Multiphysics Software Helps Build Hi-Fi Hearing Aids

Widex A/S, Vaerloese, Denmark, develops and manufactures hearing aids with integrated digital signal processing. Classical, behind-the-ear (BTE) devices include a microphone for picking up sound, a unit for amplifying and processing sound, and a receiver for playing the sound. A sound tube connects the electronics-based module to an earmold sitting in the ear channel. Because small amounts of amplified sound inevitably leak out from around the earmold, the device’s microphones might pick up the sound, causing squawks and other annoying feedback effects. Sound radiating through the tube can also lead to feedback.

In the hearing aid, feedback algorithms eliminate these negative effects. The algorithms are based on an estimate of the acoustic-transfer function from the miniature loudspeaker (receiver) to one of the microphones. The specific nature of this transfer function is dependent on many variables, including the shape of the pinna (the projecting outer portion of the ear, also known as the auricle) and the location of the hearing aid on the ear. Consequently, to get a more detailed understanding of the system, we analyze hearingaid models in Comsol Multiphysics software. This lets us understand the detailed acoustical effects and thereby develop stable, better-working algorithms.

For example, the multiphysics software lets us study details that are extremely difficult to see experimentally, including radiation from the earmold and earmold tube and internal structural vibrations. And while today all models in a given product line use roughly the same feedback algorithm, it might someday be possible to scan an individual’s ear and use mathematical modeling to design customer feedback algorithms.

We find the multiphysics software particularly useful in the modeling of thermoviscous acoustics behavior because the underlying equations are not standard, at least as far as we know, in any other commercial simulation package. The software even lets us incorporate our own systems of equations, critical for the development of the hearing-aid models. In addition, we find the freedom to specify arbitrary boundary conditions quite useful.

Basically, our modeling and analysis process works like this: We read-in Pro/Engineer CAD drawings supplied by the mechanical design department, then set up the problem in Comsol’s PDE mode. We transfer results to a specialized simulation and algorithm-development environment we have created based on Matlab. We have found a close agreement between sound-pressure plots from hearing aids using these algorithms and test setups we have constructed using plastic ears.

The simulation results help us optimize the hearing aids for mechanical stability without further feedback. Analysis also lets us determine the best place to position the microphones. These actions are possible because the software lets us better study the so-called “shadow” effects of the ear, where sound from certain directions is blocked to some extent.

Comsol helps us gain a far-better understanding of ear acoustics in general and how they affect the sound field a hearing aid can measure. We can perform virtual tests on changes to the hearing-aid geometry, and the insights we gain into the physics of the system help us determine how various parameters interact. And while the current model includes only an ear on a flat surface, we hope later models will help us see how the entire head influences incoming sound.

— Edited by Leslie Gordon

Author Mads J. Herring Jensen is a project engineer who works in the audiological research lab of hearing-aid manufacturer Widex A/S.