An inverse analytical method for extracting liner impedance from pressure measurements

Abstract

The in-situ method has been used for a long time as an easy, quick and direct method for measuring the acoustic impedance of locally reacting liners with and without flow. Several drawbacks and problems have been reported with the use of this method, which motivated the development of indirect methods to deduce the impedance from the measurement of the pressure field. A new inverse analytical technique is presented in this paper. The liner sample is placed inside a rectangular duct. The amplitude of the plane wave incident towards the lined section is measured using the two-microphone technique. The reflection coefficient at the exit plane is also measured using the same technique. These measured values are fed to an analytical model for sound propagation through the lined section, which is constructed using mode-matching. A minimization scheme is used to find the liner impedance value in the complex plane to match the calculated sound field to the measured one at the microphone positions already used for the two microphone measurements. The results show that the proposed technique can educe or measure the impedance to an acceptable accuracy. Copyright © 2004 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.