Longitudinal waves are waves such that the direction of travel of the wave, from left to right in the diagram below, is along the same line as the vibration of the wave, or the motion of the air molecules in the diagram below.
Longitudinal waves are pressure waves. Regions of high pressure – compression - and low pressure – rarefaction - alternate along the wave, moving as the wave travels.
Sound waves in air, liquid or materials, p – waves produced in earthquakes, shock waves and sonic booms are all examples of longitudinal waves. These have in common that they all need a material medium to travel through. This is a general feature of longitudinal waves. Longitudinal waves are impossible in a vacuum.
Like all waves, longitudinal waves obey the wave equationwhere
= wave speed
This is illustrated in the diagram at top.