3D-printed straw-inspired metamaterial for sound absorption

An anisotropic acoustic metamaterial inspired by straw-stacks is reported for sound absorption wherefore the physics of porous media is enriched by tailoring inner resonances. The idealized periodic arrangement of straw-like hollow tubes is described in terms of the anisotropic Darcy permeability tensor and the effective compressibility accounting for both the thermo-elastic compressibility of the air-matrix and the apparent tube-resonance-induced compressibility. The main features of such porous medium with inner resonances are the possibility for the effective compressibility to become negative around the resonance and the drastic reduction of the effective sound speed (slow sound) at very low frequency. Impedance tube measurements on a 3-D printed metamaterial layer evidence multiple absorption peaks stemming from sub-wavelength Fabry-Perot interferences in the backed layer. The experimental data is in good agreement with the theoretical model. Applications may concern sound absorption upon reflection, sound insulation for transmissions, and orientation-selective absorption due to the anisotropy.