Computational Modeling of Frequency Responses in the Outer Ear

Master's defence by Jonas Due Vesterleden, DIKU

It takes place: 17 August, 2012 at 10:00 a.m. in meeting room A + B

Abstract:

Modern day hearing aids can be fitted entirely in the human ear canal, minimizing the cosmetic impact to the wearer while remedying his or her loss of hearing. An unresolved and common issue for the wearers is a reduction in the ability to spatially localize incoming sounds. This thesis proposes the use of acoustic simulations to investigate the causes of this issue and to improve the individual fitting process for new hearing aid users.

Similar work in the field focuses on "head related transfer functions" (HRTFs), a measure of the effect of the human head on a sound field versus the unaffected sound field. The current work takes a new approach of comparing the sound fields in the human ear canal with and without the hearing aid. This gives results that are more directly applicable to the design and fitting of new hearing aids. Basing the simulation process on scans of the ear canal only as opposed to full scans of heads and ears, the work sets the stage for a full scale industrial setting, where only quick and simple scanning methods would be viable.

The thesis presents an introduction to acoustics in terms of its mathematical and physical representations. Discretization methods for these representations are explained and existing frameworks implementing these methods are examined. COMSOL Multiphysics, a commercial package, is evaluated and found highly suited for the work, but discarded on grounds of licensing issues. Next, the open source computational fluid dynamics package, OpenFOAM is evaluated, but also discarded because of missing implementations of specific features crucial to the acoustic simulations.

Finally, the open source package, Elmer is evaluated and found adequate. The physical correctness of the Elmer acoustics implementation is verified through several test cases with known analytical solutions.

Using a simple idealized model of the human head and ear, the ear canal simulation setup is tried and two methods for visualizing the results are compared. Then, a total of nine human ear canals are set up in simulations and the results are compared, showing a significant effect of the presence of a hearing aid. Furthermore, the effects are found highly angle- and frequency-dependent, but also strongly tied with the placement of the aid in the canal. This confirms that the effects of inserting a hearing aid could be connected to decreased abilities in spatial localization, and that the effects vary with individuals.

Finally, the thesis presents the next natural steps to be taken before applying the methods to real world hearing aids. Building on these, a long term vision of a method for individually corrected devices that preserve spatial cues is outlined.

Supervisors: Kenny Erleben and Sune Darkner, DIKU

Censor: Niels Jørgen Christensen, IMM DTU