Comparison of impedance eduction results using different methods and test rigs

Abstract

The last twenty years has seen a large development in inverse techniques for the determination of liner impedance under grazing flow conditions, so called impedance eduction techniques. This paper contribute to a continuing effort to gain confidence in results obtained using different test rigs as well as different impedance eduction techniques. The latter includes the use of different equations for solving for the sound field in the lined section. The DLR reference liner sample HR-S2 studied is a locally reacting single degree of freedom Helmholtz resonator liner which has previously been tested at DLR and NASA Langley. In this study it is tested in a smaller scale facility at KTH, but under similar mean flow and sound pressure level conditions as in the previous studies. A good agreement has been obtained for different method under the same plug flow assumption. The same trend but not identical effects of nonlinearity have been obtained with high levels of acoustic excitation. The effect of different flow Mach number assumptions are discussed in connection with the use of the Ingard-Myers boundary condition.