A laser-based technique to evaluate sound generation during phonation

Abstract

Investigating aeroacoustic sound generation in a measurement is challenging since the flow field needs to be highly resolved in the temporal and spatial domain. Thus, complex aeroacoustic phenomena like the mechanisms of the human voice are still not fully captured scientifically. Recent advances in optical technologies have pushed the limits of measurement resolution and accuracy, making aeroacoustic investigations potentially accessible. Therefore, as a first part of this thesis, a measurement technique is implemented that applies a hybrid acoustic PIV (HAcouPIV) approach. State-of-the-art measurement technology is combined with most recent numerical strategies in the field of acoustics. High-speed PIV is applied to determine the flow velocity, and a numerical approach computes the pressure field based on the measurement data. Then, a perturbation approach is applied to separate the sound pressure from the fluid-mechanical pressure fluctuations. In the second part of the thesis, the HAcouPIV technique is employed to investigate the sound-generating mechanisms and its radiation during human phonation. Thereby, for the first time, the acoustic source field is extracted based on a combination of experimental measurements and numerical simulations. A highly modulated source field that reaches into the supraglottal region was found as the most significant sound-generating mechanism of the phonatory process. This was shown for both broadband and harmonic content. The analysis of the vibroacoustic behavior of the vocal fold oscillation based on laser-vibrometry data revealed strong harmonic content in the glottal region of the superior vocal fold surface. Hence, it potentially contributes to a strong fluid-structure-acoustic interaction especially for the harmonic content. In conclusion, the hybrid approach of laser-based measurement techniques and numerical evaluation procedures was successfully applied. It substantiates the fundamental mechanisms that form the basis of sound generation during phonation. These results suggest to focus on the modulation of the glottal jet and its entire spatial expansion as the main component for sound generation during voiced speech.