Hearing aid in ear. A spectacle-mounted, air cond- uctor hearing aid on a man's ear. This hearing aid sits on top of the ear. The thin, transparent, plastic tube conducts sound into the ear canal. The small dial is a volume control. The switch turns the microphone on (M), off (O), and sets the hearing aid to pick up electromagnetically trans- mitted sound (T), which is often provided in tele- phones or public buildings. Hearing aids improve hearing in people with partial deafness. Sound is picked up by a tiny microphone, amplified, and then transmitted into the ear by a tiny speaker. Each hearing aid is fine-tuned to amplify the range of frequencies most needed by the patient.