Hearing instruments have a microphone which picks up sound, an amplifier which magnifies it and a receiver which delivers the amplified sound into the ear canal. The characteristics of these vary to suit different hearing losses. An audioprosthologist can then adjust the hearing instrument either manually or using a computer to get the optimum result for each individual. As hearing losses are generally very different from person to person, a hearing instrument must be individually selected for each case.
Hearing instruments must also fit snugly in the ear canal so impressions of the ears are taken to have the hearing instruments or ear molds custom made.
Some hearing instruments have multiple memories and are called programmable instruments. They can be adjusted by the wearer to suit different environments, for example, to respond better in noisy situations. Some have a volume control, either on the device itself or on a remote control unit. Others automatically adjust their volume according to the incoming sound.
All hearing instruments operate with a battery, the size and life span of which varies according to the size of the hearing instrument. Usually, each battery cell lasts for about 100 hours of usage. The battery is easily replaced by the hearing aid user.