To and fro motion
Teaching Guidance for 5-11 11-14
A fundamental property of sound is that it is produced by a source that vibrates. This to and fro motion applies to all sounds, whatever the medium.
Regions of high pressure and low pressure happen as a result of to and fro motion in air. (The to and fro causes increases and decreases in the density of the air, thus changing the pressure). Changes in pressure can only happen in fluids; sounds can travel in solids as well. So keep it simple and general: to and fro is both accurate and sufficient.