Longitudinal waves

In longitudinal waves, the vibrations are parallel to the direction of wave travel.

Examples of longitudinal waves include:

  • sound waves
  • ultrasound waves
  • seismic P-waves
  • push-pull waves on a spring

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.

Demonstrating longitudinal waves

Longitudinal waves show areas of compression and rarefaction:

  • compressions are regions of high pressure due to particles being close together
  • rarefactions are regions of low pressure due to particles being spread further apart

Longitudinal waves are often demonstrated by pushing and pulling a stretched slinky spring.

An outstretched slinky spring

When the spring is pushed from left to right, the coils bunch up. A compression is what we call the bunched up region. When the spring is pulled back, coils are stretched further apart and this region where coils are more spread out is called a rarefaction. Both compressions and rarefactions can be seen to move along the spring. However, none of the particles are transported along a longitudinal wave. Instead, they move backwards and forwards between compressions and rarefactions as the wave is transmitted through the medium.