hearing aid, device used in some forms of deafness to amplify sound before it reaches the auditory organs. Modern hearing aids are electronic. They contain a tiny receiver and a transistor amplifier, and are usually battery powered. Some are small enough to fit into an arm of a pair of eyeglasses, or into the outer ear. The bone-conduction hearing aid, placed behind the ear, channels sound waves to the adjacent bony part of the skull, which then transmits the vibrations to the auditory nerve of the cochlea. The air-conduction hearing aid amplifies sounds and directs them into the ear toward the tympanic membrane. In recent years, a number of advancements have been made to hearing aids, improving the comfort, sensitivity, and aesthetic quality of the devices. Today, many hearing aids are customized to amplify only those noises (e.g., high frequency) that the user has difficulty hearing. Cochlear implants have been developed for use by certain totally deaf people. They consist of mechanical replacements for ineffective hair cells in the inner ear, which transform sound vibrations into electronic impulses that stimulate the auditory nerve.
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