A Rayl,
rayl or Rayleigh
is one of two units of
specific acoustic impedance or, equivalently,
characteristic acoustic impedance; one an
MKS unit, and the other a
CGS unit. These have the same dimensions as momentum per volume.
The units are named after
John William Strutt, 3rd Baron Rayleigh
John William Strutt, 3rd Baron Rayleigh, (; 12 November 1842 – 30 June 1919) was an English mathematician and physicist who made extensive contributions to science. He spent all of his academic career at the University of Cambridge. Amo ...
. They are not to be confused with the
rayleigh unit of
photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always m ...
flux
Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ph ...
, which is used to measure
airglow
Airglow (also called nightglow) is a faint emission of light by a planetary atmosphere. In the case of Earth's atmosphere, this optical phenomenon causes the night sky never to be completely dark, even after the effects of starlight and diff ...
and is named after his son,
Robert John Strutt, 4th Baron Rayleigh.
Explanation
Specific acoustic impedance
When sound waves pass through any physical substance the pressure of the waves causes the particles of the substance to move. The sound
specific impedance is the ratio between the
sound pressure
Sound pressure or acoustic pressure is the local pressure deviation from the ambient (average or equilibrium) atmospheric pressure, caused by a sound wave. In air, sound pressure can be measured using a microphone, and in water with a hydrophone ...
and the
particle velocity
Particle velocity is the velocity of a particle (real or imagined) in a medium as it transmits a wave. The SI unit of particle velocity is the metre per second (m/s). In many cases this is a longitudinal wave of pressure as with sound, but it can ...
it produces.
''Specific acoustic impedance'' is defined as:
[
:
where and are the specific acoustic impedance, pressure and ]particle velocity
Particle velocity is the velocity of a particle (real or imagined) in a medium as it transmits a wave. The SI unit of particle velocity is the metre per second (m/s). In many cases this is a longitudinal wave of pressure as with sound, but it can ...
phasor
In physics and engineering, a phasor (a portmanteau of phase vector) is a complex number representing a sinusoidal function whose amplitude (''A''), angular frequency (''ω''), and initial phase (''θ'') are time-invariant. It is related to ...
s, is the position and is the frequency.
Characteristic acoustic impedance
The Rayl is also used for the ''characteristic (acoustic) impedance'' of a medium, which is an inherent property of a medium:[
:
Here is the characteristic impedance, and and are the density and speed of sound in the unperturbed medium (i.e. when there are no sound waves travelling in it).
In a viscous medium, there will be a phase difference between the pressure and velocity, so the specific acoustic impedance will be different from the characteristic acoustic impedance .
]
MKS and CGS units
The MKS unit and the CGS unit confusingly have the same name, but not the same value:
* In MKS units, 1 Rayl equals 1 pascal-second
The second (symbol: s) is the unit of time in the International System of Units (SI), historically defined as of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds e ...
per meter
The metre ( British spelling) or meter ( American spelling; see spelling differences) (from the French unit , from the Greek noun , "measure"), symbol m, is the primary unit of length in the International System of Units (SI), though its pr ...
(Pa·s·m−1), or equivalently 1 newton-second per cubic meter (N·s·m−3). In SI base unit
The SI base units are the standard units of measurement defined by the International System of Units (SI) for the seven base quantities of what is now known as the International System of Quantities: they are notably a basic set from which all ...
s, that is kg·s−1·m−2.[
:
* In CGS units, 1 Rayl equals 1 ]barye
The barye (symbol: Ba), or sometimes barad, barrie, bary, baryd, baryed, or barie, is the centimetre–gram–second (CGS) unit of pressure. It is equal to 1 dyne per square centimetre.
: = = = = =
See also
*Pasca ...
-second
The second (symbol: s) is the unit of time in the International System of Units (SI), historically defined as of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds e ...
per centimeter (ba·s·cm−1), or equivalently 1 dyne
The dyne (symbol: dyn; ) is a derived unit of force specified in the centimetre–gram–second (CGS) system of units, a predecessor of the modern SI.
History
The name dyne was first proposed as a CGS unit of force in 1873 by a Committee o ...
-second per cubic centimeter (dyn·s·cm−3). In CGS base units, that is g·s−1·cm−2.
:
*1 CGS Rayl = 10 MKS Rayl. In other words, a CGS Rayl is ten times larger than an MKS Rayl.
References
{{reflist
T. D. Rossing, ''Springer Handbook of Acoustics'', Springer, 2007, p. 60
L. E. Kinsler, A. R. Frey, A. B. Coppens, and J. V. Sanders, ''Fundamentals of Acoustics'', Fourth Edition (New York: John Wiley & Sons, Inc.,2000), p. 126
External links
Dictionary of Physics Units of Measurement
Units of measurement
Non-SI metric units