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

A method is proposed for measuring the angle-dependent absorption properties 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 traveling in all directions. The incident and reflected wave components at the boundary can be separated in the wavenumber domain, from which an absorption coefficient is deduced for each angle of incidence. Unlike existing techniques that operate in the nearfield of a sample's surface, the pressure field is measured over a three-dimensional volume to better resolve all wave propagation angles. The proposed framework makes it possible to reconstruct pressure and normal particle velocity at the boundary, and to infer an acoustic surface impedance from these two quantities. 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 classroom with suspended sound-absorbing ceiling using a programmable robotic arm to scan the sound field. The derived absorption properties are compared with theoretical values calculated from a Transfer Matrix Model (TMM), showing good agreement up to grazing incidence.