a. The process or act of making rare or less dense; increase of volume,
the mass remaining the same: now usually of gases; also, the state of
being rarified; as, the rarefaction of the atmosphere on a high mountain.
Standard, 2 b. Diminution of air pressure below normal, as in alternate half-cycles in
the transmission of a sound wave past a point. CTD
Dictionary of Mining, Mineral, and Related Terms