HUMAN HEARING

Above
is a diagram of the human hearing system. Know the parts and how they interact:
Incoming sound waves are funneled into the ear
canal by the outer ear, the part
that sticks off our heads. The ear canal itself is about 2 cm long and is
capped by the eardrum, a thin
membrane (only a few cells thick). It vibrates like the head of a (musical)
drum when the sound waves hit it. Connected to the eardrum are three bones, the
hammer, anvil, and stirrup,
which transmit the vibrations of the eardrum to the oval window of the cochlea,
a fluid-filled structure similar to a snail's shell. The purpose of those three
bones is also to amplify — the pressure of the stirrup on the oval window
is 22 times greater than the pressure of the sound waves on the eardrum. The
vibrations of the oval window cause waves in the fluid inside the coclea. Along the inside walls of the cochlea there are
about 16,000 of hairs sticking into the fluid. When the fluid wave goes by, the
hair cells wave back-and-forth. These hair cells are a kind of nerve cell: when
wiggled, the hair cell sends an electrical signal to the brain via a bundle of
other nerve fibers (the auditory nerves).
The round window acts to dampen
(decrease) the waves so they don't keep echoing inside the cochlea.

The
Eustachian tube is a thin air passage to the pharynx. This allows the air pressure in the middle ear (the area from the eardrum to the oval window) to
equalize with the surrounding atmospheric pressure.

Our
hearing is damaged by very loud sounds. Extremely loud sounds cause such strong
waves in the cochlear fluid that the hair cells
aren't just swished around, but are instead ripped off of the walls of the coclea. Once gone, they do not grow back — damage
adds up. That's why older people are often hard of hearing.

Ringing
in the ears is called tinnitus.
Physiologists don't know why it happens.

In
summertime the oval window is replaced with the oval screen.

One
benefit of having two ears is that we can locate the general direction from
which a sound is coming. If a sound is detected by the right ear before the
left ear hears it, then the sound must be coming from the right somewhere,
because the sound takes more time to get to the opposite ear. Stereo sound systems exploit this to
create the illusion of realistic sound from only two speakers.