Porogranular materials composed of elastic Helmholtz resonators for acoustic wave absorption

International audienceWe present a theoretical and experimental study of the acoustic absorption of granular porous media made of non-cohesive piles of spherical shells. These shells are either rigid or elastic, possibly drilled with a neck (Helmholtz resonators), and either porous or impervious. A description is given of acoustic propagation through these media using the effective medium models proposed by Johnson (rigid particles) and Boutin (rigid Helmholtz resonators), which we extend to the configurations studied in this work. A solution is given for the local equation of elasticity of a shell coupled to the viscous flow of air through the neck and the micropores. The models and our simulations are compared to absorption spectra measured in reflection in an impedance tube. The effective medium models and our measurements show excellent agreement for configurations made of rigid particles and rigid Helmholtz resonators that induce an additional peak of absorption at low frequency. A shift of the Helmholtz resonance toward low frequencies, due to the softness of the shells is revealed by our experiments for elastic shells made of soft elastomer and is well reproduced by our simulations. We show that microporous shells enhance and broaden acoustic absorption compared to stiff or elastic resonators