The outer ear funnels sound waves from the air to the ear drum. For humans, sounds in the range of 20–20,000 Hz are transmitted by three bones (the smallest bones in the body) resting under the ear drum to a membrane lying on the cochlear surface. Vibrations passed on by the fluid-filled spiral tube reach the hair cells inside, each of which supports a tuft of 30–150 stereocilia arranged in rows of increasing height. These cells transduce mechanical signals into chemical ones through mechano-sensitive channels (see story p. 26).