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Sounds good: Tech advances in hearing aids

21 Oct 2015  | Bill Schweber

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Some consumer electronic devices, such as smartphones and PCs, get more media mileage and attention from potential buyers, while other products that are equally beneficial to specific audiences and need a diverse and multi-disciplinary set of engineering and productions skills usually end up unnoticed. These devices often do not get media coverage.

Hearing aids are in that second group. A very readable article in a recent issue of Machine Design, "Tech Advances Upgrade Hearing Aids," detailed the latest in electronics, functionality, basic types, power supply, materials used and construction. Today's hearing aids are doing so much more than those of even just a few years ago are that any engineer involved in mixed-signal applications should check it out. (For example, do you know the five basic types of hearing aids?)

Among the features in some of these units are dual microphones (either electrets or MEMS-based types); significant computational power for their algorithms including compression, filtering and noise suppression (especially needed for transient and impulse noise); and electromagnetic-based transducers which function like tiny loudspeakers, sometimes direct mechanical conduction to bone and inner ear (interestingly, this function is called the "receiver" in the industry).

These elements are combined with specialised materials that can tolerate earwax (don't laugh!) and other body-tissue issues. Some hearing aids now have provision for wireless adjustment via smartphone of key settings such as volume or selection of different signal-processing modes to meet specific real-world situations.

Of course, all of these features and functions require power, which is very limited for obvious reasons. Surprising—at least to me—was that hearing aids rely on zinc-air batteries, rather than the more "trendy" lithium-based chemistries, but it makes sense here. These 1.35V to 1.45V batteries develop their power by oxidizing zinc with available oxygen from the air, and range from about 6mm diameter by 2mm thick, up to 11mm diameter and 5mm thick.

They can last for up to 10 days in some hearing aids, although four days is typical, and given the functions and signal processing, those are impressing ultra low-power designs. Since the oxygen comes from ambient air, the battery can be mostly zinc without internal electrolyte or oxidizer, and so has fairly high energy density; the 11mm × 5mm unit has 620mAh capacity and weighs 1.9g. (A related battery chemistry which instead uses silver oxide in place of free-air oxygen is used in some space missions.)

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