Absorption of anisotropic fibrous materials partitions shielded by thin micro-perforated panels

Recent trends in broadband noise reduction have considered the design of bio-inspired multi-layer sound absorbers. In particular, the silent flight of certain species of night bird predators can offer an inspiration for the design of acoustic partitions for the control of the airframe noise in terrestrial and aeronautic transport systems. The layout usually comprises a suitable combination of thin micro-perforates, fibrous materials and airspaces with a view to mimic the remarkably low acoustic emissions of the owl flight. One of the main features is the anisotropic texture of the highly porous material beneath the micro-perforate. This study examines how the absorption properties of rigidly-backed micro-perforated panels are modified when lined with anisotropic fibrous materials with specified inclination of the parallel fibers within the material thickness. A fully anisotropic model has been proposed that accounts for anisotropy and frequency dependence in complex densities and bulk modulus. This model has been validated against a set of measurements carried out with an in-situ pressure-velocity probe and a comparison has been performed with several analytical predictions. It has been shown that the best agreement for the prediction of both the absorption maxima and the Helmholtz resonance is the proposed fully anisotropic model.