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Description
Low-frequency sound absorption in underwater environments remains challenging due to the large acoustic wavelengths and strong fluid–structure interaction effects. While Helmholtz-type resonators generally offer a promising route toward compact subwavelength solutions, achieving efficient dissipation mechanisms in water is non-trivial because of the suppressed role of visco-thermal losses relative to air. This often implies moving beyond ideal rigid-wall assumptions toward more realistic fluid-structure interactions.In this work, a comparative study of 16 water-filled resonator configurations is presented, examining the influence of the wall material across analytical, rigid, aluminum, and compliant-polymer models. Results show that analytical and rigid-wall models remain closely aligned (frequency deviations <1%) and achieve near-perfect absorption. Aluminum implementations preserve high absorption while introducing moderate and predictable resonance downshifts (≈5–13%). In contrast, compliant polymeric walls lead to strong detuning and reduced peak absorption, highlighting that structural compliance must be treated as an integral part of the resonant mechanism rather than as a perturbation.These findings provide practical guidelines for the modeling and design of compact underwater acoustic absorbers and metamaterials, and clarify the conditions under which rigid-wall approximations remain valid and when they must be replaced by compliance-aware approaches.