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Perfect, broadband and asymmetric sound absorption is theoretically,numerically and experimentally reported by using subwavelength thickness panelsin a transmission problem. The panels are composed of a periodic array ofvarying cross-section waveguides, each of them being loaded by an array ofHelmholtz resonators (HRs) with graded dimensions. The low cut-off frequency ofthe absorption band is fixed by the resonance frequency of the deepest HR, thatreduces drastically the transmission. The preceding HR is designed with aslightly higher resonance frequency with a geometry that allows the impedancematching to the surrounding medium. Therefore, reflection vanishes and thestructure is critically coupled. This results in perfect sound absorption at asingle frequency. We report perfect absorption at 300 Hz for a structure whosethickness is 40 times smaller than the wavelength. Moreover, this process isrepeated by adding HRs to the waveguide, each of them with a higher resonancefrequency than the preceding one. Using this frequency cascade effect, wereport quasi-perfect sound absorption over almost two frequency octaves rangingfrom 300 to 1000 Hz for a panel composed of 9 resonators with a total thicknessof 11 cm, i.e., 10 times smaller than the wavelength at 300 Hz.

Rainbow-trapping absorbers: Broadband, perfect and asymmetric sound absorption by subwavelength panels with ventilation