Experimental study of self-oscillations of the trachea-larynx tract by laser doppler vibrometry

We study the acoustical and mechanical vibration fields generated when Italian speakers emit Italian vowels in order to individuate the role played by the trachea in the speech production mechanism. Specifically, experimental series regard the mechanical vibrations along the trachea-larynx tract measured by using non-contact laser Doppler vibrometry (LDV), contemporaneously we acquire the acoustical field by means of a set of microphones. The experimental apparatus consists of two LDV systems and two microphones: this allows to have synchronized recorded signals to be compared, thus to get information about the vibration field propagation. The two lasers are pointed along the thorax scanning along the tracheal tract. Weobserve that the trachea displays significant self-oscillations that cannot be avoided in the speech production. In particular, a clear self-oscillation at the frequency related to the pitch is extracted by independent component analysis. Thanks to the two synchronized LDV systems, we observe that the mechanical oscillations at the trachea start before those of the larynx and the acoustic oscillations, indicating that a mechanical oscillation is upward propagating. These experiments support the hypothesis that a further hydrodynamic instability should be advocated in the speech production mechanism at trachea level. Moreover, the pitch, typically extracted by looking at the inter-times in the acoustical signals, is directly obtained by measuring the mechanical vibrations of the trachea.