APPLICATION OF THE LOCAL PLANE WAVE METHOD TO MEASURE IN-SITU SOUND ABSORPTION USING A SPHERICAL MICROPHONE ARRAY

Former studies have shown that the Local Plane Wave (LPW) method can be used to measure the (effective) in-situ sound absorption coefficient of any surface. The LPW-method is based on a local plane wave assumption, in which the normal component of the sound field in each spatial coordinate is approximated by an incident and a reflected plane wave. This is contrary to conventional methods that rely on a known global sound field, like the impedance tube (plane wave) and reverberation room (diffuse field) methods. The LPW-method can therefore be applied in sound fields for which a model is not available. This has been numerically and experimentally validated using a probe consisting of 8 MEMS-microphones in an open cubical structure. In this paper, the LPW-method will be applied to a rigid spherical microphone array. The wellknown plane wave expansion in spherical harmonics will be used to formulate a local model of the sound field in the presence of a rigid sphere. The resulting model will be used to infer the sound field for the case the rigid sphere would not be present. The (effective) in-situ sound absorption coefficient is then computed from the parameters of this inferred sound field. Using numerical and experimental results, the validity and usefulness of the LPW-method to measure sound absorption using a spherical microphone array will then be shown.