Indirect measurement of liner impedance under grazing flow using the two-port eduction method

Acoustic wall treatments, such as liners, are commonly used to attenuate the noise propagating through duct systems. Such materials are typically characterized by their wall impedance, which depends on the sound pressure level and the flow conditions. Indirect measurement methods, relying on non-intrusive measurements and a model, are generally preferred to characterize this material parameter under representative conditions. The two-port impedance eduction method is one of these measurement techniques. It relies on acoustic pressure measurements to obtain a two-port model of a lined duct segment, which is then compared to a semi-analytical model for the acoustic transfer matrix to compute the liner impedance. Recently, an extension of this method has been proposed, which uses an iterative optimization method to reduce parameter uncertainties during both stages of the procedure. In this paper, this improved formulation of the two-port impedance eduction method is applied to a set of honeycomb liners under different grazing flow conditions to assess the accuracy, reliability and repeatability of the educed impedance.