That is, any small piece of the medium moves in a direction perpendicular to the velocity of the wave as the wave passes by. Transverse waves are perhaps most familiar to us. Water waves and waves on a string are transverse waves. A longitudinal wave displaces its medium parallel to its direction of travel. That is, any small piece of the medium moves parallel to the velocity of the wave as the wave passes by.
Sound waves are longitudinal waves.
Interference and Superposition Unlike solid objects, waves can occupy the same space at the same time. Waves obey the Principle of Superposition . That is, when two or more waves occupy the same space the net displacement of the medium at any given point is simply the sum of the displacements due to the individual waves. This superposition of waves is also referred to as interference . Waves displace the media through which they travel in both positive and negative directions.