Acoustic properties of perforates - experimental data and empirical models

Sound reducing treatment in for instance aircraft engine and IC engine applications often involve the use of perforates. In these applications they are exposed to fluid flow and high-level acoustic excitation, and this influences the acoustic properties of the perforate, as is well known from many published papers. The acoustic properties are usually described using a transfer impedance. The transfer impedance of a perforate sample has been studied using both a conventional impedance tube (two-port) setup and an innovative three-port setup. The three-port configuration made it possible to study both the effect of grazing flow and high-level excitation effects separately as well as jointly. In the present paper these recent experimental results are presented and comparisons are made with results from previously published papers and empirical models.