Sound characteristic impedance is a characteristic quantity for the sound resistance that a medium
opposes to the propagation of sound. The sound characteristic impedance is also referred to as the acoustic field impedance. In the far field, this begins at a distance of several wavelengths from the sound source
, and wherethe sound pressure and the sound velocity are in phase. The sound characteristic impedance (Z) is the ratio of the sound pressure and the sound velocity or the product of the density of the medium (kg/cbm) and the sound velocity
(m/s). It is a material-dependent characteristic value expressed in Newton-seconds per cubic meter (Ns/m3).