Download Outer Ear

Introduction
The organ that we use to hear sound is the ear. Almost all the structures in the ear are
needed for this purpose. Together, they gather and amplify sound waves and change
their energy to electrical signals. The electrical signals travel to the brain, which
interprets them as sound.
How We Hear
Figure below shows the three main parts of the ear: the outer, middle, and inner ear. It
also shows the specific structures in each part.
Outer Ear
The outer ear includes the pinna, ear canal, and eardrum.
 The pinna is the only part of the ear that extends outward from the head. Its position
and shape make it good at catching sound waves and funneling them into the ear
canal.

The ear canal is a tube that carries sound waves into the ear. The sound waves
travel through the air inside the ear canal to the eardrum.

The eardrum is like the head of a drum. It’s a thin membrane stretched tight across
the end of the ear canal. The eardrum vibrates when sound waves strike it, and it
sends the vibrations on to the middle ear.
Middle Ear
The middle ear contains three tiny bones (ossicles) called the hammer, anvil, and
stirrup. If you look at these bones in Figure below, you might notice that they resemble
the objects for which they are named. The three bones transmit vibrations from the
eardrum to the inner ear. They also amplify the vibrations. The arrangement of the three
bones allows them to work together as a lever that increases the amplitude of the
waves as they pass to the inner ear.
Inner Ear
The stirrup passes the amplified sound waves to the inner ear through the oval window
(see Figure above). When the oval window vibrates, it causes the cochlea to vibrate as
well. The cochlea is a shell-like structure that is full of fluid and lined with nerve cells
called hair cells. Each hair cell has tiny hair-like projections. When the cochlea vibrates,
it causes waves in the fluid inside. The waves bend the "hairs" on the hair cells, and this
triggers electrical impulses. The electrical impulses travel to the brain through nerves.
Only after the nerve impulses reach the brain do we hear the sound.
Hearing Loss
All these structures of the ear must work well for normal hearing. Damage to any of
them, through illness or injury, may cause hearing loss. Total hearing loss is called
deafness. Most adults experience at least some hearing loss as they get older. The
most common cause is exposure to loud sounds, which damage hair cells. The louder a
sound is, the less exposure is needed for damage to occur. Even a single brief
exposure to a sound louder than 115 decibels can cause hearing loss.
Preventing Hearing Loss
Hearing loss caused by loud sounds is permanent. However, this type of hearing loss
can be prevented by protecting the ears from loud sounds. People who work in jobs
that expose them to loud sounds must wear hearing protectors. Examples include
construction workers who work around loud machinery for many hours each day. But
anyone exposed to loud sounds for longer than the permissible exposure time should
wear hearing protectors. Many home and yard chores and even recreational activities
are loud enough to cause hearing loss if people are exposed to them for very long.
How Hearing Protectors Work
You can see two different types of hearing protectors in Figure below. Earplugs are
simple hearing protectors that just muffle sounds by partially blocking all sound waves
from entering the ears. This type of hearing protector is suitable for lower noise levels,
such as the noise of a lawnmower or snowmobile engine.
Electronic ear protectors work differently. They identify high-amplitude sound waves and
send sound waves through them in the opposite direction. This causes destructive
interference with the waves, which reduces their amplitude to zero or nearly zero. This
changes even the loudest sounds to just a soft hiss. Sounds that people need to hear,
such as the voices of co-workers, are not interfered with in this way and may be
amplified instead so they can be heard more clearly. This type of hearing protector is
recommended for higher noise levels and situations where it’s important to be able to
hear lower-decibel sounds.