Deep-subwavelength broadband underwater sound absorption: Modeling, optimization, and mechanism elucidation

Low-frequency underwater noise absorption remains challenging because the large characteristic acoustic impedance and long wavelength in water make conventional porous/fibrous treatments impractical, while thermoviscous-loss-based resonant absorbers often suffer from weak intrinsic dissipation and narrowband performance. This work proposes a deep-subwavelength underwater absorber that combines two coupled folded-cavity extended-neck Helmholtz-resonator layers via a thin rubber damping interlayer, enabling either frequency-targeted strong absorption or bandwidth-enhanced performance within the same compact footprint. A unified lumped-impedance network model is developed for efficient design and optimization, using the Johnson–Champoux–Allard formulation as the representative thermoviscous neck model and retaining short-tube and thermoviscous-duct formulations as consistency checks. COMSOL-based single-neck thermoviscous benchmarks show that these formulations yield practically indistinguishable complex neck-impedance predictions in water over 20–1000 Hz for the investigated geometries. Guided by this validated model, SQP optimization produces representative frequency-targeted and broadband designs. The optimized absorbers achieve strong deep-subwavelength absorption in the 250–450 Hz range with total thicknesses of only 32–42 mm, while a broadband design with H = 52 mm maintains SAC > 0.5 over a several-hundred-Hz band within 619–968 Hz. In addition, the rubber interlayer provides additional resonance retuning toward lower frequencies and a pronounced bandwidth gain for coupled or multi-resonant configurations, offering a simple and manufacturable means of tailoring low-frequency and broadband underwater absorption.