This paper presents the experimental results obtained with three different measurement techniques used for the characterization of the acoustic impedance of a typical 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 back-to-back compared in flight representative 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 shown the capability to characterize the liner damping and the amplification factor with increasing sound pressure level. For the “impedance eduction” approach it is highlighted the reasonable correlation obtained with both the “in-situ” technique and the “two-microphone” method.