Comparison of Broadband Mode Decomposition Techniques for Acoustic Liner Assessment in Annular Ducts

Acoustic liners are an effective means to reduce aircraft engine noise. The assessment of acoustic liners in a realistic setting, i.e., in an annular duct with a complex sound field composed of single or multiple higher-order modes, remains challenging. In this study, two different liners are tested in an annular duct under broadband excitation without flow. The sound fields on the reflection and transmission sides were measured using two arrays of approximately 100 microphones each. Various evaluation methods for broadband acoustic excitation are compared with each other and for certain cases with respective single mode excitation and are discussed based on two different liner types: a conventional Helmholtz resonator liner with a small damping bandwidth and a foam-resin liner with broadband damping.