When sound waves pass through any physical substance the pressure of the waves causes the particles of the substance to move. The sound impedance is the ratio between the pressure and the particle velocity it produces.
The impedance is 1 rayl if unit pressure produces unit velocity.
The Rayl is the SI derived unit of specific acoustic impedance.
1 MKS Rayl = 1 Pa s/m (this unit was established in honour of Lord Rayleigh).