166 - Hearing aid transducers, by Mead C. Killion (p.1979 to 1990)
\nTransducers (and thus hearing aids) have gotten dramatically smaller since the 1950s, when they were body-worn; weirdly enough, smaller transducers perform better on a number of factors.
\nMagnetic receivers are the most energy-efficient (as of the 1997 writing of this encyclopedia article).
\nThe receiver consumes 50-95% of the power in a hearing aid.
\n
Small receivers have led to the problem of internal feedback: if you've got a profound hearing loss and your receiver is consequently pumping out super-high air pressures out the business end of your device, it's not just going to vibrate your eardrum -- it's going to chug around your ear canal and vibrate the case of your hearing aid as well. And when your hearing aid shell vibrates, it pushes the air near it around, which creates sound -- which faithfully gets picked up by the microphone and amplified, and voila, feedback. Ouch. (Only true for in-the-ear aids; BTEs and other things separating mic/receiver avoid this problem.)
Researchers used to think HAs should have a narrow bandwidth (50-3000 or 50-4000 Hz) to cut out all the \"noise,\" but the extra speech cues outside that band actually get more important if you have a hearing loss.
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If you want to have the HA receiver in the canal, you need a small receiver.
\nSmall receivers have lower MPOs than large ones.
\nThe definition of a \"low\" MPO is subjective; if the user turns the gain all the way up and it's still not loud enough, it's low. While turning up the gain might make softer sounds louder (and more audible), the loudest sounds won't get any louder like they need to be because you're running into the physical limitations of the receiver. Everything just gets crushed up to the maximum-loudness ceiling, compromising intelligibility.
\nOne way of making hearing aids \"louder\" is simply inserting them deeper, closer to the eardrum. This doesn't solve all problems, though; in particular, it's not a replacement for the proper MPO.
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