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rarefaction
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Diagram with three panels showing sound wave representations: (A) evenly spaced blue lines for air at equilibrium, (B) alternating dense and sparse blue lines for compressions and rarefactions, and (C) a yellow sine wave illustrating amplitude and wavelength.
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longitudinal wave
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rarefaction

physics
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History

rarefaction, in the physics of sound, segment of one cycle of a longitudinal wave during its travel or motion, the other segment being compression. If the prong of a tuning fork vibrates in the air, for example, the layer of air adjacent to the prong undergoes compression when the prong moves so as to squeeze the air molecules together. When the prong springs back in the opposite direction, however, it leaves an area of reduced air pressure. This is rarefaction. A succession of rarefactions and compressions makes up the longitudinal wave motion that emanates from an acoustic source.

This article was most recently revised and updated by William L. Hosch.