At about 1802, F.C.Rein created a passive speaker installed on the telephone, which can provide 10dB amplification in a limited frequency band and is suitable for mild hearing loss.
1902, at this stage, the efficiency of electric carbon hearing aid is controlled by the size relationship between microphone and earphone. Compared with a miniature microphone, a larger earphone has a greater amplification effect. Carbon devices are made of "small" microphones, which are suitable for mild hearing loss. The microphones with moderate hearing loss are twice as large as those with severe hearing loss, and the microphones with severe hearing loss are four times as large.
1934 the first vacuum tube hearing aid was designed by two manufacturing companies. This device needs two batteries, but it is limited to only one day's battery life. A hearing aid consists of a microphone, an earphone/receiver, an amplifier and two batteries.
Personal hearing AIDS appeared in the 1920s. They are the earliest electric hearing AIDS. They are bulky, inconvenient to carry and consume a lot of electricity. In addition, carbon microphones are very sensitive to temperature.
Small vacuum tubes amplify or enhance electrical signals by controlling the movement of electrons in low-pressure space, which creates conditions for the emergence of small personal hearing AIDS. The first generation of small personal hearing AIDS appeared in the 1940s, with more attractive appearance, including a microphone, an amplifier and two batteries.
In the mid-20th century, transistors and glasses hearing AIDS appeared. Hearing AIDS are simplified into a single structure that can be used every day. The sales of the first generation of spectacle hearing AIDS in the US market ranged from 1953 to 1959, accounting for about 50% of the market. Patients can choose ordinary or bone conduction glasses hearing AIDS. Despite its popularity, integrated audio-visual equipment has proved impractical. If patients take off their glasses, they will also lose their hearing AIDS. Similarly, if the patient needs to update his glasses, so does the hearing aid equipment.