Editing Rarefaction (section)

{{Short description|Reduction of an object's density}}
{{about|the physical process|the ecological technique|Rarefaction (ecology)}}
{{Refimprove|date=October 2012}}
[[Image:Onde compression impulsion 1d 30 petit.gif|thumb|305px|
An example of rarefaction is also as a [[Phase (waves)|phase]] in a [[sound]] wave or [[phonon]].  Half of a [[sound wave]] is made up of the compression of the [[Transmission medium|medium]], and the other half is the decompression or ''rarefaction'' of the medium.]]
'''Rarefaction''' is the reduction of an item's density, the opposite of [[compression (physical)|compression]].<ref name="Encyclopædia Britannica">{{cite web|url=https://www.britannica.com/science/rarefaction|title=Rarefaction|publisher=[[Encyclopædia Britannica]]}}</ref> Like compression, which can travel in waves ([[sound wave]]s, for instance), rarefaction waves also exist in nature. A common rarefaction wave is the area of low relative pressure following a [[shock wave]] (see picture).

Rarefaction waves expand with time (much like sea waves spread out as they reach a beach); in most cases rarefaction waves keep the same overall profile ('shape') at all times throughout the wave's movement: it is a ''self-similar expansion''. Each part of the wave travels at the local speed of sound, in the local medium. This expansion behaviour contrasts with that of pressure increases, which gets narrower with time until they steepen into shock waves.