Wherever there is a sound, something is moving to make that sound. It is a
to-and-fro movement that makes sound. Scientists have a name for this kind of
movement. It is called vibration. Things can vibrate without making a
sound (such as vibrating your hand like a fan). In order for you to hear a
sound, something must vibrate between about sixteen times a second and about
twenty thousand times a second. The number of times which an object vibrates
in a second is called its frequency. A frequency of twenty-five means
that something is vibrating twenty-five times a second.
an object vibrates so fast that you cannot hear it. If an object has a
frequency of over 20,000, you will not hear it. Such very fast vibrations are
called ultrasonic vibrations. A dog is able to hear frequencies up to
40,000. Some people can hear better than others.
vocal chords vibrate inside your throat when you hum, speak, or sing. Sounds
have different pitches, ranging from high to low (such as humming a high note
or low note). The higher the pitch of the sound, the faster the vibrations
that make the sound. The lower the pitch, the slower the vibrations. In order
to change the pitch, the speed of the vibrations must change. A thicker object
will vibrate slower, or less frequently, than a thinner object of the same
length and the same material. The thicker object produces a lower sound than
the thinner object. When two objects are of the same thickness, the shorter
object has the higher pitch. The tightness of a vibrating object helps
determine its pitch. This tightness or looseness is called tension.
something vibrates in the air, it makes waves. The waves travel through the
air. Sound waves are carried by the air. No air means no sound waves. Sound
waves must travel through some substance. The vibrations are passed along
through the air from molecule to molecule. This movement of molecules, to and
fro, is a sound wave.
travels better through a solid. The molecules of a solid are close together.
So molecules can bump each other more easily. If molecules bump more easily,
they can carry sound better.
is the molecules in air that pass the vibrations along. Molecules next to the
vibrating object are pushed by the object as it vibrates. Those molecules push
the molecules next to them. So the push is passed along from one molecule to
the next. Each molecule moves back and forth in its place as it passes the
vibration along. What if the molecules could not move? The sound would not be
passed along. There would be no sound waves.
echo happens when sound waves bounce. One thing that is needed for an echo is
something for sound to bounce off. If molecules did not bounce, there would be
no echoes. If molecules were not able to move fast or slowly, there would be
no difference in pitch.
How Fast is Sound?
It takes time for sound to travel. After many investigations and
experiments, scientists have measured the speed of sound. Sound travels about
1,100 feet in one second of time. That’s about as long as four football
fields. If you say “hello” to a wall close to you, the sound does bounce
from the wall. But it goes to the wall and returns to you in so little time
that you cannot make it out. You cannot hear an echo from a close wall, even
though sound waves bounce from it.
Now that we know what sound is and how it behaves, we can control it
better. If we do not want the molecules of a sound wave to bounce off a wall,
we hang soft curtains on the wall. The moving molecules in the air sink in
among the molecules of cloth and do not bounce. We say that the cloth absorbs
the sound. The cloths are porous. They contain many dead air spaces
which stop or absorb sound waves. Materials are different in their ability to
absorb sound waves. The more porous the material, the better the sound waves
are other ways of controlling noise. Planning the location of a building is
one way. Sometimes bushes are planted to act as sound-wave absorbers. Floors
of rubber or cork tile absorb sound waves. A door with a control on it closes
the door quietly and slowly. Manufacturers of heating systems find it easier
to sell heating systems that work quietly.
When sound waves reach your ear, they push against the eardrum and make
it vibrate. The eardrum is made of thin material called a membrane.
When the eardrum vibrates, the vibration passes along three tiny bones into a
special fluid in your ear. The waves in this fluid travel along to nerve
endings. Then the nerve endings send messages to the brain. Your brain
receives these messages and gives meaning to them. You actually hear sounds
with your brain. Your ear and nerves act as a road to the brain to make
hearing possible. The louder the sound is, the stronger the sound waves were
produced in the air. The stronger the sound waves in the air, the harder they
push against your eardrums. The harder they push, the louder the sound you
hear. Strong sound waves can be heard farther away than weak ones.
Measuring the Volume of Sound
The unit used to measure the loudness of sound is a decibel. The
number of decibels depends on how loud a sound is. If the sound is loud, the
decibel number is high.
are some common sounds with their approximate decibel measurements:
|Rustle of leaves
Would you say that a passing train is only six times louder than a
whisper? Common sense will tell you that the train is many times louder than
that. You cannot compare decibels as you would some other units of
measurement. Decibels have a different method of comparison.