travelling waves may be either transverse waves or longitudinal waves. we have already seen the difference between these two types of motion in chapter 4. in transverse waves the change in the corresponding physical quantity, e.g. displacement, occurs in the direction at right angles to the direction of travel of the waves, as for the outgoing ripples on a pound. for longitudinal waves, the change occurs along the direction of travel. an example of this is the longitudinal compressions and rare factions of the air that occur in the propagation of a sound wave. it is easier to see the physical processes going on in a transverse wave and so we will concentrate on them in the present chapter.