Underwater metagratings for sub-kilohertz low frequency and broadband sound absorption.

• Optimal anechoic coating based on metagratings. • Meta-absorbers in low-frequency and broadband via subwavelength thicknesses. • Enhanced multiple scattering effects in tandem metagratings. • The theoretical model modulates local resonance coupling of multi-layer meta-units. The anechoic coating capable of absorbing sound energy in low frequency and broadband is essential to conceal underwater vehicles. In this work, metagrating has been demonstrated as a tunable quasi-perfect absorber in low frequency, which is constructed with the single layer viscoelastic medium embedded with the periodic cylindrical cavities and the steel plate. Upon effective medium approximation and genetic algorithm optimization, such tunable absorbers could be put forward via deep subwavelength thicknesses. Broadband underwater absorption would be further approached in the multi-layer metagratings while the longitudinal waves convert into shear waves efficiently via the local resonance coupling and multiple scattering effects. Distinct from previous studies that limit the configuration of anechoic coatings to a given law and inefficiently alter numerous geometric parameters for optimal acoustic performance, the metagrating comprising multi-layer voids with random periods is adopted to precisely modulate the surface impedance in the target spectrum, leading to quasi-perfect absorption performance in broadband. Our results will contribute to designing lightweight underwater absorbers to improve the stealth performance of vehicles. [Display omitted] [ABSTRACT FROM AUTHOR]