Effects of vocal fold anisotroy and inhomogeneity on the glottal closure pattern during vibration
An important feature of normal human phonation is the complete glottal closure during vibration. However, our early experiments using isotropic vocal fold models showed that these models often vibrated with incomplete glottal closure. To explore possible structural and material properties of the human vocal folds that may facilitate complete glottal closure, seven vocal fold models with different structural features were designed and tested (Xuan and Zhang, 2014). An isotropic model was used as the baseline model, and other models were modified from the baseline model by either embedding fibers aligned along the anterior-posterior direction in the
body or cover layer, adding a stiffer outer layer simulating the epithelium layer, or a combination
of the two features. Phonation tests were performed with both aerodynamic and acoustic
measurements and high-speed imaging of vocal fold vibration. Compared to the isotropic one-layer model, the presence of a stiffer epithelium
layer led to complete glottal closure along the anterior-posterior direction and strong excitation
of high-order harmonics in the resulting acoustic spectra. Similar improvements were observed
with fibers embedded in the cover layer, but to a lesser degree. Presence of fibers in the body
layer did not yield noticeable improvements in glottal closure or harmonic excitation.
This study shows that the presence of collagen and elastin fibers and the
epithelium layer may play a critical role in achieving complete glottal closure.
Numerical analysis (Zhang, 2014) further showed that isotropic models have a tendency to vibrate in an up-and-down swing-like motion, with the entire medial surface vibrating in-phase and a dominantly vertical motion (which does not directly modulate airflow or produce sound). Increasing anisotropy or increasing the stiffness along the anterior-posterior direction suppresses this swing-like motion and allows the vocal fold to exbihit a more wave-like motion along the medial surface. Anisotropic vocal folds also vibrate with enhanced medial-lateral motion, which directly modulates airflow, and thus a high flow-modulation or voice production efficiency. Increasing anisotropy also allows the vocal fold to move in phase along the anterior-posterior length, instead of the out-of-phase vibration pattern observed in isotropic vocal fold models. These vibrational differences between isotropic and anisotropic vocal fold models may facilitate complete glottal closure during phonation in anisotropic vocal fold models as observed in Xuan and Zhang (2014).
Xuan, Y., Zhang, Z. (2014). Influence of embedded fibers and an epithelium layer on glottal closure pattern in a physical vocal fold model, Journal of Speech, Language, and Hearing Research, in press. [pdf] [link]
Zhang, Z. (2014). The influence of material anisotropy on vibration at onset in a three-dimensional vocal fold model, J. Acoust. Soc. Am., 135, 1480-1490. [pdf] [link]