Air conductor hearing aid. Close-up of an air con- ductor hearing aid. This hearing aid fits on top of the ear, and can be mounted onto the arm of a pair of spectacles. The plastic tube is then attached to a long, thin, transparent plastic tube that runs into the ear canal and conducts sound. The small dial controls volume. The switch turns the hearing aid on (M), off (O), or sets it to pick up electromagnetically transmitted sound (T), which is often provided in public telephones and theatres. Hearing aids consist of a tiny micro- phone, amplifier and speaker. They are individual- ly tuned to amplify the range of frequencies need- ed most by each patient.