Influence of voxelization on finite difference time domain simulations of head-related transfer functions.

Sebastian Prepeliță,Lauri Savioja,Federico Avanzini,Michele Geronazzo

doi:10.1121/1.4947546

Abstract

The scattering around the human pinna that is captured by the Head-Related Transfer Functions (HRTFs) is a complex problem that creates uncertainties in both acoustical measurements and simulations. Within the simulation framework of Finite Difference Time Domain (FDTD) with axis-aligned staircase boundaries resulting from a voxelization process, the voxelization-based uncertainty propagating in the HRTF-captured sound field is quantified for one solid and two surface voxelization algorithms. Simulated results utilizing a laser-scanned mesh of Knowles Electronics Manikin for Acoustic Research (KEMAR) show that in the context of complex geometries with local topology comparable to grid spacing such as the human pinna, the voxelization-related uncertainties in simulations emerge at lower frequencies than the generally used accuracy bandwidths. Numerical simulations show that the voxelization process induces both random error and algorithm-dependent bias in the simulated HRTF spectral features. Frequencies fr below which the random error is bounded by various dB thresholds are estimated and predicted. Particular shortcomings of the used voxelization algorithms are identified and the influence of the surface impedance on the induced errors is studied. Simulations are also validated against measurements.

Highlights

One important acoustic wave scattering problem is the influence of the human body on a sound field in the vicinity of the ear canal since it contains all of the necessary cues for sound localization
For a staircase approximation of the boundary, different approaches exist: the continuous geometry is directly acquired into voxels and further processed or first scanned at fixed, discrete locations and the resulting polyhedron mesh voxelized to be used in an Finite Difference Time Domain (FDTD) simulation
This can cause voxelized external ears with holes that may result in unreliable headrelated transfer functions (HRTFs) simulations

Summary

Introduction

One important acoustic wave scattering problem is the influence of the human body on a sound field in the vicinity of the ear canal since it contains all of the necessary cues for sound localization Such influence can be modeled as linear time-invariant filters and are generally referred to as headrelated transfer functions (HRTFs) or head-related impulse responses. These direction-dependent filters are usually measured in anechoic conditions such that the transmission of an incident plane wave generated by a point source from the farfield to a point close to the ear canal is captured for each ear.. The voxelization process of a three-dimensional (3D) geometry is neither a straightforward nor trivial task: depending on the constraints and the required properties of the resulting geometry, it is possible that it will not preserve the mesh topology or that it will not be unique. Different algorithms are designed for the voxelization process that have their own parameters, which in turn may generate additional errors in the resulting sound field, independent of the FDTD update scheme and boundary implementation

Methods

Results

Conclusion