Design of sub-wavelength acoustic absorbing panels using accumulation of resonancesdue to slow sound
Résumé
We report sub-wavelength resonant panels for low-frequency sound absorptionby using the accumulation of cavity resonances due to the slow sound phenomenon. The pan-els are built by bricks of sub-wavelength thickness with periodically distributed horizontal slits,in the walls of which identical Helmholtz resonators are attached (HRs). Due to the presenceof the HRs, the propagation in the slit is highly dispersive with low group velocity that goes tozero due to the gap produced by the resonance of the HRs. In the slow-sound regime, the res-onant frequency of cavity modes in the slit are shifted down and they are accumulated beforethe HRs frequency. Near the accumulation point, simultaneously symmetric and antisymmetricmodes can be nearly critically coupled and produce nearly perfect absorption. However, due tothe distribution of the resonant modes perfect absorption is not possible in this kind of struc-tures. Therefore, we further analyze the case of different HRs in the slit, in which the structureof resonances is modified and symmetric and antisymmetric resonances can be simultaneouslycritical coupled. In this conditions we report perfect absorption for a transparent panel of 1/26wavelength thickness.