Computer simulation of a longitudinal wave in a solid. The solid is represented by spheres (atoms) connected with springs (bonds). Longitudinal waves are pressure waves, alternating regions of compression and rarefaction, in which the oscillations of the particles is in the same direction as the wave itself. Sound in air is a pressure wave, but such waves travel much more quickly in solids. In earthquakes, for instance, the longitudinal P waves travel through the crust at some six kilometres per second.