How Hearing Aids Work
The hearing aid is an instrument designed to compensate for loss of hearing in some cases of deafness. All hearing aids work by increasing the amount of sound energy that reaches the ear. Simple hearing aids, such as man's cupped hands or an ear trumpet, do this by collecting more sound energy than the ear normally receives. Electronic hearing aids work by amplifying the sound electronically. All modern electronic hearing aids now use transistors for amplification rather than vacuum tubes, which are bulky.
The transistor hearing aid consists of a magnetic microphone, one or two batteries, a transistor amplifier, and an earphone, or receiver. The microphone changes the sound waves that strike it into electrical impulses, which are then strengthened by the amplifier. The strengthened impulses flow to the earphone, which converts them back into sound waves like the original ones, but stronger. Occasionally the receiver element is set next to a bone behind the ear, and transmits the sound vibrations directly through the bone to the inner ear and the auditory nerve.
Most hearing aids have a volume control knob to allow the user to adjust the amplification as needed. Some types of hearing aids also allow the user to control the frequency range of the amplified sound.
The ear trumpet was the earliest artificial hearing aid. Shaped like a flaring horn, it had a large receiving end and a small opening at the end that was inserted into the ear. The horn collected more sound energy than the unaided ear and funneled this greater volume of sound directly into the ear.
The carbon hearing aid was developed around 1900. Like the early telephone, it had a carbon microphone and a magnetic earphone. A battery provided the energy needed to increase the sound volume. In the 1930's the electronic hearing aid was developed. This was similar to the hearing aids of today except for its larger size, due to the use of vacuum tubes instead of transistors. The transistor hearing aid was developed in the early 1950's.