Ultrathin quasi-perfect absorber for low-frequency and broadband underwater sound

We proposed an ultrathin absorber that can efficiently absorb broadband and quasi-omnidirectional underwater sound. The absorber consists of a thin viscoelastic (VE) layer embedded with thin-plate scatterers (PSs) of equal width but with gradient variation in thickness. A dynamic model considering the full interaction of the PSs as a thin plate and the VE matrix as an elastic solid is used to predict the absorption coefficient of the absorber, which is validated by an effective medium model and the finite element model. The absorber exhibits perfect or quasi-perfect absorption (QPA) in multiple bands and broad incident angles at deep subwavelength thickness, due to resonant excitation of multiple local modes of different types in the absorber. By properly adjusting the absorbers' geometric and the material properties, broadband QPAs of over 99%, 95%, and 90% at bandwidths of 202.0 Hz, 348.3 Hz, and 572.9 Hz, respectively, are achieved via combined effects of multiple locally resonant (LR) modes. It thus offers an effective method for controlling low-frequency and broadband underwater sound.