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Sound waves enter the outer ear and are transmitted through the auditory canal to the eardrum. The resulting
vibrations are moved by the three small ossicles into the cochlea, where they are detected by hair cells and sent to
the auditory nerve.
Although loudness is directly determined by the number of hair cells that are vibrating, two
different mechanisms are used to detect pitch. The frequency theory of hearing proposes
that whatever the pitch of a sound wave, nerve impulses of a corresponding frequency will be
sent to the auditory nerve. For example, a tone measuring 600 hertz will be transduced into 600
nerve impulses a second. This theory has a problem with high-pitched sounds, however, because
the neurons cannot fire fast enough. To reach the necessary speed, the neurons work together in a
sort of volley system in which different neurons fire in sequence, allowing us to detect sounds up
to about 4,000 hertz.
Not only is frequency important, but location is critical as well. The cochlea relays information
about the specific area, or place, in the cochlea that is most activated by the incoming sound.