Hearing Aid

Hearing aid is a device that amplifies sound. Many hearing-impaired people depend on this device. It may enable them to do such things as use a telephone, enjoy music, and more easily understand conversation. Millions of hearing aids are sold each year. Most are sold to people over 65 years of age.

Types of hearing aids. There are two types of hearing aids, air-conduction aids and bone-conduction aids. An air-conduction aid amplifies sound and brings it directly into the ear. Most people who use hearing aids have this type of aid. However, in some people, sound cannot be transmitted through the outer or middle ear. These people often use a bone-conduction aid, which brings sound waves to the bony part of the head behind the ear. The bone transmits the vibrations to the auditory nerves of the cochlea. Hearing is possible when these nerves are stimulated.

The parts of a hearing aid. Modern hearing aids have five major parts: (1) a microphone, (2) an amplifier, (3) a receiver or earphone, (4) a battery, and (5) an earmold that fits in the ear. The microphone picks up sound and changes it into electrical impulses or signals. The amplifier increases the strength of the signal. The receiver then changes the signal back to amplified sound that can be understood by the user. The battery supplies power for the operation of the aid, and the earmold secures the receiver in the ear canal.

The main parts of a hearing aid are shown in this diagram of a behind-the-ear aid. They are (1) a microphone, (2) an amplifier, (3) a receiver, (4) a battery, and (5) an earmold.
From The World Book^™ Multimedia Encyclopedia ©1998 World Book, Inc., 525 W. Monroe, Chicago, IL 60661. All rights reserved.
World Book diagram by Arthur Grebetz; critically reviewed by Dyn-Aura Engineering Laboratories.

History. Hearing aids have been used since the 1600's. The first hearing aids used a horn-shaped device to gather sound and funnel it into the ear. Electric hearing aids appeared about 1900. These instruments were quite large and provided little help to people with severe hearing loss. Acoustic fans, sometimes called hearing fans, also appeared about 1900. These devices were fan-shaped bone-conduction instruments made of a thin, hard rubber called vulcanite. A user held the edge of the device between the teeth and bent the fan toward the sound. The sound vibrations traveled from the teeth to the jawbone, the skull, and finally the auditory nerves.

Electronic hearing aids first appeared about 1920. The first of these devices were vacuum-tube aids. At first, these aids were bulky and inconvenient to use. By 1930, however, wearable vacuum-tube aids appeared. These aids used a crystal microphone, a vacuum-tube amplifier, and two batteries. They were more compact and easier to carry and conceal than earlier models. These factors helped many hearing-impaired people overcome their reluctance to use hearing aids.

Another type of electronic hearing aid, the transistor hearing aid, appeared in the early 1950's. These hearing aids are dramatically smaller than vacuum-tube models. They are also less expensive to operate because they usually require only one small battery. For these reasons, transistor hearing aids have completely replaced vacuum-tube models.

Modern transistor hearing aids are made in a wide range of styles and designs. They are very powerful and may be adjusted to a wide range of sound frequencies and levels of amplification. In addition, many transistor hearing aids are small enough to be concealed in the user's ear canal.

Contributor: John B. Christiansen, Ph.D., Prof. of Sociology, Gallaudet Univ.

Hearing Aid

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