Parametric excitation of the acoustic eigenmodes of the vocal tract by motion of the vocal folds: Non-aeroacoustic mechanisms of phonation.
Gordon Ramsay · The Journal of the Acoustical Society of America · 2009
Studies of voice production have focused on aeroacoustic mechanisms of phonation, describing how energy in the background flow is converted into sources of sound. This paper describes a non-aeroacoustic mechanism that may be important in explaining how sound is generated by vocal fold vibration. According to dynamical systems theory, temporal variations in the parameters of a system may result in parametric excitation of the system. Under appropriate conditions, modulation of the parameters of the acoustic wave operator governing sound propagation in the vocal tract by changes in glottal geometry should be sufficient to induce parametric excitation of the vocal tract eigenmodes, creating sources of sound additional to those predicted by classical aeroacoustics. Rapid changes in the shape of the glottis during phonation are already known to create modulations of the formants within each glottal cycle, but the role of these modulations in exciting the formants has not been established. To test this hypothesis, we use a time-domain finite-volume simulation of acoustic wave propagation in a time-varying vocal tract to compare aeroacoustic and parametric excitation mechanisms. Results show that parametric excitation terms that transfer energy between formants, mainly at the instant of glottal closure, contribute significantly to the overall glottal source.