Laryngeal flow-structure interaction during phonation

Short term visitor's project

Voice may be regarded as the final result of a chain of coupled oscillators. The vocal folds at the larynx constitute a biomechanical oscillator that acts as the sound source in phonation. The physics of the oscillation has been enunciated by the so-called Myoelastic-Aerodynamic Theory: in general terms, the oscillatory motion is caused by interaction of the airflow blowing through the glottis, the elastic structure of fold tissues (fluid-structure interaction) and the acoustics of the upstream and downstream airways (aeroacoustic interaction). This mechanism of sound production is common to most mammals and also to songbirds by action of their syrinx's membranes.

In normal conditions, the left and right vocal fold oscillators synchronise their movement in complete in-phase entrainment. Coupling is provided by the airflow blowing through the glottis, and by collision between the folds during their oscillatory cycle. Asymmetries due to polyps, nodes, paralyses and other pathological conditions may cause other entrainment regimes, toroidal and even irregular oscillations. The vocal fold motion may be also analysed in terms of coupled oscillators at different portions of the vocal folds - the upper and lower halves of each fold or the anterior and posterior halves. The pulsatile airflow may also engage the ventricular folds, as in Tuvan harmonic throat singing and ventricular dysphonia, and even the aryepiglotic folds, as in guttural voices employed by heavy metal singers.

The vocal fold oscillation is also coupled to the acoustical resonances of the upstream and downstream vocal tracts. When the oscillation frequency crosses a resonance, frequency jumps and other instabilities may occur. Such is the case of the voice breaks of yodeling. Lyric singers, on the other hand, train to avoid those breaks. They also learn to tune the vocal tract resonances to the voice frequency so as to enhance the radiated power. Still, the cycles of respiration, cardiac rhythm, and neurological oscillators associated to the innervation of laryngeal, vocal tract and pulmonary muscles affect the oscillation causing fluctuations of frequency and amplitude. Vibrato itself, the singing ornament, is a voluntary fluctuation caused by coupling to a neurological oscillator.

Here, we propose to explore the dynamics of this oscillator chain by using mathematical models of the involved physics, developed by Dr. Jorge Lucero at University of Brasília (Brazil), combined with experiments on mechanical replicas of the larynx and of vocal tract being developed by Dr. Xavier Pelorson at LEGI. Our past collaboration in this context has already considered the analysis of phase entrainment regimes in the presence of left-right stiffness asymmetries of the vocal folds. In this project, we propose a continuation of such work to focus on the characterisation of disordered or pathological vocalisation.