On the practical use of a three-microphone technique for in-situ acoustic impedance measurements on double layer flow duct liners

A three-microphone technique is presented for in-situ acoustic impedance measurements on double layer liners of the Helmholtz resonator type. It can be used under conditions of flow and arbitrary noise sources. The emphasis of this paper is the practical application of this technique, including microphone installation and the implications of flow noise on data analysis. Explicit relations are given for the impedance and its accuracy as deduced from the microphone signals. The required coherence and number of samples to be taken from the signals appear to be rather high for aero engine applications.