Rarefaction

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. Each particle moves back and forth parallel to the direction in which the sound travels and affects neighbouring particles.

Rock particles oscillate parallel to the wave’s direction, producing compressions and rarefactions. P waves can travel through solids and liquids.

Where coils bunch together there is a compression; where they spread out there is a rarefaction. These regions move along the slinky even though each coil only vibrates locally. Sound travelling through air is a longitudinal wave: air particles vibrate back and forth in the direction the sound travels.

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.