Microstructure topology optimization of underwater anechoic layers via moving morphable component (MMC) method

This paper presents an innovative MMC-based microstructure topology optimization framework designed to maximize the sound absorption performance of underwater anechoic layers. A semi-numerical solution, named the Homogenization and Transfer Matrix Method (HTMM), is introduced to calculate the microstructure's sound absorption coefficient efficiently. Three viscoelastic materials, polyurethane (PU), PU with nano ZnO (ZPU), and chloroprene rubber (CR), serve as matrix materials in the topology optimization process. Two examples, focusing on limited thickness, are explored within the low-frequency band (500-2000 Hz) and high-frequency band (2000-6000 Hz), respectively. Results indicate the effectiveness of the optimization formulation in generating microstructures with remarkable absorption performance. The corresponding optimized microstructures achieve average sound absorption coefficients of 0.82 and 0.9 at low and high frequencies, showcasing superior performance. These optimized microstructures are further explained through normalized specific impedance analysis, the effective longitudinal wave speed, and its loss factor. Furthermore, the simulation model is established to validate the results by HTMM and analyze the acoustic characteristics. The results between the HTMM and simulation show a great agreement. Moreover, we find that the displacements in both the X and Y directions occur within the optimized composites when subjected to a normal sound wave along the X direction, indicating waveform transformation. This transformation significantly enhances acoustic energy dissipation. Unlike conventional structures, where the materials involved in waveform transition do not contribute to damping, all matrix materials in these composites actively participate in dissipating acoustic energy. Beyond its promising outcomes, this introduced topology optimization framework also establishes a systematic approach for developing and refining advanced anechoic layers.