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, as shown in the video below.
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 (as shown in the video below) 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 (see the figure below which compares high-speed images of vocal fold vibration during one oscillation cycle for seven physical vocal fold models of different configurations).
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.
In our latest development, the physical vocal fold model now is able to remain completely closed for a considerablly long period of the oscillation cycle, as shown in the video below.
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]