Measuring angle-dependent sound absorption in ordinary rooms

Abstract

The effective use of sound absorbing materials in room acoustical design requires the use of angle-dependent coefficients, as opposed to random incidence coefficients, to properly account for the dissipation of sound energy at the room's boundary. In this study, a method is proposed for measuring the angle-dependent surface impedance and absorption coefficient of a boundary material in situ, in an ordinary room. The proposed method relies on the reconstruction of the pressure and normal particle velocity at the boundary using a plane-wave decomposition of the sound field measured with an array of microphones. In sufficiently reverberant environments, where sound strikes the material from nearly all directions in space, the angle-dependent properties of the boundary can be obtained simultaneously for all angles of incidence, from a single source position. In addition, the proposed methodology enables us to identify the specific directions of incidence on the material and makes it possible to estimate its operational acoustic performance. The validity of the method is examined experimentally in a reverberation chamber and in a classroom, using a robotic arm to scan the sound field in the vicinity of an absorbing boundary. Consistent absorption data are measured for various source positions and room configurations.