How You Hear
The human ear is a very intricate structure, and the system by which your ear converts the energy in sound waves into electrical signals for your brain is enormously complex. Even today some of the mechanisms are not yet clearly understood. But, in general terms the system works like this:
Hearing starts at the outer ear. The auricle collects sound waves which are directed through the ear canal and move the eardrum. As the eardrum vibrates with sound, these vibrations pass across the middle ear chamber by way of the three small bones of the middle ear. The inner ear contains the mechanisms of hearing and balance. The vibrations of the ossicles set up waves of pressure in the fluid in the inner ear. These waves pass to tiny hair cells in the inner ear that convert this to electrical signals and pass these to the brain.
A Tour of the Ear
1) The Outer Ear: The outer ear is comprised of the pinna, ear canal, and outer layer of the eardrum. Sound enters the ear canal. At the eardrum, sound energy (air pressure changes) are transformed into mechanical energy of eardrum movement.
2) Pinna: The pinna is composed of cartilage and has a relatively poor blood supply. Its presence on both sides of the head allows us to localize the source of sound from the front vs. the back. Our ability to localize from side to side depends on the relative intensity and relative phase of sound reaching each ear and the analysis of the phase/intensity differences within the brainstem. And, of course, the lobe of the pinna is handy for single or multiple hoops, loops, diamonds, etc.
3) Cartilaginous Portion of External Ear Canal: The ceruminous and sebaceous glands in the cartilaginous portion of the ear canal combine to produce cerumen. The total length of the ear canal in adults is approximately one inch, which gives it a resonance frequency of approximately 3400 Hz, an important frequency region for understanding speech.
4) Bony Portion of External Ear Canal: The bony portion of the ear canal is surrounded by the mastoid bone, occupies the inner third, and is very tender. Occasionally, completely in the canal (CIC) hearing aids will reach as far as this portion of the canal. Outgrowths of bone, called exostoses occasionally will grow in the bony portion of the ear canal in response to cold-water exposure.
5) Mastoid Air Cells: A portion of the temporal bone (surrounding the ear). Under normal circumstances these honeycombed area cells are filled with air. They can fill with fluid or pus when chronically infected.
6) Tympanic Membrane: The tympanic membrane actually has three layers, with the outer layer continuous with the skin of the outer ear canal. The upper portion of the TM is called the pars flaccida, while the lower portion is called the pars tensa. The central portion of the pars tensa provides the active vibrating area in response to sound. The TM is a continually growing structure, which allows it to close if it has a hole in it and to extrude a ventilation tube.
7) Middle Ear: The middle ear serves as an impedance-matching transformer, matching the impedance of air in the ear canal to the impedance of the perilymph of the inner ear.
8) Malleus: The malleus is the most lateral (toward the side of the head) of the three ear bones (ossicles) in the middle ear. The long process of the malleus is attached to the inner layer of tympanic membrance. When the TM vibrates in response to sound, the malleus vibrates in concert.
9) Incus: The incus is attached to the malleus, and so vibrates as the malleus vibrates. The long process of the incus is also attached to the head of the stapes. Because the long process of the incus is slightly shorter than the long process (manubrium) of the malleus, incoming sound is give a slight (2.5 dB) boose in energy. This is referred to as the lever advantage.
10) Stapes: The stapes has a footplate and a superstructure. Its footplate is seated in the oval window, which separates the middle ear from perilymph of the inner ear. As the long process of the incus vibrates, so does the footplate of the stapes. Because the vibrating area of the tympanic membrane is larger than the area of the stapes, incoming sound is given a significant boost in energy of over 20 dB. This is referred to as the hydraulic advantage.
11) Choclea: A snail shaped structure that is the sensory organ of hearing. The vibrational patterns that are initiated by vibration of the stapes footplate set up a traveling wave pattern within the cochlea. This wavelike pattern causes a shearing of the cilia of the outer and inner hair cells. The shearing causes hair cell depolarization, resulting in all or none neural impulses that the brain interprets as sound.
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