A series of mechanical compressions and rarefactions that successively propagate through air or any medium that is at least a little compressible or elastic (solid, liquid, or gas, but not vacuum). In sound waves molecules or groups of molecules are excited and transmit their energy to successive molecules in an outward direction.
A sound wave is usually represented visually by a wavy, horizontal line. The upper part of the wave (the crest) indicates compression and the lower part (the trough) indicates rarefaction.