Underwater acoustic absorbing metamaterials by material-structure-functionality collaborative optimization

Underwater acoustic metamaterials have provided new ideas for underwater acoustic stealth of various underwater vehicles. However, the existing sound absorption performances are still limited by the intuitive design or geometry-only optimization. To systematically construct the low-frequency broadband underwater absorbing metamaterials under rigorous constraints of ultrathin thickness and specific material parameter ranges, this paper proposes a material-structure-functionality collaborative optimization method that synthesizes the variational material parameters and topological configurations as the whole design variables for underwater sound absorption in the prescribed frequency range. Diverse inverse-designed metamaterials are shown to support low-frequency, broadband, and high-efficiency (>90 %) sound absorption with thin thickness. All low-frequency broadband absorption characteristics are revealed to be dominated by the resonance superposition along with a certain amount of wave mode conversion. In particular, a representative metamaterial can achieve an average sound absorption of 90 % within the range of 300∼10,000 Hz. Another ultrathin metamaterial with a thickness of 1/200 wavelength obtains the effective low-frequency absorption (>50 %) at 300 Hz. Finally, the underwater sound tube tests demonstrate the customized low-frequency broadband sound-absorbing functionality. The proposed inverse-design methodology and underwater acoustic metamaterials may offer guidance for underwater acoustic stealth technology, acoustic communication, and detection technology, etc.