In longitudinal waves, the vibrations are parallel to the direction of wave travel.
Examples of longitudinal waves include:
One way to remember the movement of particles in longitudinal waves is to use the 'P' sound: longitudinal waves such as seismic P-waves may be thought of as pressure or push waves as the particles move parallel to the wave.
Longitudinal waves are often demonstrated by pushing and pulling a stretched slinky spring.
In the diagram, the compressions move from left to right and energy is transferred from left to right. However, none of the particles are transported along a longitudinal wave. Instead, they move backwards and forwards between compressions as the wave is transmitted through the medium.