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The human ear can comfortably hear sounds up to 80 decibels. Prolonged exposure to sounds above 80 decibels can
cause hearing loss.
Audition begins in the pinna, the external and visible part of the ear, which is shaped like a
funnel to draw in sound waves and guide them into the auditory canal. At the end of the canal,
the sound waves strike the tightly stretched, highly sensitive membrane known as
thetympanic membrane (or eardrum), which vibrates with the waves. The resulting vibrations are
relayed into the middle ear through three tiny bones, known as the ossicles—the hammer (or
malleus), anvil (or incus), and stirrup (or stapes)—to the cochlea, a snail-shaped liquid-filled
tube in the inner ear. The vibrations cause the oval window, the membrane covering the opening
of the cochlea, to vibrate, disturbing the fluid inside the cochlea.
The movements of the fluid in the cochlea bend the hair cells of the inner ear, much in the same
way that a gust of wind bends over wheat stalks in a field. The movements of the hair cells
trigger nerve impulses in the attached neurons, which are sent to the auditory nerve and then to
the auditory cortex in the brain. The cochlea contains about 16,000 hair cells, each of which
holds a bundle of fibers known as cilia on its tip. The cilia are so sensitive that they can detect a
movement that pushes them the width of a single atom. To put things in perspective, cilia
swaying at the width of an atom is equivalent to the tip of the Eiffel Tower swaying by half an
inch (Corey et al., 2004). [1]
Figure 4.30 The Human Ear