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 called acoustic field impedance.
In the far field, this begins at a distance of several wavelengths from the sound source, and where the soundpressure and the sound velocity are in phase. The sound characteristic impedance (Z) is the ratio of sound pressure and sound velocity or the product of the density of the medium (kg/cbm) and the speed of sound (m/s). It is a material-dependent characteristic value expressed in Newton-seconds per cubic meter (Ns/m3).
The sound characteristic impedance is directly dependent on the speed of sound or reciprocally dependent on the sound velocity. These values are directly dependent on the air density. This means that the sound characteristic impedance for air is lower at high temperatures than at low temperatures, since the air density falls with high temperatures and increases at low temperatures. The sound characteristic impedance is thus strongly dependent on temperature, changing from about 407 Ns/cbm at 30 °C to 445 Ns/m3 at -20 °C.