Broadband low-frequency underwater sound absorption of triangular lattice metamaterials with excellent load-bearing capacity

A multifunctional triangular lattice metamaterial for broadband low-frequency underwater sound absorption is proposed, fabricated by integrating Helmholtz resonators with a triangular lattice sandwich panel. This metamaterial achieves integrated performance of broadband low-frequency underwater sound absorption, high mechanical strength, and high specific energy absorption. A theoretical model is developed to predict the acoustic impedance of the absorber for both a single unit cell and parallel combinations of multiple cells, with predictions showing good agreement with numerical simulations. The results show that adding a rubber inner lining to the wall of the Helmholtz resonator significantly enhances underwater sound absorption—this improvement is most pronounced at the resonant frequency, where the vibration of water within the chamber causes the rubber inner lining to deform and generate viscoelastic energy dissipation. Moreover, the annular embedded neck (composed of embedded necks and solid columns) can simultaneously improve sound absorption performance and load-bearing capacity. Additionally, the metamaterial features tunable absorption properties, as its resonant frequency can be adjusted by optimizing the combination of rubber inner lining thickness, embedded neck diameter, and length. To further optimize structural parameters, a hybrid algorithm (FCNN-GA) integrating a fully connected neural network (FCNN) and genetic algorithm (GA) is developed, enabling the metamaterial to achieve sound absorption coefficients <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>α</mml:mi> <mml:mo>&gt;</mml:mo> <mml:mn>0.8</mml:mn> </mml:mrow> </mml:math> across the 282–1095 Hz range. This study innovatively combines column lattice structures with sandwich panels, realizing a composite with exceptional load-bearing capacity and superior low-frequency broadband sound absorption. The proposed FCNN-GA algorithm efficiently broadens the sound absorption bandwidth, and this work provides a column lattice-sandwich composite framework for designing high-load-bearing low-frequency underwater acoustic structures.