Acoustic. The Basics of Sound
At the simplest level sound is the movement of air molecules. This movement creates very slight local differences in the air pressure which cause our ear drums to expand and contract - allowing us to perceive the air movement as sound.
Sound travels in waves - the initial movement pushes air molecules together creating a localised area of high pressure and a localised area of low pressure immediately behind. This partial vacuum then “pulls” air behind it creating a moving wave.