Hi Jacquelyn -
Whether it's in a solid, liquid, or gas, sound travels in waves.
These waves move by particles colliding with one another. One hits
another, and that one hits another, etc. It's like a domino effect.
Heat travels in this way too, but with heat the molecule motions are
random. Waves consist of collections of molecules moving back and forth
together. Some waves travel as variations in pressure, such as sound
waves, while others involve sideways motions of the molecules, relative
to the direction the wave is going. Examples of sideways wave are the
waves on the surface of water in a swimming pool. Both kinds of waves
travel in liquids.
In solids, the molecules are very close together, and they have
strong chemical bonds between them. To get a bit of solid to compress
you have to squeeze really really hard on it. This compressibility is
related to the speed of sound in the solid, for the pressure waves
described above. The density of the solid is also important -- the more
dense the material, the slower the sound will travel. For a crystal,
sound waves may even have different speeds in different directions
because it may be easier to stretch or compress the bonds along one
axis of the crystal than another.
In liquids, the molecules do not follow a rigid pattern in space
like those in a crystalline solid, and the bonds between molecules are
usually weaker and constantly break and re-form. Raising the pressure
in a tiny bit of the liquid will cause the molecules there to move to
lower-pressure regions, pushing on the molecules already there, and
raising the pressures there. The molecules have some inertia, so they
in fact go a bit farther than it would take to even out the pressure,
and the pressure in that little region where the pressure was initially
higher becomes lower than the average pressure in the liquid, and the
molecules are pulled back where they started. The whole process then
repeats until all the energy is carried away in the sound waves. This
is why you get multiple waves spreading out from the place where a rock
is dropped in a pond.
Gases are just like liquids when it comes to pressure waves. Gases
are less dense than the liquid versions of the substance, and they also
are more compressible -- it doesn't take as much pressure to squeeze a
gas into a smaller volume as it would to make a similar change in a
liquid. Sound travels faster in less dense materials and slower in
materials more easily compressed. Usually the compressibility change
has a much bigger effect than the change in density, and the net effect
is that sound travels slower in a gas than in a liquid of the same
substance.
~Bob and Tom
(published on 10/22/2007)
