(a) When a sound wave is produced by a wave in a medium, it sets the particles of the medium. Consider, for example, vibration of the diaphragm of a speaker. As the diaphragm vibrates outwards, it sends in a wave of compression and the particles of air directly in front of it. Thus, compressed air travels away from the speaker with a speed of sound. Again, when the diaphragm reverses, its motion moves outwards, it then sends in a wave of rarefaction through the medium which travels with the velocity of sound. Then the sound travels in the form of compressions and rarefactions.
(b) The graphical representation is as shown:
(c) Sound waves are called longitudinal waves because the propagation of the sound wave takes place in the direction in which particles of the medium vibrate.