[Visualization of the vibratory movements of the vocal cords under asymmetrical conditions]

In this study, we present a new system for measuring the vibration signal of the larynx which allows the vibration of each vocal cord in an excised larynx to be studied using a method of distant optical measurement. The set-up consisted of an insufflator and a device for immobilising the larynx, together with microprobes capable of creating mechanical pressure at the laryngeal ventricles. The whole system was computer-controlled. The measurements were carried out under normal and asymmetrical conditions on seven excised pig larynges. Measurement of the individual vibration of each vocal cord showed that the transient period of commencement of vibration (vocal onset) is influenced to a significant extent by the degree of asymmetry. Under conditions of extreme asymmetry, movement of the cords appeared to be aberrant, but not random because of the coupling mechanisms which cause "anchoring" of the two cords during the phase of closure. Thus one cord may vibrate at twice the frequency of the other. The method described has given us a better understanding of the mechanism of laryngeal vibration, and one which is in conflict with the theoretical harmonic model in many details.