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

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 cylin-drical 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 ab-sorption 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 geo-metric 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.

Automated summary

In this study, a metagrating is constructed with a single layer viscoelastic medium, periodic cylindrical cavities, and a steel plate, and demonstrated as a tunable quasi-perfect absorber in low frequency. By utilizing effective medium approximation and genetic algorithm optimization, tunable absorbers with deep subwavelength thicknesses can be achieved. Broadband absorption underwater is further improved by using multi-layer metagratings and converting longitudinal waves efficiently into shear waves through local resonance coupling and multiple scattering effects. The adoption of a metagrating comprising multi-layer voids with random periods enables precise modulation of surface impedance in the target spectrum, leading to quasi-perfect absorption performance in broadband. These findings will contribute to the design of lightweight underwater absorbers to enhance the stealth performance of vehicles.