[Home > Intro to Sound > Expanding Wave]
Here is a wave in two dimensions. Imagine the red dots are air molecules. When you press "play," the yellow dot at the center is going to suddenly expand and collapse. As it expands, it pushes on the air molecules that closely surround it. It is at this point that "the wave" begins.
If you watch this closely, you'll notice that the wave forms because an individual air molecule gets a push, which causes it to push on the air molecule right next to it. As each air molecule recovers from its push, the wave passes.
Ok, finally, here's what sound is, and how we hear...