A manufacturer of hearing aids found that finite element analysis (FEA) lets its engineering team simulate vibration velocity, generate acoustic holography in 2D and 3D, and measure sound-pressure levels inside and outside a hearing instrument. The capability also lets the team experiment with different types of rubber, plastic, and other materials to evaluate their damping and stiffness.

“The distance from microphone to receiver, or loudspeaker, inside the hearing aid is only 2 to 3 mm so it's easy to generate feedback, the loud squeal that comes when output sound feeds back into the microphone,” says GN ReSound Senior Acoustic Engineer Morten Sondergaard. “We are trying to produce up to a 90 dB gain, the noise-level difference between a normal office and a loud rock band, and without exceeding the feedback limit.”

Not long ago, hearing aid prototypes were tested in the lab and design modifications were based on results. Adding FEA to their R&D effort lets company engineers improve their testing and thereby reduce the number of prototypes and shorten development cycles. “Before simulation, we were limited to a trial-and-error approach for all hearing aid designs and tests,” says Sondergaard. “We were essentially working with a ‘black box’ and could only measure from the outside to get information. Simulation lets us look inside the black box to evaluate and alter its behavior.”

Sondergaard's team uses Abaqus FEA software (simulia.com) from Dassault Systèmes to experiment with new materials and geometries. “A hearing aid's feedback path has many parameters that can be difficult to accurately assess with traditional measuring equipment,” says Sondergaard. “Some vibro-acoustic behavior is impossible to measure or visualize without FEA.”

Hearing-aid components that get particular attention include behind-the-ear hearing instruments that connect to a PVC sound tubes and a coupler that represents the ear canal. Simulation looks at vibration and sound pressure stresses that approximate real-world conditions. Afterwards, prototype testing lets the design team assesses performance and validates results.

To model a hearing instrument, the engineers start with simplified geometry. Their CAD software transfers models of parts and assemblies into Abaqus/CAE. Users then define materials, mesh models, and examine results. An associative connection between CAD and FEA lets them make model changes in the CAD software, update the FEA model, and rerun the simulation.

One particular simulation focuses on the critical connection between a steel receiver housing and a tube that fits over the receiver sound port. A “shrink-fit” function in the software models the pretension in that portion of the tube that stretches over the receiver sound port.

Acoustic-resonance frequencies are important in a sound-amplifying device. Engineers simulate these with modal analysis in the software to find a structure's frequencies of natural vibration and specific vibration patterns.

Another challenge is to account for the air around and inside the hearing aid, and then to analyze the interaction between the air and the unit. The multiphysics capabilities within the software help model the hearing aid structure and couple it to the air, using surface-based tie constraints, and without matching meshes between the two, a necessity in some software.

An average model has 200,000 to 300,000 elements and 1 million degrees of freedom. The team runs the models on a computer with four Xeon CPUs and can test two models in overnight runs.

Sondergaard says close agreement between FEA models and lab tests give the team confidence and design freedom to adjust components and materials in their models for high stability — no feedback or squeal — and maximum sound gain. “We have a greater understanding of the causes for instability so we can eliminate them early in design stages,” says Sondergaard. “Once we have a working model, we can improve it before it goes into production. Simulation data also lets us give researchers advice and guidelines for developing future designs.”