Alternate compressions (compacting together) and rarefactions (spreading apart) of air particles moving away from something that is vibrating between 20 and 20,000 times a second or a similar occurrence in another substance (such as water).
Found on http://www.encyclo.co.uk/local/20447
Alternate compressions (compacting together) and rarefactions (spreading apart) of air particles moving away from something that is vibrating between 20 and 20,000 times a second or a similar occurrence in another substance (such as water).