A close look at this early 60s Zenith Diplomat hearing aid is interesting, as it shows how far electronics and transducer technology have advanced since that time.
The system consists of the following major parts:
The input and gain parts: A microphone which is at the top and has a little brass tube on the end.This is connected to
an amplifier and power source, probably two stages of bipolar transistors, in a simple circuit
seen here as a green PCB, single layer with discrete transistors, rotary potentiometer, and on-off switch.
The output: A transducer that is inside of its own little container to prevent feedback to the microphone, and connected to the audio tube, which in this case is a piece of tubing, not a glass audio tube by any means! Finally a fitted earpiece complete with residual earwax. designed to fit tight and deliver all the energy the little battery limited amp can produce.
The gain curve in a hearing aid is of interest. The idea is to help people overcome nerve deafness which is most prominent at the higher frequencies. Hearing loss prevents people from easily understanding normal speech and carrying on a conversation, especially in a noisy environment. In order to boost intelligibility, the makers emphasize the midrange. Nothing much below 300 Hz and nothing at all about 8 or 10 KHz.
Some day, implants will be able to restore hearing. Here is a link to work at UC Irvine, where small nanotube arrays are being looked at for future cochlear implants. Notice on the same page there are stories about resonant nanotubes, another area our lab pioneered.