Inverse and direct impedance eduction of liners in the MAINE Flow facility

The assessment of acoustic liners typically involves conducting experiments with airflow in a duct under various incident sound pressure levels and flow velocities. Impedance, as the most intrinsic parameter characterizing an acoustic wall (due to its independence from the duct section), is often the primary focus. Experimental techniques for its determination can be categorized into two classes: direct and inverse eduction methods. This paper contrasts these two methodological classes within the context of the MAINE Flow facility-a 150mm x 280mm rectangular duct capable of generating flow Mach numbers up to 0.63 and multimodal sound fields of Sound Pressure Levels up to 150 dB. Direct impedance determination necessitates a substantial degree of attenuation along the liner. However, in the current implementation, inverse impedance determination is constrained by the uniform flow hypothesis, which is invalid for large Mach numbers in such facilities. The findings illustrate that both methods can yield valuable impedance data, enabling the examination of the effects of flow and incident modal content on the measured impedance.