The influence of material anisotropy on vibration at onset in a three-dimensional vocal fold model

Abstract

Although vocal folds are known to be anisotropic, the influence of material anisotropy on vocal fold vibration remains largely unknown. Using a linear stability analysis, phonation onset characteristics were investigated in a three-dimensional anisotropic vocal fold model. The results showed that isotropic models had a tendency to vibrate in a swing-like motion, with vibration primarily along the superior-inferior direction. Anterior-posterior (AP) out-of-phase motion was also observed and large vocal fold vibration was confined to the middle third region along the AP length. In contrast, increasing anisotropy or increasing AP-transverse stiffness ratio suppressed this swing-like motion and allowed the vocal fold to vibrate in a more wave-like motion with strong medial-lateral motion over the entire medial surface. Increasing anisotropy also suppressed the AP out-of-phase motion, allowing the vocal fold to vibrate in phase along the entire AP length. Results also showed that such improvement in vibration pattern was the most effective with large anisotropy in the cover layer alone. These numerical predictions were consistent with previous experimental observations using self-oscillating physical models. It was further hypothesized that these differences may facilitate complete glottal closure in finite-amplitude vibration of anisotropic models as observed in recent experiments.

Regulation of glottal closure and airflow in a three-dimensional phonation model: Implications for vocal intensity control

“…This study confirmed a previous experimental observation that anisotropic vocal folds were better able to maintain their position against the subglottal pressure (Zhang, 2011(Zhang, , 2014Xuan and Zhang, 2014). Although the vocal folds were still pushed open with increasing subglottal pressure, the increase in the mean glottal area and flow rate decreased with increasing vocal fold AP stiffening (i.e., increasing anisotropy) and were much smaller than those observed in isotropic models.…”

“…The wave-like motion, particularly on the lateral surface, was also more obvious in Fig. 3 with an increased AP stiffness, which is consistent with the observation in Zhang (2014).…”

“…In addition, our previous studies using similar computational models have been able to reproduce experimental observations regarding sound production by confined pulsating jet flows (Zhang et al, 2002), dependence of phonation threshold pressure on vocal fold properties (Mendelsohn and Zhang, 2011), vocal fold vibration patterns in different vibratory regimes, and transitions between regimes (Zhang and Luu, 2012;Zhang, 2014). Based on the above, we concluded that our model captured the essential features of glottal fluid-structure interaction and was sufficient for qualitative investigations of the regulation of glottal closure and flow rate in phonation.…”

“…In other words, the vocal folds behaved mechanically as a one-layer structure for most CT/TA conditions. Thus, in this study, the vocal fold was modeled as a one-layer transversely isotropic, nearly incompressible, linear material with a plane of isotropy perpendicular to the AP direction, as in previous studies (Titze and Talkin, 1979;Itskov and Aksel, 2002;Zhang, 2011Zhang, , 2014. The material control parameters for the transversely-isotropic vocal fold included the transverse Young's modulus E t , the AP Young's modulus E ap , the AP shear modulus G ap , and density.…”

“…However, human vocal folds are inherently anisotropic, due to the presence of collagen and elastin fibers along the anteriorposterior (AP) direction. Recent studies (Xuan and Zhang, 2014;Zhang, 2014) suggest that an inherently anisotropic vocal fold may be able to better maintain its position against the subglottal pressure. Thus, one may wonder, for anisotropic vocal folds as in humans, if a typical mean glottal flow rate can be maintained by adjusting the degree of vocal a) Author to whom correspondence should be addressed.…”