I will not explain the basics of resonance here; plenty of other places do that. We will, however, talk about how it affects hearing aid designs.
First: your ear is a resonant chamber, just like everything else that's physical. Its resonance frequency is somewhere around 2.2-3.0 kHz unless you're a toddler or an elephant; things change then. Or unless you stick something like a hearing aid into your normal adult-sized ear; that changes things as well - that 2.2-3.0 kHz resonance ain't there any longer.
But you haven't removed the resonant frequency, you've changed it. You now have a differently-shaped chamber of vibrating air stuck between your eardrum and... uh, your earmold (but there's also the ports and tubes and vents and they're different for every ear and hearing aid, so that's an oversimplification). The relevant technical term here is Helmholtz resonance, which is fancy-speak for "stuff that happens to a contained mass of vibrating air." It's the same thing that happens when you blow musical notes on a soda bottle. Nothing particularly unusual. Helmholtz resonance of hearing-aid-in-ear is usually somewhere around 5-7 kHz.
Resonance is one of a bunch of things that affect a hearing aid microphone's frequency response.
- Shape! Obviously. Change the shape of a resonant chamber and you'll change the way it bounces around sound.
- Length, particularly of the sound coupling tube (whatever routes sound from the receiver to the bit nearest your eardrum). The longer the sound coupling tube is, the lower the Helmholtz frequency will be; think of a digeridoo or pipe organ. Longer instrument, lower sound.
- Hole size. I refer here to the diaphragm hole, which is the little hole in the earmold that keeps hearing aids from busting your Eustachian tube with high-pressure sound waves. The larger the hole is, the more it affects the resonance by attenuating (making-softer, muting, fading) the low frequencies.
- The damper, which is the screen in front of the microphone (which I had always thought was just to keep the dust out). It can smooth out the resonant peak.
Hearing aid designers try to adjust these things to get the resonant frequency to be somewhat like an ear's (~2.2-3.0 kHz, again), which is tough because... well, hearing aids are shaped significantly differently from ears. One thing that sometimes happens is to simply have a peak in that section of the frequency response of the receiver (remember, that's the "speaker" bit of a hearing aid) that replaces that natural resonance.
Our next (and last) post on the topic of transducers will talk about Uncomfortable Things.