The normal incidence surface impedance is a complex coefficient given by the acoustical pressure to velocity ratio at the surface of the tested sample when it is excited by a normal incidence acoustical wave. It measures the resistance and inertia encountered by the acoustical wave trying to penetrate the material. Its real part is the acoustical resistance, and its imaginary part is the acoustical reactance. It is particularly useful when studying the effects of an air cavity backing a resistive layer.
Tested frequency range
- Small impedance tube (29 mm diameter): 200 to 6600 Hz
- Medium impedance tube (44.44 mm diameter): 100 to 4300 Hz
- Large impedance tube (100 mm diameter): 35 to 1900 Hz
The normalized surface impedance (Zsn) is often used. It is defined by Zsn=Zs/Zo, where Zo is the specific impedance of air.
Zo is given by Zo=ρc, where ρ and c are the density and sound speed in air.
 J.Y. Chung and D.A. Blaser, “Transfer function method of measuring in-duct acoustic properties: I. Theory,” J. Acoust. Soc. Am. 68(3), 907-913 (1980).
 J.Y. Chung and D.A. Blaser, “Transfer function method of measuring in-duct acoustic properties: II. Experiment,” J. Acoust. Soc. Am. 68(3), 907-913 (1980).