Mechanism of initiation of oscillatory motion in human glottis

Phonatory onsets of 17 normal subjects under usual speech conditions were investigated by measuring the cross-sectional area of the glottic aperture, using a photoelectric device. During a normal soft or breathy onset of phonation, the vocal fold vibration begins with an open glottis; the pattern of the optical signal is a sine wave of increasing amplitude with one to eight cycles before steady state oscillation is achieved. The first deflection of the base-line is either towards adduction or towards abduction. The classically assumed--since van den Berg et al. (1957)--retro-aspiration phenomenon, consisting in a narrowing of the glottic chink due to the accelerated air flow, according to Bernoulli's law, is incompatible with these observations. An aerodynamic study, with accurate calibration of the photoelectric transducer in one trained subject, as well as flux and subglottic pressure measurements, suggests that the gas flux (air as well as a helium-oxygen mixture) reaches the condition of turbulence at the level of the glottic nozzle, just before vocal folds are set into oscillatory motion. The setting in motion of the free edge of the vocal folds in normal soft or breathy onset of phonation can be explained by a sudden modification of flow conditions within the expiratory gas : the flow is laminar in the trachea and suddenly becomes turbulent at the level of the glottic nozzle. On the other hand, approximately normal atmospheric pressure values are attained due to a Bernoulli-effect at that level, allowing the vocal folds to oscillate sinusoidally about their vibration axes, corresponding to their virtual resting position, like a forced oscillator.