when the diaphragm of a speaker vibrates, a sound wave is produced that propagates through the air. The sound wave consists of small motions of the air molecules toward and away from the speaker. If you were able to look at a small volume of air near the speaker, you would find that the volume of air does not move far, but rather it vibrates toward and away from the speaker at the frequency of the speaker OU vibrations. This motion is very much analogous to waves propagating on a string. An important difference is that, if you watch a small portion of the string, its vibrational motion is transverse to the direction of R propagation of the wave on the string. The motion of a c small volume of air in a sound wave is parallel to the direction of propagation of the wave. Because of this, the sound wave is called a longitudinal wave.