Absorption and transmission of boundary layer noise through flexible multi-layer micro-perforated structures

This paper describes analytical and experimental studies conducted to understand both the absorption and transmission performance of non-fibrous micro-perforated partitions excited by a turbulent boundary layer (TBL). This is of relevance in surface and air transport systems for the airframe design of external liners. A fully-coupled modal formulation is established that predicts the absorption coefficient and the transmission loss (TL) of finite-sized and infinite-sized partitions made up of a micro-perforated panel (MPP) backed by an air cavity and a thin plate. The front MPP undergoes random wall-pressure fluctuations with prescribed auto- and cross- correlation properties such as a TBL and/or an acoustic diffuse field (ADF). An experimental methodology is proposed to evaluate in a closed wind tunnel both the absorption and insulation performance of a TBL-excited MPP partition. This leads to an approximation of the frequency-averaged power flow injected by the TBL into the MPP partition. Both the predictions and the measurements show significant differences in the absorption and TL curves when considering either a low speed TBL or an ADF excitation. These properties are examined considering a weighted contribution of the acoustic and turbulent components, as it is the case for an aeroacoustic excitation. A hole-based transitional Strouhal number is found below which low-back scattering of the wall-pressures occurs and above which the MPP apertures efficiently convert turbulence into back-scattered sound whatever the magnitude of the acoustic component. As for the TL, a minute increase of the acoustic component generates most of the TL decrease. Finally, the effects of adding a second MPP within the cavity are assessed for both the absorption and the TL.