Elastomers used in hearing aids have three purposes: suspensions for microphones and receivers, sound tubes, and combination components.

In the beginning

Their original purpose—suspensions—relies on the material’s ability to absorb energy. If a hearing aid is dropped, the rubber component should be able to absorb the majority of the energy in the fall, not the electronic component. In addition, the suspension has to absorb the vibration associated with the sound waves in the audible hearing range to allow hearing aid manufacturers to develop more powerful hearing aids without feedback. This is being accomplished through advances in the programming of the electronic components and careful selection of elastomeric suspensions. Material choices today included fluorosilicone, fluorocarbon, and Q-Flex compounds by Chicago-based Flexan.

Sound choice

The second typical use of elastomers in hearing aids is for sound tubes. Inside a hearing aid, sound is generated by the electronics and often has to be transmitted through the hearing aid and eventually into the ear canal. Hearing aid manufacturers have often found it best to use elastomeric materials for these applications. The materials chosen for these applications are primarily selected for their ability to perform without acoustic loss. If the materials were to contribute to acoustic loss there could be problems with generating feedback loops inside the hearing aid as well as the loss of some of the acoustic gain generated by the receivers. The material that is most frequently used in this application is a fluorocarbon. Fluorocarbon materials are quite dense and usually much stiffer than other materials used in hearing aids, but they have proven to have low vibration, good energy absorption, and they do a good job preventing acoustic leak.

Over the past 10 years the fastest growing use of elastomers in hearing aids has been propelled by a change in technology in Behind The Ear (BTE) hearing aids. BTE hearing aids today typically have all of the electronic components built into a plastic shell that sits outside the ear with a connection into the ear, typically through a plastic tube, and then a silicone dome is attached to the end of the tube and is worn in the ear canal. The sound is typically transmitted into the ear either acoustically through the tube or electronically with the receiver directly in the ear. In either case domes sit in the ear itself. While other materials have been used in this application, silicone is by far the most popular material for the domes that sit in a patient’s ear. The reason for this is comfort and biocompatibility with the human body.

Domes are now made in a variety of different shapes and sizes depending on the needs of the patient. For high-gain hearing aids there are domes with two, or even three bulbs on them. These domes typically occlude the ear canal and help the hearing aid deliver the most powerful sound into the ear canal. Other applications will find domes with very open structures and large “windows” in the skirt that allow for maximum comfort for the patient who needs a lower gain hearing aid.

Complex combinations

The third use for elastomers in hearing aids is the materials’ design flexibility, which enables the development of combination components. As hearing aids have gotten smaller, the need to both combine and miniaturize components has grown. To build a suspension and a sound tube and then sell them as two components is no longer desirable as doing so simply takes up too much room inside the plastic shell. Now, instead, it is typical for a suspension to include a sound tube in its design.

In addition, this component has to be designed in such a way as to allow it to be used to fit the component into the shell providing both cushioning in the event the hearing aid is dropped and damping to absorb acoustic vibration. This has led to the development of some difficult-to-manufacture components, and difficult decisions on the best material to use to maximize the performance of the hearing aid itself. These components today are typically designed with several ribs, or “nubs” that cause the electronic components to be suspended inside the hearing aid when assembled.

This leads the molded parts manufacturer to consider yet another property of the rubber being used in these applications—creep. When the manufacturers assemble the components into place it is critical they remain there to avoid feedback loops. If the rubber parts move over time it can lead to serious problems with the hearing instrument. Materials used here are typically fluorosilicone, Q-Flex , and fluorocarbon.

With the development of ever smaller components and increasingly complex designs, manufacturers of hearing aid components are challenged when it comes building complex tools that result in the tight-toleranced, thin-wall components required by the hearing aid industry.

To accomplish this, it is necessary to envision several aspects of making the component early in the part design and development stages. Part functionality or even the functionality of a specific area of a part must be looked at closely. Technique-related concerns having to do with de-molding must be considered when designing and building the toog. Gate locations, parting lines, and tool wear are integral factors to consider during the initial review.

These challenges have driven manufacturers of elastomeric components during the past 50 years (see sidebar) and this will continue to be the case as smaller, more powerful devices continue to be developed.

The ’60s is when it all began

Hearing aid companies started to explore the use of various elastomers in hearing aids beginning in the 1960s. The primary use for elastomers early on was to use them to protect the electronic components—primarily microphones and receivers—in the hearing aids. At that point in time there were really only two choices for materials: butyl rubber compounds and chloroprene rubber compounds. Both of these materials could be compounded soft enough, typically in the range of 20 – 30 durometer, Shore A, to allow them to cushion the electronics in the event a hearing aid was dropped. It was soon discovered that there was a second benefit associated with these materials. They provided a bit of acoustic damping and helped eliminate feedback in the hearing aid. With this discovery, the use of elastomers in hearing aids took off.

The problem with the early materials used is that while they both could be made to do a very effective job of damping, they weren’t well suited to withstand the chemical rigors of UV light, ear wax, hair spray, or cologne, leading to early failure of the elastomeric components and a high incidence of hearing aids needing to be returned to have these components replaced.

While some of these same materials are used in hearing aids components that sit deep inside the hearing aids away from harmful environmental conditions, newer elastomeric materials are enabling better designed, more responsive aids (see main article).