Hearing Aid - Electronic Hearing Aids

Hearing Aid - Electronic Hearing Aids

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Electronic hearing aids. an electronic hearing telephone by Alexander Graham Bell in 1876 made it possible to receive sound pressure waves, convert them to a varying electric current, and reconvert the varying varying current to sound pressire waves. By about 1920 a telephone type of hearing aid had been developed. basically, it consisted of a carbon-granule microphone that converted sound to a varying electrical current; an electrical circuit that carried the current to a coil around a magnet; a magnetically actualed diaphragm that moved back and forth and acted as a sound pressure generator; and a battery that supplied additional energy, so that amplification was not limited by the energy that could be obtained from the original sound pressure wave. The electrical part of this type of hearing aid made it possible to adjust the amplification and to amplify only in desired frequency regions. One disadvantage, however, was that the carbon microphone produced considerable background noise.

Electronic Hearing Aids. An electronic hearing aid consist basically of a microphone; a vacuum tube or a transistor amplifier; volume and tone contrils; an earphone with a diaphragm that act as a sound pressure generator; and a battery.

In the 1920's, improvement of the vacuum tube made it possible for hearing aids to have electronic amplification and to use a magnetic microphone instead of a carbon microphone. Miniature vacuum-tube amplifiers with increased efficiency were introduced in the early 1930's, making it possible to use smaller batteries and to package the microphone and the batteries in a single case.

In 1948 the invention of the transistor greatly reduced the battery size required for a hearing aid, and the very small size of the transistors made it possible to use several of them for implification, increased stability, and frequency response control. With the size and weight advantages provided by transistors in the early 1950's, the entire electrical circuit of the hearing aid could be made as a small unit and placed inconspicously behind the ear or in eyeglass frames. As a result, the transistor hearing aid replaced the vacuum-tube hearing aid, which itselft had supplanted the telephone type.

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