标签: Hearing

Some products, such as the latest smartphones, get lots of attention and have a lot of glamour associated with them. Yet there are large groups of products which are extremely beneficial to specific audiences and require a diverse and multidisciplinary set of engineering and production skills, but are often unrecognized in the hot-buzz world of media and product 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 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 specialized materials that can tolerate ear wax (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 1.35 to 1.45 V batteries develop their power by oxidizing zinc with available oxygen from the air, and range from about 6 mm diameter by 2 mm thick, up to 11 mm diameter and 5 mm 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 ultralow-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 11×5-mm unit has 620 mAh capacity and weighs 1.9 g. (A related battery chemistry which instead uses silver oxide in place of free-air oxygen is used in some space missions.) This is about twice the density factor of lithium cells (although frankly, the many numbers I found online for zinc and lithium chemistries each spanned a wide range, and sometimes contradicted each other). Certainly, zinc-air batteries are pretty impressive, and they do not overheat and catch fire even if their terminals are shorted together (lithium batteries, from small to large, have many fire-related safety issues).   While today's hearing aids are not designed to use rechargeable (secondary) batteries for various reasons, including their reduced capacity compared to non-rechargeable primary cells, some vendors are working on wireless recharging. They are looking at the various approaches being adopted by smartphones and other devices to see what can be adapted at both power-sourcing and power-receiver ends, although the small size of the hearing aid makes such recharging very difficult to do efficiently. Perhaps energy harvesting the body's heat or vibration will become the source in the future? It's easy to get caught up in the latest, "hottest" product arena that gets all the attention – think of the hype, hope, and hysterics at industry events such as the Consumer Electronics Show (CES). Yet the less-visible advances that may be of interest to only a smaller segment of the population (but apparently an increasing one, at least in the case of hearing aids) also deserve attention and recognition. They implement an amazing display of truly integrated engineering creativity and innovation, supported by sophisticated manufacturing and evaluation. Have you seen any such less well known yet extremely impressive products, or been involved with their design and implementation?