Rayl
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. It has the same dimensions as momentum per volume. 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 and the particle velocity it produces. "Specific acoustic impedance" is defined as: where formula_2 and formula_3 are the specific acoustic impedance, pressure and particle velocity phasors, formula_4 is the position and formula_5 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 formula_7 is the characteristic impedance, and formula_8 and formula_9 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 formula_10 will be different from the characteristic acoustic impedance formula_7. MKS and CGS units. The MKS unit and the CGS unit confusingly have the same name, but not the same value: References. 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