Achieving low-frequency and broadband underwater sound absorption (SA) is technically challenging and practically important. To tackle the problem, a new acoustic meta-structure is proposed in this paper. The proposed meta-structure is composed of a fluid-loaded thin inhomogeneous elastomer layer coated over an elastic plate. Each unit cell of the meta-structure contains a cluster of elastic plate scatters of different thicknesses, embedded into a homogenous elastomer matrix. This original design enables a large number of local resonance modes inside the unit cell, which by the same token enriches the coupling among them when subject to incident sound waves. By design, optimization, fabrication and experiments, we demonstrate that the proposed meta-structure with a deep subwavelength thickness, can entail rich local and coupled resonance responses inside the structure and in turn produce exceptional and even quasi-perfect SA at multiple frequencies and within a broad low-frequency range. This research provides a novel solution to tackle the generic broadband low-frequency underwater SA problems using ultra-thin sub-wavelength structures, which can trigger inspirations for designing other low-frequency sound control devices.

underwater sound absorption; low-frequency; broadband; meta-absorber; plate scatterer; multiple scales; subwavelength; quasi-perfect absorption; experiment validation