Outer and Middle Ears

The cartilaginous pinna on the outside of the skull has a set of characteristic folds and curves, different for each individual, which set up a series of shadowings and reflections of the sound wave. The result is that the spectrum of the sound, as transmitted to the concha (the opening of the ear canal), is modified according to the direction and elevation of the sound's source. Although the main cue for sound localization comes from comparing the relative intensities and times of arrival of the stimuli at the two ears, that comparison does not give us information on the elevation of the sound source, or whether the source is behind or in front of the head; that information is provided by the pinna. Moreover, the outer ear means that sound localization of a sort can be undertaken with only one ear.

The middle and inner ears are protected from the outside world by an ear canal, and the inner ear is further protected by a middle ear cavity. The closed cavity of the middle ear, however, is a common site for infection, in which pus and secretions in early stages, and the formation of fibrous tissue in later stages, reduce the efficiency of transmission of vibrations to the cochlea.

The relatively dense, incompressible cochlear fluids, enclosed in a bony canal, with their movement limited by the membranes of the inner ear, need a higher pressure of vibration for a certain amplitude of movement than do sound waves in air. One job of the outer and middle ears is to transform the ratio (pressure/amplitude) of vibration from a low value suitable for the external air to a much higher value able to drive the cochlear fluids efficiently. This is undertaken by (1) acoustic resonances in the outer ear canal, (2) the footplate of the stapes in the oval window being much smaller than the tympanic membrane (so that forces from the sound vibration are concentrated in a small area), and (3) a lever action in the vibration both of the middle ear bones and of the tympanic membrane.

Vibration through the middle ear is affected by the middle ear muscles, the tensor tympani and the stapedius muscle, with contraction of the muscles reducing sound transmission. These muscles contract in response to self-produced activity such as vocalizations, but also in response to loud sounds, to give some partial protection of the inner ear against acoustic trauma.