Perhaps the earliest use of hearing assistance was the cupped hand behind the ear. This is still an effective way of increasing the sound to the ear. In fact, just cupping your hand behind your ear can give you a 12 dB boost in volume at 1,000 Hz, and somewhat less at 2,000 to 3,000 Hz where we hear much speech.
One of the earliest form of hearing aids was the ear trumpet or ear horn. The general theory behind ear trumpets is to capture more sound and to provide some directionality towards the wanted sounds, while at the same time sheltering the ear from the unwanted background sounds.
Ear trumpets were most effective when used close up with the person speaking directly into the opening. By necessity, they could also be used to listen to sounds from a distance, such as a lecture or concert, but naturally suffered from the same limitations as older modern hearing aids. For example, they would pick up more background and environmental sounds.
Ear trumpets came in many shapes and sizes. Modern references to hearing trumpets really begin at the beginning of the 19th century, and there were a number of hearing aid manufacturers throughout the century.
Hearing trumpets remained the only viable form of assistance to hard of hearing people throughout the first two decades of the 20th century since the early electrical (carbon) hearing aids were not terribly effective and were slow to catch on.
Electric hearings aids came into play at the beginning of the 20th century, with the advent of the carbon microphone. While based on the telephone principle, Alexander Graham Bell was not involved in their development. The carbon microphone reproduced sound by using sound waves to compress carbon against a diaphragm. As such, these carbon models were ineffective for serious hearing loss.
By the 1940s, the more portable vacuum tube was introduced. Supposedly invented by Earl C. Hanson, he called his hearing aid the “Vactuphone.” These models were able to address more severe hearing loss, however they required two batteries, so costs were rather high at the time. Vacuum tubes became smaller but remained awkward over the next decade, until transistors appeared.
The introduction of transistors was a massive step forward, allowing for the production of far more portable hearing aids. The prototypes for today’s Behind and In the Ear (BTE/ITE) arrived on the market, containing analog technology. While analog technology allowed for far more comfort, discreetness and sound quality, their ability to filter noise and speech was quite limited. Analog hearing aids can still be found to the present day.
By the time the 1980s rolled around, companies started introducing digital signal processors (DSPs) into their hearing aid designs. The form factor shrank substantially and the proliferation of channels and bands allowed for vastly more granular sound filtering and amplification. DSPs are the cornerstone of today’s hearing aids, making up the vast majority of sales.