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About The Ear
The Outer Ear
The Middle Ear
The Inner Ear
Hearing Loss
The ear is an advanced and very sensitive organ of the human body. The major
task of the ear is to detect and analyse noises by transduction. Another
very important function is to maintain the sense of balance.
The best way to describe the functioning of the ear is to describe
the path which the sound waves take on their way through the ear. The ear
is divided into three different parts:
The outer ear
The middle ear
The inner ear
The only visible part of the ear is the pinna (the auricle) which
- with its special helical shape - is the first part of the ear that
reacts with sound. The pinna acts as a kind of funnel which assists
in directing the sound further into the ear. Without this funnel
the sound waves would take a more direct route into the auditory
canal. This would be both difficult and wasteful as much of the sound
would be lost making it harder to hear and understand the sounds.
The pinna is essential due to the difference in pressure inside and
outside the ear. The resistance of the air is higher inside the ear
than outside because the air inside the ear is compressed and thus
under greater pressure.
In order for the sound waves to enter the ear in the best possible
way the resistance must not be too high. This is where the pinna
helps by overcoming the difference in pressure inside and outside
the ear. The pinna functions as a kind of intermediate link which
makes the transition smoother and less brutal allowing more sound
to pass into the auditory canal (meatus).
Once the sound waves have passed the pinna, they move two to three
centimetres into the auditory canal before hitting the eardrum, also
known as the tympanic membrane.
The Eardrum
The eardrum (tympanic membrane), which marks the beginning of the
middle ear, is extremely sensitive. In order to protect the eardrum,
the auditory canal is slightly curved making it more difficult for
insects, for example, to reach the eardrum. At the same time, earwax
(cerumen) in the auditory canal also helps to keep unwanted materials
like dirt, dust and insects out of the ear.
In addition to protecting the eardrum, the auditory canal also functions
as a natural hearing aid which automatically amplifies low and less
penetrating sounds of the human voice. In this way the ear compensates
for some of the weaknesses of the human voice, and makes it easier
to hear and understand ordinary conversation.
Three bones
The eardrum is very thin, measures approximately 8-10 mm in diameter
and is stretched by means of small muscles. The pressure from sound
waves makes the eardrum vibrate. The vibrations are transmitted further
into the ear via three bones: the hammer (malleus), the anvil (incus)
and the stirrup (stapes). These three bones form a kind of bridge,
and the stirrup, which is the last bone that sounds reach, is connected
to the oval window. The oval window is a membrane covering the entrance
to the cochlea in the inner ear. When the eardrum vibrates, the sound
waves travel via the hammer and anvil to the stirrup and then on
to the oval window.
When the sound waves are transmitted from the eardrum to the oval
window, the middle ear is functioning as an acoustic transformer
amplifying the sound waves before they move on into the inner ear.
The pressure of the sound waves on the oval window is some 20 times
higher than on the eardrum. The pressure is increased due to the
difference in size between the relatively large surface of the eardrum
and the smaller surface of the oval window. The same principle applies
when a person wearing a shoe with a sharp stiletto heel steps on
your foot: The small surface of the heel causes much more pain than
a flat shoe with a larger surface would.
The Eustachian tube
The Eustachian tube is also found in the middle ear, and connects
the ear with the rearmost part of the palate. The Eustachian tube
equalises the air pressure on both sides of the eardrum, ensuring
that pressure does not build up in the ear. The tube opens when you
swallow, thus equalising the air pressure inside and outside the
ear.
In most cases the pressure is equalised automatically, but if this
does not occur, it can be brought about by making an energetic swallowing
action. The swallowing action will force the tube connecting the
palate with the ear to open, thus equalising the pressure.
Built-up pressure in the ear may occur in situations where the pressure
on the inside of the eardrum is different from that on the outside
of the eardrum. If the pressure is not equalised, a pressure will
build up on the eardrum, preventing it from vibrating properly. The
limited vibration results in a slight reduction in hearing ability.
A large difference in pressure will cause discomfort and even slight
pain. Built-up pressure in the ear will often occur in situations
where the pressure keeps changing, for example when flying or driving
in mountainous areas.
Once the vibrations of the eardrum have been transmitted to the oval
window, the sound waves continue their journey into the inner ear.
The inner ear is a maze of tubes and passages, referred to as the
labyrinth. In the labyrinth can be found the vestibular and the cochlea.
The Cochlea
In the cochlea, sound waves are transformed into electrical impulses
which are sent on to the brain. The brain then translates the impulses
into sounds that we know and understand.
The cochlea resembles a snail shell or a wound-up hose. The cochlea
is filled with a fluid called perilymph and contains two closely
positioned membranes. These membranes form a type of partition wall
in the cochlea. However, in order for the fluid to move freely in
the cochlea from one side of the partition wall to the other, the
wall has a little hole in it (the helicotrema). This hole is necessary,
in ensuring that the vibrations from the oval window are transmitted
to all the fluid in the cochlea.
When the fluid moves inside the cochlea, thousands of microscopic
hair fibres inside the partition wall are put into motion. There
are approximately 24,000 of these hair fibres, arranged in four long
rows.
The hair fibres are all connected to the auditory nerve and, depending
on the nature of the movements in the cochlear fluid, different hair
fibres are put into motion. When the hair fibres move they send electrical
signals to the auditory nerve which is connected to the auditory
centre of the brain. In the brain the electrical impulses are translated
into sounds which we recognise and understand. As a consequence,
these hair fibres are essential to our hearing ability. Should these
hair fibres become damaged, then our hearing ability will deteriorate.
The Vestibular
Another important part of the inner ear is the organ of equilibrium,
the vestibular. The vestibular registers the body's movements, thus
ensuring that we can keep our balance. The vestibular consists of
three ring-shaped passages, oriented in three different planes. All
three passages are filled with fluid that moves in accordance with
the body's movements. In addition to the fluid, these passages also
contain thousands of hair fibres which react to the movement of the
fluid sending little impulses to the brain. The brain then decodes
these impulses which are used to help the body keep its balance.
Hearing loss, or hearing impairment, happens when there is a problem
with one or more parts of the ear (or ears) and it prevents a person
from hearing properly.
Impairment is a word that means something is not working correctly or
as well as it should. You may also hear the words "deaf," "deafness,"
or "hard of hearing," when people talk about hearing loss.
Hearing loss may affect one or both ears, and a person may be able
to hear some sounds or nothing at all. About two out of 100 babies are
born with hearing impairment. Six out of 100 school-age kids develop some
kind of hearing loss.
There are a few different types of hearing loss:
- Conductive hearing loss happens when there is a problem with a part of the outer
or middle ear. If the outer ear canal, eardrum, hammer, anvil, stirrup,
or other parts of the middle ear aren't doing their jobs correctly, it
can cause this
kind of hearing loss. Most kids with conductive hearing loss have a mild
hearing loss and it is usually temporary because, in most cases, medical
treatment can
help.
- Sensory hearing loss happens when the cochlea is not working correctly
because the tiny hair cells in the cochlea are damaged or destroyed.
It can affect one
ear or both ears. Depending on the loss, a kid may be able to hear
most sounds but they would be muffled (a mild loss), some sounds
slightly (a moderate loss),
or no sounds at all (a severe to profound loss), and because of this,
a kid's ability to talk may be affected. Sensory hearing impairment
is almost
always
permanent.
- Mixed hearing loss is a combination of conductive and sensory
hearing
loss.
- Neural hearing loss happens when there is a problem with
the connection from the cochlea to the brain. If the nerve (neural
means related
to nerves) that
carries the messages from the cochlea to the brain is damaged,
there may be a neural hearing loss
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