  We theoretically and experimentally report sub-wavelength resonant panels for
low-frequency quasi-perfect sound absorption including transmission by using
the accumulation of cavity resonances due to the slow sound phenomenon. The
sub-wavelength 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 bandgap below the resonance
frequency of the HRs. Near this accumulation frequency, simultaneously
symmetric and antisymmetric quasi-critical coupling can be achieved. In this
way, using only monopolar resonators quasi-perfect absorption can be obtained
in a material including transmission.
