Longitudinal waves

In a longitudinal wave, particles of the medium oscillate back and forth parallel to the direction in which the wave transfers energy. Each particle oscillates about its equilibrium position rather than travelling along with the wave.

As particles move, they affect neighbouring particles, passing the disturbance and its energy through the medium. The wave can therefore transfer energy without the medium as a whole travelling with it.

Sound in air

Sound in air is a longitudinal wave. As air particles oscillate, they produce alternating regions called compressions and rarefactions:

• In a compression, particles are closer together, so particle density and air pressure are higher than their equilibrium values.
• In a rarefaction, particles are farther apart, so particle density and air pressure are lower than their equilibrium values.

These regions travel through the air as the sound wave transfers energy, while the air particles oscillate back and forth about their equilibrium positions.