Rarefaction is the reduction of an item's density, the opposite of
compression.
Like compression, which can travel in waves (
sound wave
In physics, sound is a vibration that propagates as an acoustic wave through a transmission medium such as a gas, liquid or solid.
In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by the ...
s, for instance), rarefaction waves also exist in nature. A common rarefaction wave is the area of low relative pressure following a
shock wave
In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a me ...
(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.
Physical examples
A natural example of rarefaction occurs in the layers of
Earth's atmosphere
The atmosphere of Earth is composed of a layer of gas mixture that surrounds the Earth's planetary surface (both lands and oceans), known collectively as air, with variable quantities of suspended aerosols and particulates (which create weathe ...
. Because the atmosphere has
mass
Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...
, most atmospheric
matter
In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic pa ...
is nearer to the Earth due to the Earth's
gravitation
In physics, gravity (), also known as gravitation or a gravitational interaction, is a fundamental interaction, a mutual attraction between all massive particles. On Earth, gravity takes a slightly different meaning: the observed force b ...
. 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 (loss of oils and other impurities) in the cellular structure of the soundboard, a rarefied guitar top produces a tonal decompression affecting the sound of the instrument, mimicking aged wood.
See also
*
Longitudinal wave
Longitudinal waves are waves which oscillate in the direction which is parallel to the direction in which the wave travels and displacement of the medium is in the same (or opposite) direction of the wave propagation. Mechanical longitudinal ...
*
P-wave
A P wave (primary wave or pressure wave) is one of the two main types of elastic body waves, called seismic waves in seismology. P waves travel faster than other seismic waves and hence are the first signal from an earthquake to arrive at any ...
*
Prandtl–Meyer expansion fan
A supersonic expansion fan, technically known as Prandtl–Meyer expansion fan, a two-dimensional simple wave, is a centered expansion process that occurs when a supersonic flow turns around a convex corner. The fan consists of an infinite numb ...
* Rarefied gas dynamics
Citations
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Sound
Acoustics
Waves
Conservation equations