The impedance of perforated plates subjected to grazing gas flow and backed by porous media

Abstract Experimental data of the acoustic impedance of multiply perforated plates exposed to fully developed grazing gas flow in duct, both with and without a backing layer of porous material, are presented. Both circular perforations and “louvres” are included in the investigation. Empirical formulae are presented for the impedance of plates with no porous backing layer, in terms of dimensionless parameters, and an heuristic method of including the effects of the porous medium is described. It is observed that the porous backing layer has a large effect on the impedance of plates with circular perforations, and little or no effect on louvred perforates.