset frequency.

Figure 9

MDS coordinates for stimuli in series III as a function of asymmetric body stiffness in the physical models.