A method for measuring the angle-dependent absorption coefficient and surface impedance in situ

A method is proposed for measuring the angle-dependent surface impedance and absorption coefficient of a boundary material in situ. The method consists in expressing the sound field measured in the vicinity of the boundary as a superposition of plane waves travelling in all directions (i.e., using a spatial Fourier transform). The incident and reflected wave components at the boundary’s surface are separated in the wavenumber domain, from which sound pressure, normal particle velocity, and surface impedance are reconstructed for each angle of incidence. In sufficiently reverberant environments, the angle-dependent properties of the
boundary can be obtained for all angles of incidence simultaneously from a single set of measurements (one source position). Experiments are conducted in a reverberation chamber and in the mock-up of a classroom using a programmable robotic arm to scan the sound field in a three-dimensional domain in the vicinity of the boundary. The derived absorption properties are compared with theoretical values calculated from a Transfer Matrix Model (TMM), showing good agreement up to grazing incidence.