Multimodal characterisation of acoustic liners using the MAINE Flow facility

MAINE Flow is a large-scale duct facility that allows investigating the acoustic properties of liners with flow and acoustic conditions close to the ones typically found in nacelles of aircraft engines: the incident acoustic level can go up to 150 dB and the flow velocity up to Mach 0.63. Compared to other large-scale duct experiments, this facility permits a precise control of the modal content and amplitude, as well as the measurement of the scattering matrix of the liner. In this paper, we compare three methods that allow calculating the insertion losses of a test liner by measuring the transmission losses with and without liner. Two are direct methods that use noise level measurements upstream and downstream the sample: one with an uncorrelated broadband excitation and one with a modal sine-sweep excitation. In the broadband case, robust measurements up to 10 kHz are obtained but the modal content is not controlled. The third method uses the coefficients of the liner scattering matrix to compute transmission losses. More physical insights are provided by this method which works on a more limited range of frequency.