A hearing aid or deaf aid is an electroacoustic device which is designed to amplify sound for the wearer, usually with the aim of making speech more intelligible, and to correct impaired hearing as measured by audiometry. In the United States, Hearing aids are consideredmedical devices and are regulated by the Food and Drug Administration (FDA). Ordinary small audio amplifiers or other plain sound reinforcing systems cannot be sold as "hearing aids".
Earlier devices, known as ear trumpets or ear horns, were passive funnel-like amplification cones designed to gather sound energy and direct it into the ear canal. Similar devices include the bone anchored hearing aid, and cochlear implant.
Hearing AIDS :