Absorption and Transmission through Functionally-Graded Micro Perforated Panels

To enhance absorption of noise without the introduction of active or massive components in the low frequency range, layouts of panels with micro-slits or micro-perforations have been considered as a non-fibrous alternative to conventional silencers and wall absorbents. These treatments are resonance absorbers composed of Micro-Perforated Panels (MPPs) with sub-millimetric apertures backed by air cavities with an absorption peak centred on the frequency of interest. For multilayer partitions, the parameters of each individual MPP can be tuned to broaden the global absorption performance. In this work we will further investigate the optimisation of multilayer MPP partitions considering not only the absorption but also the transmission performance of the control device. A theoretical model based on modal matching formulation and Fourier-Floquet expansion of the diffracted orders has been used for the optimisation of the graded structure constitutive parameters that maximises the acoustic dissipation of the system. It is found to be valid over a wider range of holes radii and separation distance with respect to effective impedance models, assuming MPPs with a constant holes pitch. A comparison against optimal impedance relationships from coherent perfect absorption criterion has also been carried out.