Hearing aids are miniaturized sound-amplifying devices that contain a microphone to receive sound, an electronic processor (amplifier) that manipulates the qualities of sound to make it more usable to the listener, a speaker (receiver) directed toward the ear, and a power source. Hearing aids may be bone or air conduction. Bone-conduction hearing aids deliver the acoustic output to the mastoid process of the temporal bone. The bone vibrator is held in place with a headband. Recently, a surgically implanted bone conducting device has been very successful (see below). Candidates include patients in whom air-conduction aids would be difficult such as patients with chronic otorrhea, canal stenosis or atresia, or chronic external otitis.

Because more energy is required to deliver bone-conducted signals than air-conducted signals, air-conduction aids are preferred when feasible. Five styles of air-conduction hearing aids exist: the body-worn aid, the behind the ear (BTE), the in the ear (ITE), in the canal (ITC), and the completely in canal (CIC). Eighty percent of hearing aids dispensed in the United States are in the ITE to CIC size range.

There are special types of hearing aids for unaidable ears. CROS (contralateral routing of signal) is for unilateral, unaidable hearing loss. A CROS aid picks up the signal on the poor side and reroutes it to the normal ear to a nonoccluding ear mold.

BiCROS is for bilateral hearing losses in which only one ear is aidable. A CROS system is combined with a conventional hearing aid such that the better ear receives input from microphones from both ears.

Bone-anchored hearing aids (BAHA^TM) consist of a conventional microphone and amplifier, a specially designed transducer, and a coupling arrangement to attach the device to the skin-penetrating and temporal bone-anchored implant. The degree of conductive loss is of no importance because the middle ear is bypassed by the bone-conducting sound. Patients with a draining ear in need of amplification comprise the largest group for whom these devices are indicated. Other indications include ear canal atresia, maximal conductive hearing loss in the only hearing ear, and external ear canal problems. Recently, bone-anchored hearing aids have been used for unilateral sensorineural
hearing loss to provide sound awareness from the nonhearing side.

Important advances are being made in the area of implantable hearing aids. There are currently several available devices that are partially implanted. Improvements in design to achieve a fully implantable hearing aid are likely to emerge in the near future.

Cochlear implants bypass the function of the sensory receptors by directly stimulating ganglion cells in the auditory nerve. The external microphone picks up sounds and transmits it to a wearable microcomputer (speech processor) that analyzes the sound and converts it to electrical code that is in turn transmitted through the skin by a radiofrequency-transmitting coil. This signal is picked up by the internal radiofrequency-receiving coil and sent to a computer chip that selects the appropriate electrode and stimulates it at the appropriate intensity.