Causality-inspired robust low-frequency broadband underwater sound absorption

The dynamic properties of viscoelastic damping material are unstable and unpredictable, restricting its applications in underwater sound absorption. This paper investigates a novel causality-inspired design strategy for viscoelastic damping material enhanced underwater absorbers. This design strategy leads to low-frequency broadband absorption spectrum robust to material and geometric parameter uncertain variation. The causality-inspired robust sound absorption for passive system is first demonstrated. Based on this, an optimization strategy in the face of viscoelastic dynamic parameter variation is put forward. The design strategy is further implemented to the viscous damping material enhanced Helmholtz resonator (VDME-HR) to formulate a robust underwater low-frequency sound absorption. A lumped element method based on linearization is applied to predict the power and sound absorption characteristics of the VDME-HR. Results show that the theoretical predictions agree well with the numerical simulations. The experiment verifies the robustness of the design with a mean absorption coefficient at 0.86 in the frequency range [500 Hz, 900 Hz], even when the viscoelastic dynamic parameter varies by four times. This innovative robust design strategy has great potential in the future of noise control.