Compression wave or longitudinal wave shown travelling from left to right in a medium. The particles are first shoved forward by the vibrating surface (the white bar at the left, representing, for example, the surface of a hifi speaker) and then they return to their original positions (they oscillate in same the direction that the wave travels, hence longitudinal). This creates a region of higher pressure and density (compression) that travels along as a wave. This is immediately followed by a zone of lower pressure (or rarefaction, where the particles are further apart). This is how sound propagates in air or water and how p-waves travel during earthquakes. The phase of the particles determines their colour in this animation, giving a sense of the wave progression.