Methodologies for duct liner impedance eduction

Abstract

Methodologies and techniques are reviewed that are currently employed at Langley Research Center to educe (from primitive measurements) the impedance of acoustically absorbing liner structures. These structures are of interest for suppressing noise emission from aircraft engine nacelles. The accuracy and precision of the primitive measurements and their impact on the educed impedance of liners when exposed to high speed grazing flows is of special interest for aircraft engine nacelle applications. The test setups range from the classical standing wave tube for which the primitive measurement is a complex transfer function between two judiciously chosen locations, to an elaborate grazing flow duct arrangement (the Langley Grazing Incidence Tube) for which the primitive measurements (acoustic pressure and phase) are compromised by increased flow noise contamination due to high speed grazing flows up to a Mach number of 0.5. Results of different techniques/methodologies are compared on the basis of how the primitive measurements are processed and mapped into impedance spectra for different test setups. The results are compared on the basis of bias and precision errors that are specific to the impedance eduction methodology employed.