I though some background in hearing aid history might be important at this point.
When my Dad first noticed his hearing loss, he was told that hearing aids wouldn’t help much. Most hearing aids then (into the 1950’s) were “body” style with a flat frequency response that rolled off at about 2000Hz. There wasn’t a lot of difference between hearing aids, so there wasn’t a lot of choice available.
A big event in hearing aid history was the introduction of the transistor, replacing the vacuum tube, in the 1950’s. This meant that hearing aids could be made smaller, and there was more scope to have different frequency responses. Since then the same drivers have been behind hearing aid development – the goals have been to make them smaller, and to have more choices in setting up the frequency response. In the mid 70’s non-linear circuits became available, introducing more scope in dealing with reduced dynamic range. This also marked the beginning of a research industry in working out how to predict how to set up hearing aids for people. The next big step change came with digital programmable, and then digital signal processing. More on that later, but the latter has provided us with the opportunity to really think outside the box.