Causally-guided acoustic optimization of rigidly-backed micro-perforated partitions: Case studies and experiments

A causal-based optimization criterion has been theoretically derived in an earlier study for a single-layer micro-perforated absorber (MPA) under normal incidence. In this study, it is extended to multiple and parallel MPA systems as well as to general incidence. It is shown that the ultimate wideband performance of multiple (resp. parallel) MPAs scale on their overall (resp. effective) cavity depths, with adverse effect of the incidence angle. The multi-variate constrained optimization process requires maximising the sensitivity of the total reflected intensity, integrated over all wavelengths, with respect to the parameters constitutive of the micro-perforated panels. It leads to perfect absorption and broad bandwidth efficiency at one or several resonant states of the MPAs. As for serial MPA systems, it is a suitable sub-optimal alternative to total absorption and inverse-frequency weighted absorption maximizations. Concerning MPA arrays, the causal-based optimal design brings near-unit absorption over a wide bandwidth in the low-frequency domain due to efficient grouping and merging of cross-coupled resonant states. It brings similar performance to direct maximization of the total inverse-frequency weighted absorption that puts weight on the low frequencies. These optimal performances have been verified through standing wave tube absorption measurements for single and double-layer MPAs and through finite element simulation for MPA arrays.