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How We Hear: A Look at the Ear and How
it Processes Sound
Brittany O’Shea
Written For: Communication Sciences & Disorders
Majors taking Introduction to Audiology
February 26, 2012
Audience and Scope
This document was created to inform students majoring in Communication
Sciences and Disorders about the hearing process and how the ears works.
Specifically this document will be used in an Introduction to Audiology class where
hearing and hearing disorders are discussed. Before one can learn about hearing
disorders however, they must understand the basic process of hearing in a healthy,
normal functioning ear. This document could be used in a class lecture or as
supplementary notes to a lecture.
Introduction
The ear is the portion of the body that converts sound waves into electrical
and chemical energy making it possible for sound to be perceived by the brain. The
three major portions of the ear (outer, middle and inner) are all used in this process.
This document will take you through the steps of how all sounds travel from the
environment and are processed by your brain.
How the Ear Works
The process of hearing can be quite a complex task, however when broken
down into basic steps it is much easier to understand. Sounds travel through the ear
and to the brain in the following manner:
1. Sound is funneled in by the outer ear into the ear canal.
2. Sounds waves cause the tympanic membrane (the ear drum) to vibrate.
3. The ossicles of the middle ear vibrate.
4. The vibrations cause fluid waves to be sent up the cochlea.
5. Hair cells in the cochlea convert the chemical energy to electrical energy
6. The electrical impulse is sent up the auditory nerve to the brain where the
sound is perceived and processed.
Let’s Take a Closer Look…
The Human Ear
Figure 1
http://www.hearingcentral.com/howtheearworks.asp
1. Sound is funneled in by the outer ear into the ear canal
As you can see in Figure 1 the shape of the outer ear resembles a
funnel. This is not just for cosmetic purposes, but rather to “funnel” in
sound waves as a way to direct the sound where it needs to go. The
sounds waves are channeled from the outer ear and the ear canal and
directed to the tympanic membrane, which is more commonly known as
the eardrum.
2. Sound waves cause the tympanic membrane to vibrate.
The energy from the sound waves causes the tympanic membrane to
vibrate. The tympanic membrane is the barrier that separates the outer
and middle ear. This vibration is then continued into the middle ear.
3. The ossicles of the middle ear vibrate
The energy from the vibration of the tympanic membrane causes the
ossicles of the middle ear to vibrate as well. On the other side of the
tympanic membrane (in the middle ear) there are three tiny bones known
as ossicles. These bones are called the malleus, incus and stapes ( see
Figure 1) and when the vibrations hit them they begin to rock back and
forth which triggers fluid in the inner ear to be set in motion.
4. The vibrations cause fluid waves to be sent up the cochlea
The vibrations on the middle ear can be felt in the inner ear, which
causes fluid there to create wave like motions in the cochlea (the inner
ear). The cochlea, which is snail-shaped, is known as the organ of
hearing and is lined with tiny hair cells. When the fluid is in motion the
hair cells are activated. The cochlea is known to be tonotopically
organized, meaning that each area on the cochlea responds to different
frequencies of sound.
Figure 2
http://www.clarkaudiology.com/tinnitus.html
5. Hair cells in the cochlea convert the chemical energy to
electrical energy
As you can see from Figure 2 the hair cells in the cochlea lay on
what is known as the basal membrane or the base of the cochlea. The
chemical energy activates the hairs cells causing them to move in a
back and forth motion. When the hair cells become activated they then
transform this chemical energy into electric impulses. The sound
energy is now ready to be sent to the brain.
The loss or damage of hair cells which can be seen in
Figure 2 causes sounds in those certain frequency areas
to not be heard, this is known as sensorineural hearing
loss.
6. The electrical impulse is sent up the auditory nerve to the
brain where the sound is perceived and processed.
The electrical impulse that is created in the cochlea contains
information about the sound and its frequencies. This information
carrying impulse is sent from the cochlea and up the auditory nerve.
From the auditory nerve the impulse goes to the brain. The area known
for comprehension of sound is the temporal lobe, here the sound is
perceived and processed and the hearing process is complete.
Conclusion
While it is possible to function without one’s hearing, it presents a major
communication barrier. If one step in the hearing process goes wrong sound
will not be transmitted correctly and the message could be misinterpreted.
In order for the correct message to be relayed from the environment to the
brain the sound needs to first be funneled into the ear canal, then cause
vibrations in the tympanic membrane and the ossicles of the middle ear. The
vibrations must then be strong enough to create fluid waves up into the cochlea.
From there the activated hair cells will convert the energy into an electric
impulse, allowing the message to be sent to the interpreting center in the brain.
If these steps are not followed or there is damage to one or more areas of the
ear, a hearing loss will be the result.
Figure Citations:
Figure 1: Hearing Central, LLC, 2012
http://www.hearingcentral.com/howtheearworks.asp
Figure 2: Clark Audiology, 2011
http://www.clarkaudiology.com/tinnitus.html