Numerical design of a low-frequency broadband underwater sound-absorbing metastructure with a folded water channel and air cavity

Abstract

Achieving low-frequency underwater sound absorption within a subwavelength thickness remains challenging. This numerical study uses a coupled acoustic-solid finite-element model to investigate a folded rubber-lined water channel backed by a sealed air cavity under normal incidence and zero hydrostatic prestress. A one-dimensional transmission-line model is calibrated to the equivalent straight-channel FEM response for reduced-order interpretation, and a bounded RBF-surrogate-assisted search is used to identify a dual-unit geometry that is re-evaluated using the complete coupled FEM. The numerical fields support pressure-driven compliant-wall deformation and rubber viscoelastic dissipation, while the retrieved band-averaged complex velocity is consistent with reduced effective sound velocity. For a 50-mm thickness, the final configuration maintains α ≥ 0.8 from approximately 62 to 935 Hz, giving B 0.8 ≈ 873 Hz and H / λ L ≈ 1/484. Relative to the rounded Unit 1 baseline, the lower band edge is reduced by 72.6% and B 0.8 is increased by 16.4%. These predictions are conditional on the adopted frequency-independent rubber model and zero-hydrostatic-prestress assumption and remain to be validated experimentally.