Elasticity and modal effects on the optimal performance of micro-perforated multi-layered absorbers

Multi-layered wall-treatments with thin lightweight micro-perforated panels offer a potential solution to achieve broadband noise absorption while complying with mass saving and compactness. This study examines how vibrational effects, often neglected, may impede the wideband acoustical performance of such partitions, made up of a distribution of either periodic or graded micro-perforated membranes or panels. The impedance translation method and a scattering matrix analysis have been implemented. It is found that the elasticity behavior of thin micro-perforated membranes has a beneficial effect on the acoustical efficiency of the partitions. The contributions of the individual acoustical resonances tend to merge if the elasticity effects are accounted for. If the partition is optimized, near-constant high absorption and low transmission values are obtained over a wide frequency range. A more robust design involves partitions made up of micro-perforated panels. It was observed that the first volumetric volumetric mode of the panels modifies the absorption properties of the partition. It cross-couples with the acoustical resonances and redistributes their spectral locations. This structural resonance sets an upper frequency bound, below which the partition achieves a high broadband performance. It should be accounted for when setting the frequency limits of the total dissipation to be optimized.