Longitudinal wave

Sound travelling through air is a longitudinal wave. A vibrating source, such as a loudspeaker, pushes nearby air particles closer together and then allows them to spread further apart. This produces alternating compressions and rarefactions.

In a longitudinal wave, the oscillations are parallel to the direction of travel. Imagine pushing and pulling the end of a stretched spring along its length. Each coil moves backwards and forwards in the same line as the wave travels.

In a longitudinal wave, the vibration is parallel to the direction of travel. Push and pull one end of a horizontally stretched slinky along its length. The disturbance travels along the slinky, and each coil moves forwards and backwards along that same line.

In a longitudinal wave, the disturbance is parallel to the direction of travel. Pushing and pulling the end of a slinky along its length produces moving compressions, where the coils are closer together, separated by rarefactions, where they are farther apart. Individual coils move backwards and forwards while the compression pattern travels along the slinky.