A battery-powered acoustic device intended to compensate for impaired hearing by transmitting amplified sound waves to the eardrum through air. It typically consists of a microphone, an amplifier, and a speaker (receiver) contained in a case worn on the body (e.g., in a pocket or on a belt). The microphone receives sound waves and converts them into electrical signals which are increased by the amplifier and sent as sound waves, by the speaker, to the eardrum through an earmould connected to the case by a cord. Because it's relatively large size permits high amplification, this device has been used for profound hearing loss; it has largely been replaced with behind-the-ear (BTE) devices.