A complete sound-reproduction system that increases the sound intensity at the ear. Modern hearing aids use a small crystal microphone, battery-powered amplifier, and earpiece. The development of integrated circuits has allowed hearing aids to become very small and light and they may be hidden in a spectacles frame. Good sound quality at sufficient power output for most cases is obtained with volume and tone controls available to the wearer. Special types of hearing aids include those designed to amplify a specific frequency band only and those in which the output from the amplifier is used to produce vibrations in the mastoid bone behind the ear, thus bypassing a defective outer or middle ear.