Theoretical consideration of the flow behavior in oscillating vocal fold.

Self-excited oscillation of the vocal folds produces a source sound of the human voiced speech. The mechanism of the self-excitation remains elusive partly because characteristics of the flow in rapidly oscillating vocal folds are unclear. This paper deals with theoretical considerations of the flow behavior in oscillating constriction based on general flow equations. The cause-and-effect relationships between time-varying glottal width and physical variables such as glottal pressure, velocity, and volume flow are analytically derived as functions of oscillatory frequency through perturbation analysis. The result shows that the unsteady effect due to convective acceleration of vocal fold wall-induced flow becomes comparable in magnitude to the Bernoulli effect at a high but physiological frequency of phonation. Consequently, a phase difference between the vocal fold motion and glottal pressure appears, enabling self-excited oscillation. The phase-lead of the pressure compared to wall motion is described as a monotonically increasing function of the Strouhal number. The above two effects essentially play the dominant role in the glottal flow. These explicit descriptions containing flow-related variables are useful for understanding of the glottal aerodynamics particularly at high frequency range of the falsetto voice register.