Validation of an Improved Experimental Method for Use in Impedance Eduction

Abstract

Results from impedance eduction methods developed by NASA Langley Research Center are used throughout the acoustic liner community. Occasional anomalies persist with these methods at frequencies where the liner produces minimal attenuation. An approach to educe impedance spectra with increased confidence is demonstrated, by combining results from successive tests with different cavity depths. First, a raylometer is used to measure the DC flow resistance of three wire-mesh facesheets. These facesheets are then mounted onto frames and a normal incidence tube is used to determine their respective acoustic impedance spectra. Next, each facesheet is successively mounted onto three frames with different cavity depths, and a grazing flow impedance tube is used to educe their respective acoustic impedance spectra with and without mean flow. Since the resonance frequency varies with cavity depth, each sample provides robust results over a different frequency range. Hence, a combination of results can be used to determine the facesheet acoustic resistance. When combined with the acoustic reactance (weakly dependent on the source sound pressure level and grazing flow Mach number), the acoustic impedance can be educed with increased confidence. Representative test results are discussed, and the complete database is available in electronic format upon request.