Rayl

 ( Units Of Measurement ) 

A rayl (symbol Rayl) is one of two units of specific acoustic impedance and 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. They are not to be confused with the unit of photon flux, the rayleigh.

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Explanation

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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 and the particle velocity it produces.

Specific acoustic impedance is defined as:

$\{\backslash underline\{Z\}(\backslash mathbf\{r\},\backslash omega)\; =\; \backslash frac\{\backslash underline\{p\}(\backslash mathbf\{r\},\backslash omega)\}\{\backslash underline\{v\}(\backslash mathbf\{r\},\backslash omega)\}\}$

where $\backslash underline\{Z\},\; \backslash underline\{p\}$ and $\backslash underline\{v\}$ are the specific acoustic impedance, pressure and particle velocity , $\backslash mathbf\{r\}$ is the position and $\backslash omega$ is the frequency.

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Characteristic acoustic impedance The rayl is also used for the characteristic (acoustic) impedance of a medium, which is an inherent property of a medium:

$\{Z\_0=\; \backslash rho\_0\; c\_0\}$

Here, $\{Z\_0\}$ is the characteristic impedance, and $\{\backslash rho\_0\}$ and $\{c\_0\}$ 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 $\backslash underline\{Z\}$ will be different from the characteristic acoustic impedance $\{Z\_0\}$.

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MKS and CGS units Subscripts are used in this section to distinguish identically named units. Texts often refer to "the MKS rayl" to ensure clarity.

The MKS unit of specific acoustic impedance is the pascal-second per meter, and is often called the rayl (MKS: 1 Rayl = 1 Pa·s·m⁻¹).

The MKS unit and the CGS unit confusingly have the same name, but are not the same quantity (or unit):

• As an MKS unit, one rayl equals one pascal-second per meter (Pa·s·m⁻¹), or equivalently one newton-second per cubic meter (N·s·m⁻³). Expressed in SI base units, that is kg·s⁻¹·m⁻²:
• : 1 Rayl = 1 N⋅s/m = 1 Pa⋅s/m = 1 kg/(s⋅m)
• As a CGS unit, one rayl equals one barye-second per centimeter (ba·s·cm⁻¹), or equivalently one dyne-second per cubic centimeter (dyn·s·cm⁻³). Expressed in CGS base units, that is g·s⁻¹·cm⁻²:
• : 1 Rayl = 1 dyn⋅s/cm = 1 ba⋅s/cm = 1 g/(s⋅cm)
• The CGS unit rayl is ten times larger than the MKS unit rayl:
• : 1 Rayl = 10 Rayl

Citations

Sources

• Bjørn Angelsen (2025). 9788299581103, Emantec. ISBN 9788299581103
• Richard S. C. Cobbold (2025). 9780195168310, Oxford University Press. ISBN 9780195168310

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External links

• Dictionary of Physics Units of Measurement
• A discussion of the history and ambiguity of the rayl

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