Great question! The idea of sound traveling in waves can be difficult to grasp. When we think waves, we to tend picture long, wiggly strings moving through the air. In fact, the motion of sound waves is a little more complicated.
There are two kinds of waves we should consider. Those wiggly ones I mentioned before are called "transverse waves", and they represent the way in which sound might travel across a string, for example.
In the air, however, sound waves are actually pressure variations -- regions of molecular compression pushing onward. Imagine sound traveling from a stereo to your ears. Speakers like the one pictured vibrate, causing molecules in their path to vibrate in the same way and push together into compressed regions. These regions naturally push outward, causing a chain reaction. Eventually, the reaction reaches your ear, where the molecules cause your ear drum to vibrate, detecting sound.
So here, people use pictures of string-like waves to visualize patterns of compression, in which there's really no sideways movement. If we had a way of counting exactly how many molecules are in each region, a graph of those numbers would look just like that wave pattern, with the upper peaks representing areas with more molecules, and the lower ones areas with fewer molecules.
(published on 10/04/2009)