The human ear, comprising the outer ear, the middle ear, and the inner ear. Beyond the eardrum is the air-filled middle ear cavity bridged by a chain of three small bones (auditory ossicles) forming a mechanical link between the drum and the so-called oval window, which is the entrance to the inner ear. The semicircular canals in the inner ear are concerned with bodily equilibrium, while the fluid-filled spiral shaped like a snail’s shell and divided by a partition, serves for hearing. Located on the partition is the organ of Corti, a complex structure in which the auditory receptor cells are embedded. These cells are connected to the ends of the auditory nerve fibers. Defective, hearing may be caused by a functional disorder, due to an accident or disease, at any point in this system, including the auditory nerve and indeed the auditory center in the brain. In some cases impairment can be cured or alleviated by medical treatment or surgery. In other cases it may be possible to obtain improvement by means of a hearing aid.
In general, a hearing aid is a sound amplifier. The earliest form was the ear trumpet; which amplifies sound by collecting it with a large mouth and leading it down a tapering tube to a narrow orifice which is inserted into the ear. A modem hearing aid is a transistorized electronic device serving to amplify sound by means of electrical amplification. The sound is picked up by a microphone, which converts it into weak electrical currents. These are amplified and passed to the receiver, which converts them back into sound of greater loudness than the original sound. The power for the amplifier is supplied by a battery, which may be of the ordinary dry-cell type or a rechargeable storage battery. Present day hearing aids may comprise several amplification stages combined into a single unit (integrated semiconductor circuit) and are adjustable in various ways to suit them to individual requirements. The user can switch the apparatus on and off, as desired, and he can vary the volume (loudness) to suit the acoustic conditions. Tone control, i.e., a choice of frequency response (the variation of amplification with frequency), may also be provided. The intensity range in which speech is understood may be wide for some deaf people, but narrow for others. In some of these latter cases a hearing aid with automatic volume control may help by smoothing out the variations in sound intensity; this kind of control varies the amplification automatically, so that the output intensity is kept constant.
The user of the early type of electrical hearing aid wore earphones held in place by a headband. Later, this external receiver was replaced by the insert receiver clipped to a molded plastic insert in the outer ear. Sometimes a so-called hone-conduction receiver is used, which functions by vibrating the bones of the skull rather than by generating a sound wave. The hearing aid can be carried in a pocket, the receiver being connected to the aid by a flexible wire. Other types of hearing aid are small enough to be worn on the head—e.g., behind the ear, or built into an eyeglass frame, or in the ear. In these small devices all the component parts, including the receiver, are built into one unit.