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. The units are named after John William Strutt, 3rd Baron Rayleigh, and not to be confused with the rayleigh unit of photon flux, used to measure airglow, and named after his son, Robert John Strutt, 4th Baron Rayleigh.
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:
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
- In MKS units, 1 Rayl equals 1 pascal-second per meter (Pa·s·m−1), or equivalently 1 newton-second per cubic meter (N·s·m−3). In SI base units, that’s kg∙s−1∙m−2.
- In CGS units, 1 Rayl equals 1 barye-second per centimeter (ba·s·cm−1), or equivalently 1 dyne-second per cubic centimeter (dyn·s·cm−3). In CGS base units, that's 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 due to the way they are defined.
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