This paper is a contribution to the Physical Review Applied collection titled Phononics and Metamaterials. Underwater noise control has become increasingly important recently due to its tremendous impact on marine ecosystems. The various proposed solutions have been hampered by their insufficient noise reduction level, low-frequency wideband inefficiency, or transportation and deployment inconvenience. In this work, we present an acoustic metamaterial targeted at ultrawideband low-frequency noise attenuation in underwater environments. The design employs air-entrained channels to establish waveguides with soft boundaries that suppress acoustic wave propagation below a designed cutoff frequency. This mechanism enables effective low-frequency sound insulation over a broad bandwidth with a deep subwavelength scale, and employs a lightweight and foldable structure. Underwater experiments confirmed our metamaterial prototype can achieve an average insertion loss of 29.108 dB and a maximum noise reduction of 60.362 dB across the predicted band gap from 1.5 to 15 kHz over more than three octaves. This design has demonstrated its great potential for large-scale applications, such as offshore wind-farm construction, bridge piling, and deep-sea oil operations.

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• Acoustic modeling
• Acoustic wave phenomena
• Hydrodynamic noise
• Noise control
• Underwater acoustics
• Acoustic metamaterials
• Acoustic techniques