Influence of edge diffraction on the in situ measurement of impedance using microphone arrays

When measuring the surface impedance and/or the sound absorption coefficient of a material sample in situ, one of the main concerns is the sample size effect due to edge diffraction. In recent years, several in situ impedance measurement techniques using arrays of microphones have been proposed. This paper aims at analysing the the effect of edge diffraction on the in situ measurement with arrays of microphones based on a numerical simulation framework.

A simplified Boundary Element Method (BEM) is used to emulate the measurement performed by two types of arrays: (i) - a double layer array, consisting of two parallel layers of uniformly spaced microphones; (ii) - a three-dimensional randomized array, consisting of an irregular distribution of measurement points within a parallelepiped.

The study examines different array configurations (size and number of sensors) and sample properties (size and absorption). The wave-number spectrum is used to examine the effect of edge diffraction. The influence of edges on the estimation of surface impedance and the absorption coefficient is also analysed and compared to the estimation made with a single pressure-velocity probe.