Effects of horizontal-to-vertical stiffness ratio on the vibration characteristics of a two-mass vocal fold model

Abstract

In order to evaluate the effects of stiffness ratio of vocal fold on three-dimensional behavior of vocal fold vibration, numerical simulations were performed by using a two-mass vocal fold model with different stiffness ratio of the horizontal and vertical direction. A ratio of the spring constants for the same direction were fixed, while it for the orthogonal directions was changed. To simulate the airflow through the glottis, the one-dimensional Bernoulli’s equation was solved with an equivalent circuit for the compressible flow propagation. The vocal fold shape was represented by a smooth shape with cylindrical masses and tangential faces. The flow separation point was estimated using the boundary layer theory. The results showed that overall fundamental frequency of the vocal folds increased with increasing horizontal stiffness, whereas the vertical stiffness changes had few effects on it. We also found that there were topical conditions in which fundamental frequency rapidly changed with a few changes in stiffness ratio. The stiffness ratio at that time was from 0.80 to 1.08. Before and after the change, there was difference whether the motion trajectory of mass was circular or linear. These results indicate that the vertical stiffness largely affects the oscillation modes of vocal folds.