Hearing aid. Girl wearing a behind-the-ear (BTE) hearing aid. This type of hearing aid comprises a small casing located behind the ear, which contains the battery and microphone, and a tube that enters the ear through a custom-made ear mould. Hearing aids improve hearing in people with partial deafness. Sound is picked up by the microphone, amplified and transmitted to the ear by the tube. Such hearing aids can offer dramatic improvements to 95% of people with hearing losses. This design is ideal for children because, as the ear grows, only the mould needs replacing, and not the whole instrument.