Sound Generation and Propagation in the Human Upper Airways

Abstract: The human upper airways embrace the source of phonation and affect the modulation of the voice, which is of vital importance for communication. Moreover, unwanted sounds may be generated in the upper airways due to elastic, collapsible parts that are susceptible to flow-induced vibration and resonance. The sound resulting from fluid-structure interaction in the upper respiratory tract, commonly known as snoring, can be an important indicator for underlying breathing disorders, such as obstructive sleep apnea (OSA). The scope of this work is the assessment of acoustic sources and the conditions for sound being produced in the upper airways in healthy and diseased state. For the study of the vocal tract under phonation, both low- and high-order numerical methods are applied and the obtained results are compared to experimental data from collaborators. The geometries of the vocal tract are based on magnetic resonance imaging (MRI) data for the different vowel pronunciations under voiced speech of a healthy male subject. Unsteady, direct compressible flow simulations using a finite volume solver are carried out for the computation of the pressure fluctuations and the associated distribution of frequency peaks as a result of the modulation through the static vocal tract. The peaks of the envelope of the far-field Fourier spectrum, which are characterising the spoken tone, are extracted and compared to the Helmholtz eigenfrequencies of the airway volume. The effect of variations of vocal fold closure, fundamental frequency, and vocal tract length on the computed acoustic signal is investigated. Thus, an estimation of the impact of vocal disorders on the ability of vowel production is attempted. A particular advantage of the presented approach is the attainment of time-resolved pressure and velocity fields of the flow, which allows for analysis of the coherent structures at the level of the vocal fold constriction, responsible for sound output during unvoiced speech. Furthermore, fluid-structure interaction simulations are performed for the study of the influence of critical parameters, such as Reynolds number of the flow or elasticity of the structure, on the onset of oscillations for a simplified model. The acoustic sources involved in the generation of the dominant frequencies of tissue vibrations are identified by application of an acoustic analogy. The obtained results of this work contribute to the development of a computational tool that assists physicians in the assessment of the airway function and the effectiveness of treatment plans prior to their application.