Low-frequency broadband underwater metasurface with embedded curved rough neck

The rapid progress in underwater equipment has created a focus on underwater low-frequency noise reduction technology. Highly efficient sound absorption at low frequencies below 500[Formula: see text]Hz is difficult to achieve with conventional underwater acoustic materials. Based on this, a Helmholtz tunable underwater sound-absorbing metasurface with an embedded curved rough neck (HMCRN) is proposed. The metasurface introduces a rubber coating and a curved rough metal neck in the metal hexagonal Helmholtz cavity. Compared to traditional Helmholtz resonators, the peak frequency of the metasurface is reduced to 151[Formula: see text]Hz at a thickness of 50[Formula: see text]mm. The external geometric parameters of the metasurface unit are unchanged (wall thickness and side length, etc.), and the internal geometric parameters (rubber coating thickness and neck roughness, etc.) are adjusted to achieve the frequency tunability of the metasurface. To meet the broadband low-frequency noise reduction requirements in underwater equipment, this paper designs four imperfect acoustic absorption metasurface units at discrete frequencies using complex frequency plane analysis. The four units are arranged in parallel to form a composite acoustic metasurface. The 50[Formula: see text]mm-thick composite acoustic metasurface realizes efficient sound absorption ([Formula: see text]) with a bandwidth of 298[Formula: see text]Hz in the range 98–396[Formula: see text]Hz due to the weak coupling between units. The sound-absorbing metasurface with the embedded curved rough neck is not only great theoretical guidance but also engineering application in underwater low-frequency noise control.