Introduction to Computer Music: Volume One

A sound wave, which is not impeded by another object, propagates (or spreads) out from the source as a sphere.		The figure below illustrates the cross-section of a sound wave expanding outward from its source.
Pond ripples and vibrating strings are examples of transverse waves, where the displacement of the medium is perpendicular to the direction the wave is traveling in.

While transverse waves (such as those created by a vibrating string or pond ripple) displace the medium perpendicularly to the direction the wave propagates in, sound waves in air are longitudinal waves, in that the pulsating motion of the air is in the direction the sound wave travels. Physicists classically demonstrate this with the "Slinky" model, in which a quick push on one end of a Slinky will cause a longitudinal wave to travel down its length. The wave can be seen as areas where the coils are closer or farther apart from each other than they would normally be in the Slinky’s state of rest, corresponding the compression and rarefaction of air molecules in sound. In a sound wave, the actual air molecules do not travel far, but spread their kinetic energy or force to adjacent molecules before bouncing back near their original position, much like a cue ball striking another in billiards. A sound wave is also a form of a traveling wave, in that the air molecules disturbed by the sound source are unlikely to be the ones hitting your eardrum, but transfer their energy to other neighboring molecules.

For further study, see Hyperphysics->Sound waves