In-situ impedance and absorption coefficient estimations using a double-layer microphone array

Acoustic impedance is typically estimated using an impedance tube, which requires a sample physically fitted to the tube. However, the estimated impedance can vary greatly between the material mounted in the tube, and the material located in a real environment where the boundary conditions are likely different. In this paper, we investigate the use of a double-layer microphone array to estimate in-situ impedance and absorption coefficient. The measurement configuration is similar to other approaches: a source emits a broad-band noise toward a material sample, and a microphone array is placed at some distance between the source and the sample. Part of the emitted acoustic energy is reflected, and another part is absorbed by the material under test. The array processing consists in calculating pressure and velocity at the material surface using acoustical holography techniques such as SONAH or ESM. Those acoustic quantities can then be used to estimate the impedance over a selected area. A set of tests has been carried out in an anechoic chamber as well as in an ordinary room. Different materials and sizes, as well as source orientations have been considered. The results show consistency between the measurements in the anechoic chamber and the ordinary room.