The ear is made up of the outer, middle and inner ears. The outer ear consists of the pinna, the external auditory canal and the tympanic membrane. The middle ear is air-filled and contains the middle ear ossicles. The inner ear is fluid-filled and contains the cochlea, the semicircular canals and the vestibule.
Sound in the external environment is channelled into the auditory meatus by the pinna and impinges on the tympanic membrane causing it to vibrate. These vibrations are transmitted to the inner ear via the middle ear ossicles. The ossicles act as an impedance-matching device and amplify vibrations between the outer and inner ear. They also function in preventing damage to the inner ear by very loud sounds via the middle ear reflex.
The inner ear contains the cochlea which has three compartments: the scala tympani, the scala vestibuli and the scala media (cochlear duct). Inside the cochlear duct is the organ of Corti. The organ of Corti contains the sensory receptors that are called hair cells, sits on top of the basilar membrane and is covered by the tectorial membrane.
The stapes connects to the scala vestibuli via the oval window. Movement of the stapes in response to sound causes the fluid in the scala vestibuli to vibrate. This causes the basilar membrane to move. The motion is described as a travelling wave. The base of the membrane is 5 times narrower and about 100 times stiffer than the apex.
The basilar membrane has a frequency-to-place conversion for pure-tone stimuli. High-frequency sounds cause greatest vibration near the base of the membrane, and low frequencies cause greatest vibration near the apex.
The basilar membrane acts like a band-pass filter. Each point on the membrane corresponds to a band-pass filter with a different centre frequency. This means that sounds of different frequency result in maximal displacement at different points along the membrane.