Perception of Sound
Your ears are extraordinary organs. They pick up all the sounds around you and then translate
this information into a form your brain can understand. One of the most remarkable things about
this process is that it is completely mechanical. Your sense of smell, taste and vision all involve
chemical reactions, but your hearing system is based solely on physical movement.
We will trace the path of a sound, from its original source all the way to your brain, to see how
all the parts of the ear work together. When you understand everything they do, it's clear that
your ears are one of the most incredible parts of your body!
To hear sound, your ear has to do three
basic things:
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Direct the sound waves into the
hearing part of the ear
Sense the fluctuations in air
pressure
Translate these fluctuations into
an electrical signal that your
brain can understand
The pinna, the outer part of the ear,
serves to "catch" the sound waves. Your outer ear is pointed forward and it has a number of
curves. This structure helps you determine the direction of a sound. If a sound is coming from
behind you or above you, it will bounce off the pinna in a different way than if it is coming from
in front of you or below you. This sound reflection alters the pattern of the sound wave. Your
brain recognizes distinctive patterns and determines whether the sound is in front of you, behind
you, above you or below you.
Your brain determines the horizontal position of a sound by comparing the information coming
from your two ears. If the sound is to your left, it will arrive at your left ear a little bit sooner
than it arrives at your right ear. It will also be a little bit louder in your left ear than your right
ear.
Since the pinnae face forward, you can hear sounds in front of you better than you can hear
sounds behind you. Many mammals, such as dogs, have large, movable pinnae that let them
focus on sounds from a particular direction. Human pinnae are not so adept at focusing on sound.
They lay fairly flat against the head and don't have the necessary muscles for significant
movement. But you can easily supplement your natural pinnae by cupping your hands behind
your ears. By doing this, you create a larger surface area that can capture sound waves better. In
the next section, we'll see what happens as a sound wave travels down the ear canal and interacts
with the eardrum.
Humans, and primates such as the orangutan and chimpanzee, have ear muscles that are
minimally developed and non-functional, yet still large enough to be easily identifiable. These
undeveloped muscles are vestigial structures. An ear muscle that cannot move the ear, for
whatever reason, can no longer be said to have any biological function. This serves as evidence
of homology between related species. In humans there is variability in these muscles, such that
some people are able to move their ears in various directions, and it has been said that it may be
possible for others to gain such movement by repeated trials In such primates the inability to
move the ear is compensated mainly by the ability to turn the head on a horizontal plane, an
ability which is not common to most monkeys—a function once provided by one structure is
now replaced by another.
The Eardrum
Once the sound waves travel into the ear canal, they vibrate the tympanic membrane, commonly
called the eardrum. The eardrum is a thin, cone-shaped piece of skin, about 10 millimeters (0.4
inches) wide. It is positioned between the ear canal and the middle ear. The middle ear is
connected to the throat via the eustachian tube. Since air from the atmosphere flows in from your
outer ear as well as your mouth, the air pressure on both sides of the eardrum remains equal. This
pressure balance lets your eardrum move freely back and forth
The eardrum is rigid, and very sensitive. Even the slightest air-pressure fluctuations will move it
back and forth. It is attached to the tensor tympani muscle, which constantly pulls it inward. This
keeps the entire membrane taut so it will vibrate no matter which part of it is hit by a sound
wave.
This tiny flap of skin acts just like the diaphragm in a microphone. The compressions and
rarefactions of sound waves push the drum back and forth. Higher-pitch sound waves move the
drum more rapidly, and louder sound moves the drum a greater distance.
The eardrum can also serve to protect the inner ear from prolonged exposure to loud, low-pitch
noises. When the brain receives a signal that indicates this sort of noise, a reflex occurs at the
eardrum. The tensor tympani muscle and the
stapedius muscle suddenly contract. This pulls the
eardrum and the connected bones in two different
directions, so the drum becomes more rigid. When
this happens, the ear does not pick up as much noise
at the low end of the audible spectrum, so the loud
noise is dampened.
In addition to protecting the ear, this reflex helps
you concentrate your hearing. It masks loud, lowpitch background noise so you can focus on higherpitch sounds. Among other things, this helps you
carry on a conversation when you're in a very noisy
environment, like a rock concert. The reflex also
kicks in whenever you start talking -- otherwise, the sound of your own voice would drown out a
lot of the other sounds around you.
The eardrum is the entire sensory element in your ear. As we'll see in the coming sections, the
rest of the ear serves only to pass along the information gathered at the eardrum.
Amplifying Sound
We saw in the last section that the compressions and rarefactions in sound waves move your
eardrum back and forth. For the most part, these changes in air pressure are extremely small.
They don't apply much force on the eardrum, but the eardrum is so sensitive that this minimal
force moves it a good distance.
As we'll see in the next section, the cochlea in the inner ear conducts sound through a fluid,
instead of through air. This fluid has a much higher inertia than air -- that is, it is harder to move
(think of pushing air versus pushing water). The small force felt at the eardrum is not strong
enough to move this fluid. Before the sound passes on to the inner ear, the total pressure (force
per unit of area) must be amplified.
This is the job of the ossicles, a group of tiny bones in the middle ear. The ossicles are actually
the smallest bones in your body. They include:
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The malleus, commonly called the hammer
The incus, commonly called the anvil
The stapes, commonly called the stirrup
The malleus is connected to the
center of the eardrum, on the
inner side. When the eardrum
vibrates, it moves the malleus
from side to side like a lever. The
other end of the malleus is
connected to the incus, which is
attached to the stapes. The other
end of the stapes -- its faceplate -rests against the cochlea, through
the oval window.
When air-pressure compression
pushes in on the eardrum, the
ossicles move so that the
faceplate of the stapes pushes in
on the cochlear fluid. When airpressure rarefaction pulls out on the eardrum, the ossicles move so that the faceplate of the stapes
pulls in on the fluid. Essentially, the stapes acts as a piston, creating waves in the inner-ear fluid
to represent the air-pressure fluctuations of the sound wave.
The ossicles amplify the force from the eardrum in two ways. The main amplification comes
from the size difference between the eardrum and the stirrup. The eardrum has a surface area of
approximately 55 square millimeters, while the faceplate of the stapes has a surface area of about
3.2 square millimeters. Sound waves apply force to every square inch of the eardrum, and the
eardrum transfers all this energy to the stapes. When you concentrate this energy over a smaller
surface area, the pressure (force per unit of volume) is much greater.
The configuration of ossicles provides additional amplification. The malleus is longer than the
incus, forming a basic lever between the eardrum and the stapes. The malleus moves a greater
distance, and the incus moves with greater force (energy = force x distance).
This amplification system is extremely effective. The pressure applied to the cochlear fluid is
about 22 times the pressure felt at the eardrum. This pressure amplification is enough to pass the
sound information on to the inner ear, where it is translated into nerve impulses the brain can
understand.
Fluid Wave
The cochlea is by far the most complex part of the ear. Its job is to take the physical vibrations
caused by the sound wave and translate them into electrical information the brain can recognize
as distinct sound.
The cochlea structure consists of three adjacent tubes separated from each other by sensitive
membranes. In reality, these tubes are coiled in the shape of a snail shell, but it's easier to
understand what's going on if you imagine them stretched out. It's also clearer if we treat two of
the tubes as one chamber. The membrane between these tubes is so thin that sound waves travel
as if the tubes weren't separated at all.
The stapes moves back and forth, creating pressure waves in the entire cochlea. The round
window membrane separating the cochlea from the middle ear gives the fluid somewhere to go.
It moves out when the stapes pushes in and moves in when the stapes pulls out.
The middle membrane, the basilar membrane, is a rigid surface that extends across the length of
the cochlea. When the stapes moves in and out, it pushes and pulls on the part of the basilar
membrane just below the oval window. This force starts a wave moving along the surface of the
membrane. The wave travels something like ripples along the surface of a pond, moving from
the oval window down to the other end of the cochlea.
The basilar membrane has a peculiar structure. It's made of 20,000 to 30,000 reed-like fibers that
extend across the width of the cochlea. Near the oval window, the fibers are short and stiff. As
you move toward the other end of the tubes, the fibers get longer and more limber.
This gives the fibers different resonant frequencies. A specific wave frequency will resonate
perfectly with the fibers at a certain point, causing them to vibrate rapidly. This is the same
principle that makes tuning forks and kazoos work -- a specific pitch will start a tuning fork
ringing, and humming in a certain way will cause a kazoo reed to vibrate.
As the wave moves along most of the membrane, it can't release much energy -- the membrane is
too tense. But when the wave reaches the fibers with the same resonant frequency, the wave's
energy is suddenly released. Because of the increasing length and decreasing rigidity of the
fibers, higher-frequency waves vibrate the fibers closer to the oval window, and lower frequency
waves vibrate the fibers at the other end of the membrane. In the next section, we'll look at how
tiny hairs help us hear sound.
Hair Cells
In the last section, we saw that higher pitches vibrate the basilar membrane most intensely near
the oval window, and lower pitches vibrate the basilar membrane most intensely at a point
farther down the cochlea. But how does the brain know where these vibrations occur?
This is the organ of corti's job. The organ of corti is a structure containing thousands of tiny hair
cells. It lies on the surface of the basilar membrane and extends across the length of the cochlea.
Until a wave reaches the fibers with a resonant frequency, it doesn't move the basilar membrane
a whole lot. But when the wave finally does reach the resonant point, the membrane suddenly
releases a burst of energy in that area. This energy is strong enough to move the organ of corti
hair cells at that point.
When these hair cells are moved, they send an electrical impulse through the cochlear nerve. The
cochlear nerve sends these impulses on to the cerebral cortex, where the brain interprets them.
The brain determines the pitch of the sound based on the position of the cells sending electrical
impulses. Louder sounds release more energy at the resonant point along the membrane and so
move a greater number of hair cells in that area. The brain knows a sound is louder because more
hair cells are activated in an area.
The cochlea only sends raw data -- complex patterns of electrical impulses. The brain is like a
central computer, taking this input and making some sense of it all. This is an extraordinarily
complex operation, and scientists are still a long way from understanding everything about it.
In fact, hearing in general is still very mysterious to us. The basic concepts at work in human and
animal ears are fairly simple, but the specific structures are extremely complex. Scientists are
making rapid advancements, however, and they discover new hearing elements every year. It's
astonishing how much is involved in the hearing process, and it's even more amazing that all
these processes take place in such a small area of the body.
(exerpted from Tom Harris “How Hearing Works” howstuffworks.com and Wikipedia)