Hearing aids are devices that amplify sound so a person who has a hearing impairment can enjoy sounds from a musical symphony to a lively conversation to the rustle of leaves. Two types of hearing aids either conduct sound through the air or through bone. Usually a person who is hearing impaired can use an air-conduction hearing aid, as it amplifies sound and brings it directly to the ear. The bone-conduction hearing aid brings sound waves to the bony part of the head behind the ear and uses the bone to transmit sound waves to the nerves of the ear. A typical hearing aid contains a microphone that picks up sounds and converts them into electric signals, an amplifier, which increases the strength of the electric signals, and the receiver, which converts the signals back into sound waves that can be heard...