What is the correct ear-canal boundary condition for simulating HRTFs for open-ear listening?

Abstract

Emerging open-ear binaural reproduction methods for virtual and augmented reality are an opportunity to rethink how we pose the simulation problem for head related transfer functions (HRTFs). Historically, researchers have experimentally measured HRTFs using a simplifying blocked meatus condition, then used those HRTFs to reproduce spatial audio using low-latency head tracking, an HRTF convolution engine, and carefully calibrated free-air-equivalent coupling over-the-ear headphones. This blocked meatus boundary condition was then carried over as researchers have moved toward numerical simulation of HRTFs, allowing for direct comparison of simulation and experiment. Now that the industry is trending toward numerically-simulated HRTFs and open-ear playback methods, a more realistic ear-canal input impedance needs to be further investigated. In this paper, we report changes in the sound pressure at the entrance to the ear-canal from open to blocked meatus conditions, using a Multiphysics Finite Element model. The blocked meatus boundary is replaced with a nominal frequency-dependent ear-canal input impedance and the pressure-division ratio of open to blocked condition is evaluated. These modeling results suggest that it may not be necessary to perform detailed geometry capture of the ear-canal and that a simplified input impedance is sufficient to simulate the open ear effect.