The Mechanism of Hearing
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The sound waves we hear travel through air just like the waves travel through the slinky. The above illustration shows the relative conformations of air molecules as they transmit a sound wave downward. Soud travels through the air and along the outer and middle ear as a series of compressions (crests) and rarefractions (troughs) of air molecules. These patterns of molecules stimulate parts of the ear as described below to create the perception of sound.
The audirory canal (a.k.a. the outer auditory meatus) brings what you hear from the outside of the ear to the middle ear. At the end of the auditory canal, there is a thin layer of skin called the tympanic membrane (more commonly called the ear drum). The waves of sound hit the ear drum, and get further transferred onto the three small bones in the middle ear collectively known as the auditory ossicles: incus (anvil), stapes (stirrup), and malleus (hammer). These structures act as a chain, which lead through an opening in the bone between the middle and the inner ears. The middle ear is filled with air, and the inner ear is filled with fluid, so this opening is covered by a thin membrane to keep them separate. This membrane allows the sound waves to be transmitted into the inner ear, and finally to a bundle of 30,000 nerve fibers each representing a different frequency. Noise is filtered out of this signal and the brain interprets the signal.
The brain's interpretation of a sound gives it an added property: pitch. This is basically how the brain interprets the frequency. The higher the frequency, the higer the pitch. Since frequency is the inverse of the period, the longer the wavelength, the lower the pitch. The amplitude of the wave translates into how loud the brain takes the sound to be.
Wave addition contributes to the rich complex sounds the we hear each day. A voice is just the addition of many simpler waves to give a unique sound. If two waves are added together, and they happen to have the same amplitude, the compressions of one are in the same position as the rarefractions of the other (and vice versa) the end result is no sound. This is how your noise cancelling headphones work. They take in sounds from the outside, and emit a wave that has just the right properties to cancel them out.
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