rarefied

　

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1. 《時におどけて》非常に純粋な,えり抜きの・選ばれた人の, 選良の; 深遠な, 秘義の; 〈地位・性質など〉きわめて高い, 格調の高い, 高尚な; 希薄な

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2016/01/21 09:09:35」(JST)

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This article is about the physical process. For the ecological technique, see Rarefaction (ecology).

An example of rarefaction is as a phase in a sound wave or phonon. Half of a sound wave is made up of the compression of the 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. Like compression, which can travel in waves (sound waves, 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 gases 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 is in contrast to the behaviour of pressure increases, which get 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. Instead of seeing compressed loops seeming to move through the spring, spaced-out loops move through it: rarefaction waves.

See also

• Longitudinal wave
• P-wave
• Prandtl-Meyer expansion fan

Citations

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Related Links

• Rarefied - definition of rarefied by The Free Dictionary

rar·e·fied also rar·i·fied (râr′ə-fīd′) adj. 1. Not dense; thin: the rarefied air of the high peaks. 2. Belonging to or reserved for a small select group; exclusive: accepted in the most rarefied social circles. 3. Refined or esoteric: rarefied ...

• Rarefied | Rarefied Definition by Merriam-Webster

Rarefied was formed from the verb "rarefy," which derives from a combination of Latin rarus ("thin" or "rare") with "facere" ("to make") and has meant "to make thin" since the 14th century. In its original uses back in the 1600s, the ...

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