Introduction to Computer Music: Volume One

2. What is Sound? | page 2

A sound wave, which is not impeded by another object, propagates (or spreads) out from the sound source as a sphere.
 
The figure below illustrates the cross-section of a sound wave expanding outward from its source as a sphere if unimpeded by another object.

Pond ripples are examples of a combined transverse and longitudinal wave, where the displacement of the individual particles is in a circular (or eliptical) motion with the radius of the circle being parallel to the outward direction of the wave.

While transverse waves (such as those created by a vibrating string) 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 to 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.

It should be noted that transverse waves from vibrating strings themselve makes very little sound, and it is usually the transfer and reorientation of this motion through the mechanics of an instrument (bridge, sound board, etc.) that produce the bulk of the sound we ultimately hear.

For further study, see Hyperphysics->Sound waves

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