Comparative analysis of two liner acoustic impedance characterization techniques

This study presents the comparison of two methodologies of small size liner acoustic impedance characterization. The first one is based on a normal incidence broadband acoustic excitation up to 160 dB inside a 30-mm diameter circular tube, and uses the two microphones method, with microphones flush-mounted inside the waveguide. This bench, called NTMM and developed at Airbus, allows to directly compute the acoustic impedance at the surface of the sample. The second one lies on a measurement inside the B2A wind tunnel at ONERA, where the sample is mounted as one of the vein walls, leading to a grazing aero-acoustic excitation, up to Mach 0.3 and 155 dB with a broadband excitation. The impedance is then educed by an inverse method. During the MAMBO research project, dedicated samples have been designed and tested with the two benches as well as on the CANNELLE multimodal test rig developed by Airbus. The full paper will analyze the results and offer a cross comparison of the methodologies, highlighting the advantages and limitations of each approach.