Dual-Mode Elastic-Wall Underwater Acoustic Absorber Enabled by Metal Diaphragm for Low-Frequency Broadband Performance

Abstract To counteract sonar detection threats, the application of acoustic coatings on the surface of submersibles has become a primary technique for enhancing their security. Such coatings effectively absorb sound waves and improve stealth, thereby ensuring the safety of underwater operations. This paper presents a channel-type underwater sound absorber incorporating metal spacers as vibration isolation elements, featuring an elastic inner wall segmented by these spacers. Simulation results demonstrate that the introduction of metal spacers enhances the independent action of dual absorption peaks, significantly improving dual-peak sound absorption performance. Energy consumption distribution analysis further reveals that the lower segment of the elastic material primarily contributes to the first absorption peak, while the upper segment mainly functions toward the second absorption peak. Through modal control and parameter matching of the dual-segment elastic wall, the designed composite structure achieves sound absorption coefficients above 0.6 within the frequency range of 630-4000 Hz. Measured results from the fabricated sample show strong agreement with simulation predictions. The acoustic absorption performance of the samples was tested under atmospheric pressure and four hydrostatic pressure points (0.5 MPa, 1 MPa, 2 MPa, 3 MPa). During the gradual increase of hydrostatic pressure, the parametric gradient balance was disrupted, resulting in the gradual separation of a single peak toward the lower frequency range with near-unity absorption coefficient. Within the frequency range maintaining broadband absorption, the absorption coefficient remained extensively above 0.8.