Experimental evidences of perfect acoustic absorption in deep sub-wavelength structures in ventilation problems
Résumé
In this work, we report a mechanism providing symmetric perfect acoustic absorption by sub-wavelength structures in the ventilation problem. While the mechanism consists in critically coupling a single resonance independently of its nature in the problem of reflection, the problem in transmission becomes more complicated and a degenerate resonator with symmetric and antisymmetric resonances should be designed. Both resonances should thus be critically coupled if symmetric perfect absorption has to be obtained. The system analyzed in this work is made of a panel with a periodic distribution of slits, the upper wall of which is loaded by Helmholtz Resonators. In this work, the unit cell consists of two Helmholtz resonators loaded slits. The first one achieves the symmetric problem and the other one the antisymmetric problem. By controlling the geometry of the resonators, the viscothermal losses can be tuned to exactly compensate the leakage of the system and fulfill the critical coupling condition for both the symmetric and the antisymmetric components, thus activating the symmetric perfect absorption of sound in the ventilation problem.