TiCT-Based Cross-Scale Laminated Structural Structures: Enabling Sub-Wavelength Impedance Modulation and Underwater Broadband Sound Absorption.

Designing underwater acoustic absorbing materials with sub-wavelength thickness in the mid-to-low frequency range (400-4000 Hz) remains a challenge, especially for broad frequency applications. Most current designs focus on a single spatial scale, limiting their frequency range. To address this, a composite material (MPSFn-SBR) made of TiCT-polyvinyl alcohol self-assembled films(MPSFn) and styrene-butadiene rubber (SBR) is designed, featuring a cross-scale laminated structure. On a macroscopic scale, the MPSFn-SBR has a sandwich structure, with the SBR layer serving as a protective layer and the MPSFn core modulating impedance. On a mesoscopic scale, the MPSFn layers transform longitudinal waves into shear waves, improving sound absorptionat mid-to-low frequency range. On a microscopic scale, the MPSFn's two-phase coexistence system with a defect phase enriches vibration modes, broadening the absorption bandwidth. Results demonstrate that the cross-scale laminated structure enables effective sound absorption at sub-wavelength thickness (10 mm, ≈1/375 of the wavelength). A broad peak is observed from 1200 to 4000 Hz, with an average absorption coefficient of 0.91. The maximum absorption in the 400-1200 Hz range is 0.7. This study expands the design perspective of underwater sound-absorbing materials, transitioning from a single spatial scale to a comprehensive strategy, fostering innovation and development in the field.