1. the outer ear

2. the middle ear

3. the inner ear

Air-conducted sound waves must move through these three parts in order for sound to be heard. The outer ear serves to channel the sound waves into the middle ear which is composed of three bones. These three bones mechanically transmit these waves to the oval window, which is part of the inner ear. The oval window vibrates inwardly, creating pressure waves in an incompressible fluid which fills the inner ear. This fluid pressure excites the membranes in the cochlea, a section of the inner ear shaped like a snail shell which contains the basilar membrane. The basilar membrane has tiny hair cells which transform the mechanical motion of the pressure waves into nerve impulses. These impulses are then transmitted to the brain where they are decoded and interpreted as sound.