Sound arrives in the pinna, the visible part of the ear on the outside of the head.

Sound waves travel the 1-inch length of the ear canal and stimulate the tympanic membrane ("eardrum").

The vibrations of the eardrum are passed on to the three bones of the middle ear, which amplify the sound and send it into the inner ear, a snail-shaped tube—the "cochlea"—filled with liquid.

In the cochlea, the sound becomes a fluid wave, stimulating approximately 7,000 "hair cells" that line the cochlear walls.

The hair cells transform the wave into electrochemical signals. These signals fire the nerves that travel to the brain. (What frequency hair cells receive depends on where they are located in the cochlea.) The failure or destruction of hair cells is the leading cause of deafness.