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

One way to remember the movement of particles in longitudinal waves is to say the waves move along the same way as the particles.

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

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.

Sound Waves

Sound waves are longitudinal waves. They cause particles to vibrate parallel to the direction of wave travel. The vibrations can travel through solids, liquids or gases, by a series of compressions and rarefactions.

Sound cannot travel through a vacuum because there are no particles to carry the vibrations.