Rayl

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A rayl is a unit used for two related ideas in acoustics: specific acoustic impedance and characteristic (or intrinsic) acoustic impedance. There are two versions, in the MKS (SI) system and in the CGS system. They have the same basic dimensions as momentum per volume. The rayl is named after John William Strutt, 3rd Baron Rayleigh, and it should not be confused with the unit rayleigh, which measures photon flux.

When sound waves travel through a material, the pressure makes the particles move. The specific acoustic impedance is the ratio of sound pressure to particle velocity (Z = p/v). The characteristic impedance Z0 is a property of the medium itself; for a simple, undisturbed medium, Z0 = ρ0 c0, where ρ0 is the density and c0 is the speed of sound. In real, viscous media, pressure and velocity can be out of phase, so the actual impedance Z can differ from Z0.

In the MKS (SI) system, the unit of specific acoustic impedance is the pascal-second per meter, called the rayl: 1 Rayl = 1 Pa·s·m−1. The CGS version also uses the name rayl but is a different unit, so the two should not be confused.