Back-to-back comparison of impedance measurement techniques applied to the characterization of aero-engine nacelle acoustic liners

The aim of this paper is to assess three different measurement techniques applied to the characterization of the acoustic impedance of a Single-Degree-of-Freedom (SDOF) liner installed in nacelle ducts of turbofan engines. The “two-microphones” method, the “in-situ” impedance measurement technique and the “impedance eduction” approach are respectively compared in representative flight environment, characterized by normal and grazing incidence sound, with and without grazing flow. Goal of the study is to collect evidences of the unique and complementary features offered by these techniques, providing deeper insight into their strengths and limitations.The experimental results obtained with the three methods are in global agreement. For the “two microphones” method it is demonstrated the necessity to insert the liner samples into the impedance tube in order to perform accurate measurements at low frequencies on low porosity liners. For the “in-situ” technique it is highlighted the feasibility of implementation on aeronautical liners and the reasonable correlation achieved with the “impedance eduction” approach and with the impedance tube.