Enhance underwater sound absorption through artificial acoustic soft boundary and intelligent optimization

Traditional underwater absorption materials operate at frequencies of several thousand Hz when their thickness is low, resulting in a technical contradiction between low-frequency sound absorption and low thickness requirements. Because of the limited dissipative capacity of cavity metamaterials, even if the absorbing frequency band can be reduced to the required range, it is challenging to improve the magnitude of sound absorption. To this end, we propose a design method that can reduce the thickness of sound-absorbing structures and effectively improve the absorption magnitude. Using the space folding design, the large size in the thickness direction required for low-frequency absorption is transferred to an extremely thin plane layer, which solves the problem of the large thickness of sound-absorbing structures. Then, by arranging soft materials like rubber on the hard wall inside the cavity of metamaterials, an underwater acoustic version of an artificial acoustic soft boundary is formed to solve the problem of low sound absorption magnitude. The addition of the artificial acoustic soft boundary enhances the ability of thin-layer structure to dissipate acoustic energy, considerably reduces the sound velocity and reflection, and produces large elastic strain energy at the interface between two different media, which makes the structures realize effective low-frequency broadband sound absorption. The experimental results show that the average sound absorption coefficient of the structure increases from 0.07 to 0.6 in the range of 1–1000 Hz and from 0.38 to 0.76 in the range of 1–4000 Hz after the introduction of soft boundary. To further improve the magnitude of sound absorption, we use metaheuristic algorithm to realize the intelligent optimization design of sound-absorbing structure. The average sound absorption coefficient is 0.96 at 60-1000 Hz with 24 mm thickness and 0.98 at 1000–4000 Hz with 20 mm thickness.