148 A Study of Simultaneous Measurement of Vocal Fold Vibrations and Flow Velocity Distributions Just Above the Glottis

Human voices are originated by vibration of vocal folds in the larynx. Vocal folds are mainly composed of internal (muscle) bands and mucous tissue. It is considered that this tissue vibrates responding to breath airflow from the lungs and the resulting oscillation makes the origin of vocal sound, i.e., phonation. The vocal fold tissue is relatively flexible and an easily deformable substance. However its movement cannot be controlled at will, but the vocal fold may be either tensed or relaxed by controlling the internal laryngeal muscles. Such control may change the vibrating part and its frequency so that the radiated voice tone can be changed. The oscillation of the human vocal fold during phonation can be directly observed in a stroboscopic way or by direct picturing with the help of a high-speed video-scope. Further, the oscillation frequency can be determined by analyzing a sequence of pictures so obtained. However it has been unsuccessful to show clearly the general belief that human phonation originates in consequence of the wave-like motion of the vocal fold tissue induced by the interaction of the tissue and the expiratory airflow (simply, airflow, hereafter). In other words, the complex feature of this phenomenon may not be sufficiently captured by planar observation. Detailed study of this airflow is believed to be important to elucidate mechanism of human phonation with the help of simultaneous observation of vocal fold movement, more specifically, glottal opening by means of ultra-high-speed digital camera. Thus in the present study experimental analysis of this airflow just above glottis was made to clarify phonation mechanism and seek better modeling of vocal folds, focusing on direct measurement of airflow velocity by means of a tiny hot wire probe. Voice was also recorded simultaneously with a microphone located near the mouth of the subject.