Phase-averaged, frequency dependence of jet dynamics in a scaled up vocal fold model with full and incomplete closure

This study focuses on frequency dependence effects on glottal jet dynamics with a focus on the physiological condition in which the vocal folds do not fully close. Incomplete closure occurs naturally in children and adult females. But there are also pathological conditions that can be problematic. Experiments were conducted using a 10× scaled-up model in a free surface water tunnel. Two-dimensional vocal fold models with semicircular medial surfaces were stepper motor driven inside a square duct with constant opening and closing speeds. Cases with complete vocal fold closure and incomplete closure to only 15% of the maximum gap were examined. Time-resolved digital particle image velocimetry and pressure measurements along the duct centerline were made at Re=7200 over equivalent life frequencies from 52.5 to 97.5 Hz. Phase-averaged and cycle-to-cycle analysis of key contributors to sound production were conducted. As would be expected, acoustically relevant parameters, e.g., fluctuations in volume flow rate and transglottal pressure, are attenuated when vocal folds do not close completely. The key findings of this study, however, lie in statistical and dimensional scaling analysis of frequency dependencies of terms in the streamwise integral momentum equation. Specifically, the unsteady inertial term appears to become increasingly significant with increasing frequency and may be a key differentiator between lower frequency phonation, i.e., male voices, and the higher frequencies of children and adult females. These frequency effects, however, do not appear to be relevant to pathological conditions characterized by incomplete vocal fold closure.