Influence of structural resonances on the causal-based optimisation of micro-perforated absorbers

Abstract

Fire-proofness, cleanability and lightweight constraints make micro-perforated panels an alternative to classical porous materials in demanding environments. However, the selection of their physical parameters constitutes a combinatorial optimization problem that involves the maximization of the absorption coefficient within a frequency band, imposing restrictions on the total thickness of the partition. In this work, we propose an alternative formulation based on the principle of causality. The optimization of micro-perforated absorbers (MPAs) by this criterion enables to identify the MPA constitutive parameters that achieve both critically coupled (CC) resonant states and the smallest thickness-to-bandwidth ratio. It includes the specific resonant state that maximizes the total absorption for a given overall thickness of the absorber. However, the optimization of lightweight absorbers requires that the parameter space involves sub-millimetric panel thicknesses. In this case, the effect of elasticity and finite-sized structural resonances are hardly negligible on the absorption performance. In this study, the causality criterion developed for rigid MPAs has been extended to account for the effect of the panel vibrations on the perforations impedance. Of interest is to examine how this influences the thickness-to-bandwidth performance of the MPA and the related CC resonances, especially when they strongly couple with the cavities Helmholtz-type resonances.