THE IMPEDANCE OF PERFORATED PLATES SUBJECTED TO GRAZING GAS-FLOW AND BACKED BY POROUS-MEDIA

Experimental data of the acoustic impedance of multiply perforated pla tes exposed to fully developed grazing gas flow in a duct, both with a nd without a backing layer of porous material, are presented. Both cir cular perforations and ''louvres'' are included in the investigation. Empirical formulae are presented for the impedance of plates with no p orous backing layer, in terms of dimensionless parameters, and an heur istic method of including the effects of the porous medium is describe d. It is observed that the porous backing layer has a large effect on the impedance of plates with circular perforations, and little or no e ffect on louvred perforates. (C) 1998 Academic Press.