Quasiperfect absorption by subwavelength acoustic panels in transmission using accumulation of resonances due to slow sound

We theoretically and experimentally report subwavelength resonant panels for low-frequency quasiperfect sound absorption including transmission by using the accumulation of cavity resonances due to the slow sound phenomenon. The subwavelength panel is composed of periodic horizontal slits loaded by identical Helmholtz resonators (HRs). Due to the presence of the HRs, the propagation inside each slit is strongly dispersive, with near-zero phase velocity close to the resonance of the HRs. In this slow sound regime, the frequencies of the cavity modes inside the slit are down-shifted and the slit behaves as a subwavelength resonator. Moreover, due to strong dispersion, the cavity resonances accumulate at the limit of the band gap below the resonance frequency of the HRs. Near this accumulation frequency, simultaneously symmetric and antisymmetric quasicritical coupling can be achieved. In this way, using only monopolar resonators quasiperfect absorption can be obtained in a material including transmission.