Extreme low-frequency ultrathin acoustic absorbing metasurface

We introduce a multicoiled acoustic metasurface providing quasiperfect absorption (reaching 99.99% in experiments) at an extremely low-frequency of 50 Hz, simultaneously featuring an ultrathin thickness down to λ/527 (1.3 cm). In contrast to the state of the art, this original conceived multicoiled metasurface offers additional degrees of freedom capable of tuning the acoustic impedance effectively without increasing the total thickness. We provide analytical derivation, numerical simulation, and experimental demonstrations for this unique absorber concept, and discuss its particular physical mechanism. Furthermore, based on the same conceptual approach, we propose a broadband low-frequency metasurface absorber by coupling unit cells exhibiting different properties.