Answer:
The sound
waves reaches the eardrum, the pressure on the outside of membrane
increases and pushes the eardrum inward. Similarly, when a rarefaction falls on
the eardrum, the pressure on the outside of the membrane decreases and it moves
outward. In this way, the eardrum begins to vibrate.
The vibrations of eardrum causes the vibrations in the small bone, called
hammer. From hammer these vibrations are passed to second bone anvil and finally
to third bone stirrup. The vibrating stirrup strikes on the membrane of the
oval window and passes its vibrations to the liquid in the cochlea. The vibrating
liquid of cochlea converts the pressure variations of sound waves into electrical
signals. These electrical signals are sent to the brain via the auditory nerve.
The brain interprets these electrical signals as sound and we get the sensation
of hearing.
Human beings hear only sounds with frequencies from about
20 Hz to 20,000 Hz. These are the limits of audibility; the upper limit
decreases with age. A hearing aid can be used to send an amplified sound to the
eardrum when someone can hear only quite loud sounds. If the eardrum of the
middle ear is damaged, an aid can pass on vibrations to the cochlea directly
through the skull bones. Deafness occurs if the cochlea or auditory nerve is
damaged.
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