The reflection coefficient of a material at oblique incidence is measured in a free field. The sound pressure distributions are measured at discrete points on two measuring lines and decomposed into plane wave components by using spatial Fourier transform. The incident and reflected plane wave components are obtained from a set of "decomposition equations" using the plane wave propagation theory. Numerical simulation and experiment are performed to see the effect of finite size of measurement area. To reduce this effect, window function is used. The reflection coefficient obtained from this method is compared with those obtained from other methods; 2 microphone methods in a duct and an empirical equation determining the characteristic impedance pc and propagation constant k of a material from flow resistance information.
