Compression, which is the "squeezing" together of molecules, and rarefaction, which is the expansion of the "squeezed" molecules back toward their original position, is the oscillatory behavior that...
As a sound wave travels it compresses the air particles around it together in order for it to travel because a wave cannot travel if the matter it uses isn't connected.
Sound waves are a little part of the atmospheric pressure in air. The instantaneous sound pressure is the deviation from the local ambient pressure po caused by a sound wave at a given location and...