hearing aid(redirected from eyeglass model hearing aid)
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a sound-amplifying device used by persons afflicted with deafness or impaired hearing. The earliest hearing aids were purely acoustic devices, such as ear trumpets, whose narrow end was inserted into the ear. Modern hearing aids are electroacoustic devices consisting of a microphone, an amplifier, and an earphone. The sound enters the microphone, where it is converted into electric voltage. Amplified by electronic tubes or transistors, this voltage enters the earphone, where it is reconverted into amplified sound.
Hearing aids differ in design according to the nature of the auditory impairment. The standard earphone, inserted into the ear, transmits amplified sound into the external ear, and the bone conduction receiver, pressed against the mastoid process of the temporal bone, transmits sound into the inner ear.
Miniature hearing aids have a miniature earphone that is attached to an individually molded insert in the external ear. Some hearing aids are built into eyeglass frames or hair clips. Pocket hearing aids are manufactured as well. Hearing aids are prescribed by a physician and individually fitted in specialized laboratories after exhaustive hearing tests.