A hearing aid is an FDA approvable medical device that needs the FDA approval to be sold. Makers of these devices zealously guard their turf with FDA police(whatever enforcement branch this is, I do not know) and items with an intrinsic cost of materials of $50 or so, go for thousands via ranks of licenced audiologists. A dispensed hearing aid user has their audio sensitivity mapped via audiometers, and the amplifier is made to map to their remaining response sensitivity curve - in theor to give them normal hearing (if possible, some frequency hair cells may have atrophied or been killed by intense sound at some point - in which case they do the best they can. Recently a number of patents have expired, so lower cost mapped hearing aid have emerged. You can buy from China for $50-150 (some have mappability, the cheap ones are just an in ear microphone and amplifier. Care must be taken to make sure that you do not crank up the volume too much = more damage. these guys are in the middle. https://www.bose.com/en_us/products/headphones/earbuds/sound...