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.
In this edition of HowStuffWorks,
we'll look at the mechanical systems that make hearing
possible. We'll 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!
Sound Basics To understand how your ears
hear sound, you first need to understand just what sound is.
An object produces sound when it vibrates in matter. This
could be a solid, such as earth; a liquid, such as water; or a
gas, such as air. Most of the time, we hear sounds traveling
through the air in our atmosphere.
When something vibrates in the atmosphere, it moves the air
particles around it. Those air particles in turn move the air
particles around them, carrying the pulse of the vibration
through the air.
To see how this works, let's look at a simple vibrating
object: a bell. When you hit a bell, the metal vibrates --
flexes in and out. When it flexes out on one side, it pushes
on the surrounding air particles on that side. These air
particles then collide with the particles in front of them,
which collide with the particles in front of them, and so on.
This is called compression.
When the bell flexes away, it pulls in on the surrounding
air particles. This creates a drop in pressure, which pulls in
more surrounding air particles, creating another drop in
pressure, which pulls in particles even farther out. This
pressure decrease is called rarefaction.
In this way, a vibrating object sends a wave of pressure
fluctuation through the atmosphere. We hear different sounds
from different vibrating objects because of variations in the
sound wave frequency. A higher wave frequency simply
means that the air pressure fluctuation switches back and
forth more quickly. We hear this as a higher pitch.
When there are fewer fluctuations in a period of time, the
pitch is lower. The level of air pressure in each fluctuation,
the wave's amplitude, determines how loud the sound is.
Catching Sound We saw in the last section
that sound travels through the air as vibrations in air
pressure. To hear sound, your ear has to do three basic
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
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.
In the next section, we'll see how your ear senses the
sound once the wave makes it into the ear.
Drum Set 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
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
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, low-pitch background
noise so you can focus on higher-pitch 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
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.
Bone Amplifier 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
volume) 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:
The malleus, commonly called the hammer
The incus, commonly called the anvil
The stapes, commonly called the stirrup
Sound waves vibrate the eardrum, which moves
the malleus, incus and
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 air-pressure 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. To learn more about this hydraulic
multiplication, check out How
Hydraulic Machines Work.
The configuration of ossicles provides additional
amplification. The malleus is longer than the incus, forming a
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
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
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,
the scala vestibuli and the scala media, 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 piston action of the stapes moves the
fluid in the cochlea. This causes a vibration wave to
travel down the basilar
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
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
In the next section, we'll see how the brain detects this
wave pattern to determine the pitch and volume of different
Hair Hearing 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
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
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.