Impedance Eduction of Acoustic Liners Under Grazing Flow Using the Mode-Matching Method and the Two-Port Method

With the increase in aircraft transportation and, consequently, aircraft noise in the last decades, measurement of acoustic liner impedance under grazing flow has become a point of interest. Different indirect methodologies have been developed by independent research groups to solve this problem. The Mode-Matching technique and, more recently, the Two-Port method are examples of developed methodologies that use acoustic pressure measurements in a test rig where a liner sample is subject to grazing mean flow to educe its impedance. In this paper, both methods are explained, implemented and used to educe the acoustic impedance of different liner samples in a recently developed grazing flow impedance eduction test rig. Additionally, both methods are compared based on their computational cost and limitations.