Ultra-broadband underwater meta-absorber with gradient impedance-matched composite material

Abstract We have developed an underwater acoustic meta-absorber that exhibits ultra-broadband absorption, with most of absorption coefficients exceeding 0.9 across the frequency range of 1 kHz−20 kHz. This performance is achieved through the localized trapping of acoustic waves and efficient energy dissipation, facilitated by local resonances within meticulously designed, impedance-matched composite materials. The designed gradient structure of the absorber allows for a reduced width of 84 mm (0.05 λ at 1 kHz) and a customizable length, determined by the number of unit cells, each with a length of 15 mm (0.01 λ at 1 kHz). This different design approach yields an ultra-broadband meta-absorber of compact dimensions, offering a promising alternative for the development of underwater absorption metamaterials and their practical applications.