Beam-type structure for low-frequency and broadband underwater sound absorption

Low-frequency and broadband sound absorption have been a longstanding challenge for acoustic stealth of underwater vehicles, as the intrinsic energy dissipation of materials is weak. In this paper, we present a beam-type structure that can achieve impedance matching and generate both low-frequency and high-frequency sound absorption peaks efficiently, owing to its capability of simultaneously producing transverse and longitudinal vibrations. Theoretical analysis and numerical simulation results show that the first-order derivative of the beam element's displacement is discontinuous, and the elastic strain energy intensity is sufficiently high at the location of sound absorption peaks. The choice of material and its geometrical parameters also have crucial influences on tuning the amplitudes and frequencies of the sound absorption peaks. By integrating five beam units in a single structure with different vibration eigenfrequencies, we achieve deep-subwavelength (λ/50) broadband sound absorption from 500 Hz to 10 kHz. The present work provides an appealing strategy to improving acoustic energy density and designing underwater structures for low-frequency and broadband noise reduction.