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A free field method is reported for the measurement of the normal impedance and the absorption coefficient of a material. The pressure and phase of an incident “plane” wave in steady state are measured at a point in the absorptive surface and compared with the pressure and phase measured at the same point in space at the surface of a perfectly reflecting material, or at the same point in free space without any panel. From these data are determined the phase and magnitude of both the incident and reflected waves, and this information in turn yields both the normal impedance and the free wave absorption coefficient for the particular angle of incidence involved. Tests have been made with an 8-ft square sample of material mounted in an anechoic chamber and rotated about a vertical axis in the presence of a fixed horizontal beam of an approximately plane wave. As a check of the validity of the method, measurements have been made using two types of material with known properties, one with the normal impedance essentially independent of the angle of incidence and the other with a considerable angular dependence. The experimental values are shown to be in good agreement with calculations.
Ingård et al. (Sat,) studied this question.