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In sound reducing treatment for aircraft engine and IC engine applications it is common to use perforates. They can then be exposed to fluid flow and high level acoustic excitation which will change their acoustic properties. To study the nonlinear high level excitation effects separately it is possible to use plane wave impedance tubes where the perforate is mounted either at an open end or in the middle of the duct with microphones on both sides. The normalized transfer impedance defined as the acoustic pressure difference over the sample divided by the particle velocity through the perforate can then be determined. The transfer impedance can for the case that the sample is mounted at the end of the tube be estimated as the difference between the impedance at the sample cross section with or without the perforate in place. When the sample is sitting in the middle of the impedance tube the acoustic pressure and particle velocity can be measured at both sides of the sample which can then be used to estimate the transfer impedance. It is then assumed that the downstream (passive termination) does not change the measured nonlinear transfer impedance meaning that the nonlinear effects are occurring only locally at the perforate because of the flow separation effects caused by the high acoustic particle velocities in the holes. Some preliminary experimental results indicates that this may not be the case. The aim of the present study is therefore to clarify this using an experimental campaign backed up by numerical simulations.
Bodén et al. (Thu,) studied this question.