The earliest and most practical "hearing aid" for human beings was probably the palm of the deaf person's own hand. By placing the palm of the hand over the ear in the shape of a semicircular trumpet, sound could be well collected. Although the gain effect of this method is only about 3dB and it is not a hearing aid in the modern sense of the word, it is the most natural way of hearing aid. Until now it is still possible to see some older people using the palm of their hand to collect sound when listening to others. Many mammals have large ears, so they can hear much better than people.
Inspired by the palm of the hand to collect sound, some people have invented a variety of shapes, simple mechanical devices, such as like a trumpet or conch horn like the "ear trumpet", wooden "hearing board", "hearing tube "ear trumpets" like mouthpieces or screw horns, wooden "hearing boards" and "hearing tubes", "hearing hats" and "hearing bottles" like hats and bottles, "ear flaps" like fans and animal wings, and very long "stethoscopes" like "stethoscopes". The long "speech tubes" like stethoscopes, and so on. Because people think that the longer the hearing tube set sound effect is better, so some of the hearing tube is as long as tens of centimeters, or even more than a meter. Listen to other people's speeches with a hand holding the hearing tube to other people's mouths, the appearance of funny and ridiculous, but it makes the deaf people to improve their hearing. At the same time, it also reminds the speaker to speak as loudly as possible. This simple mechanical hearing aid was used for hundreds of years until the nineteenth century, when it was gradually replaced by the charcoal-refined telephone-type hearing aid.
In 1878, Bell, an American scientist, invented the charcoal telephone hearing aid. This type of hearing aid is assembled by charcoal microphone, headphones, batteries, wires and other components.
In 1890, Austrian scientist Ferdinant Alt prepared a generation of electronic tube hearing aid.
In 1904, Danish Hans Demant and American Resse Hutchison*** invested in the mass production of hearing aids. By the 1940s, there were two types of hearing aids, air-conduction and bone-conduction. This period of hearing aids in technology has been a greater development and improvement, although able to meet the needs of some deaf people, however, there are still many shortcomings, such as too much noise, bulky volume such as 17-inch TV sets, not easy to carry, etc..
In 1920, the thermionic vacuum tube (hot cathode tube) was introduced shortly after the emergence of the vacuum tube hearing aid. With the continuous development of vacuum tube technology, hearing aids gradually become smaller, to achieve the separation of the host and the battery. 1921, the United Kingdom produced a commercial electronic tube hearing aid. Since the electron tube requires two power supplies (one is to heat the filament in the tube to release electrons; the other is to drive the electrons through the grid to the anode), this hearing aid is large and bulky, and is almost impossible to carry around, although the gain and clarity are better. Over time, mercury batteries replaced zinc batteries, resulting in a significant reduction in battery size, and batteries and hearing aids could finally be combined into one. During World War II, new technological materials such as printed circuits and ceramic capacitors made it possible to significantly reduce the size of one-piece hearing aids so that they could be carried around. Gradually, hearing aids also adopted technologies such as peak clipping (PC) and compression (automatic gain control, AGC).