The ear itself is as complicated as you may expect for anything related to biology, but it can be broken down to three major areas; the inner ear, middle ear and outer ear.
Outer Ear
The outer ear is the visible part from the outside with the most prominent feature being the pinna. The pinna acts as a funnel, which focuses sound down to the auditory canal, essentially amplifying the sound. The shapes of the ear lobe are not an accident, the shape filters the incoming sound allowing the frequencies that humans are most concerned with (the ranges in which human speech lay) to pass into the ear canal more easily.
The main purpose of the ear canal is actually to provide defensive measures against infection, but it also acts as a passageway for sound to travel through in order to strike the eardrum. Much like a digital sound system, the brain can't comprehend sound from sound waves. Like digital systems, it needs sound to be convered into a form it can process, although unlike digital systems this form is still an analogue form. The ear drum, also known as the tempanic membrane, is the only part of the ear that deals with actual sound waves. It
s vibrated by them. This converts the sound into a form that the middle ear then has to deal with.
The Middle Ear
The middle ear consists of a mechanism made up of the three smallest bones in the human body. These three bones are the malleus (hammer), incus (anvil) and stapes (stirrup). The purpose of this mechanism, along with the tempanic membrane, is to convert sound waves into a mechanical form which is then passed on to the inner ear.
So why is this step even necessary? It's due to the nature of the inner ear. The inner is filled with fluid, while the middle and outer ears are usually filled with air. A sound travelling through air will be largely reflected off the surface of something much denser, such as a liquid. The result is very little of the sound makes it into the liquid itself. The middle ear circumvents this by directly stimulating the inner ear with mechanical vibrations.
The Inner Ear
The inner ear consists of the cochlea, whose task it is to turn the mechanical vibrations from the middle ear into electrical signals that are sent to the brain to be processed. It also needs to be able to transmit different frequencies along different parts of the nerves in order for the brain to quickly discern frequencies.
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| The basilair membrane, which resides inside the cochlea |
The main component that achieves this effect is the basilair membrane. It is a non-uniform membrane which varies in two ways from base to apex; the base is thinner and stiffer than the apex. This allows vibrations of differing frequencies to vibrate the membrane in different ways, which in turn allows it to stimulate different arrays of tiny hairs which in turn stimulate different nerve endings. The base vibrates stronger with higher frequencies. The whole membrane actually vibrates at lower frequencies, but the peak of the vibrations occur towards the apex, allowing the actual frequency to be determined.
Below is a series of images roughly demonstrating the response of the membrane to different frequencies. The coloured shape represents the size of the vibrations at given points.
This is the physical limiter to the range of frequencies that we can hear. If the frequency is too high for any part of the membrane to vibrate, or too low to determine where the peak is, then we simply won't hear the sound.
To pick out multiple frequencies from a given wave is easy, as the brain can discern where each of the peaks reside on the membrane.
The electrical signals generated from this are then processed by the brain as sound.
http://library.thinkquest.org/05aug/00386/hearing/index.htm
http://openlearn.open.ac.uk/mod/oucontent/view.php?id=398672§ion=2.1
http://openlearn.open.ac.uk/mod/oucontent/view.php?id=398672§ion=3.3
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