Folded metaporous material for sub-wavelength and broadband perfect sound absorption

This Letter reports a folded metaporous surface optimized to achieve sub-wavelength and broadband perfect absorption. Its unit cell is composed of four different helicoidal cavities filled by porous media, which are structured and quasi-isotropic micro-lattices with a variable lattice constant. The effective thickness and intrinsic losses of each helicoidal cavity can be adjusted independently by varying their macro- and micro-structures, namely, the number of revolution of the folded structure and the lattice constant of the micro-lattice. An analytical model predicting the physical properties of this metaporous surface is developed. The macro- and micro-structures are then jointly optimized for sub-wavelength broadband perfect absorption. Finally, the system is 3D printed and experimentally tested. The experimental results are found to be in good agreement with the theory and show an almost perfect absorption over a frequency range out of reach for the homogeneous constitutive porous medium and the only helicoidal cavities.