Harnessing normal-shear coupling in metabarriers for deep sub-wavelength underwater noise control

Anthropogenic underwater noise poses a significant threat to marine ecosystems, disrupting key biological functions. Common mitigation strategies include enclosing noise sources within acoustic barriers. Current designs include locally resonant absorbers, which offer narrow-band performance, and reflective systems with limited effectiveness at low frequencies. In this work, we propose an approach to design thin anisotropic metamaterial-based acoustic barriers for broadband underwater noise attenuation at deep sub-wavelength scales using topology optimization to maximize the coupling between normal stresses and shear strains. Unlike conventional methods, the proposed optimization is formulated in the static regime, relying solely on the homogenized elastic properties of the structured material and not on the characteristics of the surrounding fluid. The resulting metabarriers achieve a high sound transmission loss (STL, 100 dB peak) above 2 kHz, while maintaining a thickness-to-wavelength ratio as low as 1/70 below 1 kHz and STL of approximately 20-30 dB. The influence of hydrostatic pressure on performance is also evaluated, and structural modifications for practical deployment are proposed. The results demonstrate the potential of anisotropy-driven metamaterials as compact and efficient solutions for the control of underwater noise, offering a promising avenue for future acoustic insulation technologies.