Evaluation of Acoustic Propagation Paths into the Human Head

Abstract

Abstract : The overall goal has been to develop an acoustic wave propagation model using well-understood and documented computational techniques that track and quantify an air-borne incident acoustic wave propagated around, into and in the human head. This model serves as a computational tool to elucidate the acoustic wave propagation around, into and in the human head. Specifically, the model determines two features: (1) alternate acoustic propagation paths to the cochlear shell that exist besides the normal air-borne acoustic propagation path (eardrum-ossical path) through the auditory canal and (2) sound pressure amplitude in the cochlear shell (relative to the air-borne sound pressure amplitude) via the alternate propagation paths. A 3D finite-element solid mesh was constructed using a digital image database of an adult male head. Coupled acoustic-mechanical finite-element analysis (FEA) was used to model the wave propagation through the fluid-solid-fluid media. The pressure field in fluid media and the displacement field in solid structures were computed at each time step. Instantaneous acoustic pressure waveforms were recorded at various positions inside and outside of the head model, and propagation trajectories (ray paths) were constructed and evaluated from wavefront normals as a function of frequency and incidence angle. The acoustic loss across the skull was estimated to be approximately 33 dB, consistent with theoretical estimates. The computational ray-path results and the theoretical solutions calculated using Snell's law gave a 0.7 difference for low-angle incidence; 10 difference was obtained for larger angle incidence.