Experimental evaluation of impedance of perforates with grazing flow

Abstract

This paper is concerned with measurement of grazing flow impedance of a perforated plate. The measurements have been made for various grazing flow velocities with an impedance tube facility, at different frequencies. The effects of the thickness of the plate and the diameter of the holes on the impedance of the perforate have been included. It is found from the results that resistance increases with the grazing flow velocity, but is more or less independent of thickness and hole diameter, whereas reactance increases with thickness and diameter of the hole. Finally an empirical formula for computing the normal incidence impedance of an orifice or cluster of orifices (perforated plate) is given, and verified for use in prediction of noise reduction across concentric tube resonators.