A control mechanism of voice intensity in relation to expiratory and laryngeal functions

Abstract

A model explanation of controlling voice intensity in relation to expiratory and laryngeal functions is reported. Using the phonatory function analyzer PS-77 (Nagashima Medical Instruments, Tokyo), a log-linear relationship between the vocal intensity I* (dB) and the mean airflow rate U(cm3/s) was obtained during a stepwise increment of intensity with a constant pitch. More precisely, a piecewise linear relationship with an S-shaped pattern was obtained in an I* vs U* display, where U*equals; 20 log (U/U0). At most, three line segments with different slopes are necessary to approximate these relationships. To examine, theoretically, the controlling mechanism of the intensity for the different slope values in I* vs U* display, a simple intermittent triangular wave was assumed as the glottal sound. Using the model waveform, the vocal intensity can be expressed by a functional relationship with the mean airflow rate U and the open quotient. Based on this functional relationship and speculation on the physiology voice production, the contribution of laryngeal control and expiratory control on vocal intensity is discussed. Results of the experiment indicate that laryngeal control is more dominant in the middle intensity region than in either of the higher or the lower intensity regions. These findings may contribute to developing an effective means of noninvasive voice assessment.