Acoustic perturbation equations and Lighthill’s acoustic analogy for the human phonation

In speech, air is driven through the larynx by compression of the lungs. Thereby, air flows through the glottis which forces the vocal folds to oscillate which in turn results in a pulsating air flow. This air flow is the main source of the generated sound—the phonation. The acoustic wave then passes through the vocal tract, which acts as a filter modulating the propagated sound leaving the mouth. We model the fluid-structure-acoustic interaction with a so called hybrid approach. The air flow in the larynx, together with a prescribed vocal fold motion, is simulated with help of the open source solver OpenFOAM. Based on the resulting fluid field, acoustic source terms and the wave propagation is calculated within the finite element solver CFS++. Two methods are available to choose from, Lighthill’s acoustic analogy and an aeroacoustic analogy based on a perturbation ansatz. Additionally, the simulation domain is extended by a realistic but geometrical fixed vocal tract and connected to a propagation region. The different acoustic approaches are compared, by analyzing the acoustic pressure in the glottis (source region) and outside the vocal tract. Moreover, to illustrate the effects of the vocal tract an alternative geometry is used for comparison.