Hello blackvision and thank you for your question.
One of my favorite sites for answering this type of question is "How
Stuff Works" http://www.howstuffworks.com
If we search through their site we find they have an excellent section
that describes how we hear:
"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.
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 things:
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.
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, 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 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.
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."
To read the full article and view the explanatory diagrams visit the
site here:
http://www.howstuffworks.com/hearing.htm
Another site that gives a shorter explanation as to how we hear is
"Cool Quiz"
http://www.coolquiz.com/trivia/explain/docs/hearing.asp
Intensity, or loudness of sound,is measured in decibels. To
understand the ranges that the human ear can hear visit the sites
below:
"Decibel: (abrreviated as db, dB, and DB) a measure of the relative
loudness of a sound. The threshold of human hearing is 0db and the
threshold of pain for humans is around 120-140db. A decibel is on
tenth of a 'Bel', a seldom used unit of measurement named for
Alexander Graham Bell."
http://zeroworks.org/index.php?do=dic&l=D
"The normal human ear can hear sounds down to 0 dB. The roar of a
rocket lifting off registers over 140 dB. Experts consider noise
levels above 85 dB hazardous, particularly when you're exposed to them
for long periods. The louder the noise, the less time it takes to
damage your hearing. Many rock concerts are 110 dB or more. If you
attend often, you can damage your hearing before you know you are in
danger."
http://www.ktca.org/newtons/13/hear.html
Thank you for the question and if you need any clarification regarding
my answer do not hesitate to ask.
Best regards
THX1138
Search strategy:
"How do we hear"
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