Boundary Layer Noise Induced Through Thin Micro-Perforated Structures
Résumé
Predicting and reducing the aerodynamic noise is a major step in the design of automotive and aircraft cabins to meet the standard requirements on speech intelligibility and noise annoyance levels. In order to comply with fuel efficiency in surface and aeronautic transports, the trend is to use lighter and stiffer structures with a lower critical frequency, but which contribute to increase the transmission of several noise sources towards the cabin such as the noise induced by Turbulent Boundary Layer (TBL) wall-pressures through the wall partitions. Passive conventional reduction of the TBL-induced noise at low and mid-frequencies requires thick and cumbersome porous materials, hardly compatible with the design of lightweight and small acoustic treatments. A passive solution of interest would be to micro-perforate the existing multi-layer partitions in orderto create a surface distribution of resonators able to dissipate over a broad frequency range the acoustical energy induced by the TBL with out added mass, nor fibrous materials. This paper presents physical investigations into the sound absorption and transmission properties of thin Micro-Perforated Panels (MPP) backed by an air cavity and a thin plate and excited by a TBL. A fully-coupled modal formulation is proposed to calculate the TBL-induced vibro-acoustic response, the transmission loss and the sound power dissipated by a finite-sized MPP-Cavity-Panel (MPPCP) partition. The model accounts for the effects of the sub-structures volumetric resonances on the acoustical properties of the partitions. The acoustical performances of the MPPCP partition are compared when the MPP undergoes either aerodynamic or acoustic excitations. Theeffects are analysed in terms of spatial matching between the boundary layer convective scales and the coupled vibro-acoustic modes of thepartition.