A lightweight compact metastructure for simultaneous load-bearing, energy absorption and broadband low-frequency underwater sound attenuation

This study addresses the long-standing inherent conflict between low-frequency broadband underwater sound attenuation, structural load-bearing capacity, and lightweight design in conventional underwater acoustic structures, by proposing a lightweight compact multifunctional metastructure: a micro-perforated sandwich panel with a rubber-lined hierarchical honeycomb core (PRHH), which enables simultaneous mechanical load-bearing, energy absorption, and broadband low-frequency underwater sound attenuation. To resolve the physical contradiction between acoustic stealth and structural integrity, a design paradigm of acousto-mechanical functional decoupling is adopted within the integrated architecture. Systematic theoretical modeling and numerical simulation are performed to investigate the load-bearing, energy absorption, and broadband sound absorption performance of the proposed metastructure. An analytical model integrating the micro-perforated panel (MPP) theory and a complex viscoelastic model of rubber is established to predict the acoustic performance of the PRHH, while a theoretical model based on the super folding element theory is developed to characterize its load-bearing and energy absorption characteristics. Finite element (FE) simulations are conducted to validate the theoretical predictions, with excellent agreement achieved. The results demonstrate that the PRHH achieves perfect sound absorption ((Formula presented) ) at 339 Hz with a deep subwavelength thickness of only 1/70 of the operating wavelength. The introduction of the rubber lining provides additional acoustic resistance and reactance for optimal impedance matching, significantly enhancing the energy dissipation capacity and low-frequency sound absorption performance. In terms of mechanical properties, the PRHH exhibits a specific modulus and specific strength comparable to those of conventional hexagonal honeycombs, as well as significantly superior specific energy absorption enabled by its hierarchical topological design. Furthermore, a deep neural network (DNN) based inverse design framework is developed to optimize the PRHH, achieving quasi-perfect sound absorption ((Formula presented) )in the wide frequency range of 310-1109 Hz. A five-objective optimization is finally performed via the third-generation non-dominated sorting genetic algorithm (NSGA-III) to obtain a lightweight acousto-mechanical balanced configuration. By realizing acousto-mechanical functional decoupling in an integrated architecture, this work provides a novel design reference for developing lightweight, compact, high-performance multifunctional metastructures for demanding underwater vehicle applications.