Rarefaction

Rarefaction is the reduction of an item's density, the opposite of compression. Like compression, which can travel in waves, rarefaction waves also exist in nature. A common rarefaction wave is the area of low relative pressure following a shock wave.
Rarefaction waves expand with time ; in most cases rarefaction waves keep the same overall profile 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.

Physical examples

A natural example of rarefaction occurs in the layers of Earth's atmosphere. Because the atmosphere has mass, most atmospheric matter is nearer to the Earth due to the Earth's gravitation. Therefore, air at higher layers of the atmosphere is less dense, or rarefied, relative to air at lower layers. Thus, rarefaction can refer either to a reduction in density over space at a single point of time, or a reduction of density over time for one particular area.
Rarefaction can be easily observed by compressing a spring and releasing it.

In manufacturing

Modern construction of guitars is an example of using rarefaction in manufacturing. By forcing the reduction of density in the cellular structure of the soundboard, a rarefied guitar top produces a tonal decompression affecting the sound of the instrument, mimicking aged wood.

In philosophy

Distinctions between rarefaction and condensation, or between "the rare" and "the dense", were important in the early development of western philosophy, for example in the thinking of the ancient philosopher Anaximenes, and in Aristotle's Physics.