A sound wave is a pressure wave that results from the vibration of the particles o the medium from the source. The motion of the particles in the medium is parallel to the direction of the energy transport. The type of wave formed by a sound wave is the longitudinal wave. A longitudinal wave is characterized by rarefactions. A longitudinal wave is a wave motion wherein the particles in the wave medium are displaced parallel to transport. When motion is detected from the source, the particle next to it vibrates from its rest position and a progressive change in phase vibration is observed at each particle within that wave. The result is that the energy is transported from one region to the other. These combined motions result in the movement of alternating regions of rarefaction in the direction of propagation.