Sound is a type of wave. Another example of a wave would be a water
wave traveling in the ocean. As the wave comes by the water raises up
and then lowers down. The water in the wave is called the medium. A
medium is what scientists call the thing that waves move through.
Another example of a wave is the waves people make in a stadium. As
the wave comes by the person stands up and sits back down. Notice that
the people don't move around the stadium, they only move up and down,
but the wave does move around the stadium.
Getting back to sound, sound is a wave of pressure that can travel
in gasses, liquids or solids. The crests of the wave, which are
correspond to the high points on a water wave, are areas of high
pressure. The troughs, which are like the low spots on a water wave,
are areas of low pressure. Sound is created by a vibration. If you hum
a note and press your hand agains your throat you will notice your
throat is vibrating. This is your voice box wiggling back and forth
making the waves of pressure. You might notice that the higher the
pitch note you hum the faster your throat seems to vibrate. These waves
of pressure travel through the air and then wiggle the ear drum of the
listener. Your ear is designed to detect these wiggles in the ear drum
and that is how you can hear.
How fast sound moves is
determined by the medium it is traveling in. In general, sound travels
faster in materials that are hard to compress and not very dense, and
slower in materials that are easy to compress and very dense. Steel is
much harder to compress than air, but also much more dense, so the two
factors partly cancel. It ends up that sound travels twice as fast in
steel as in air.
Unlike light waves, sound needs to have atoms
there to travel. In space, where there are not many atoms to pass along
the wiggles sound cannot travel (contrary to what you might see in the
movies with things blowing up loudly in space).
(published on 10/22/2007)