Deep subwavelength hybrid metamaterial for low-frequency underwater sound absorption by quasi-Helmholtz resonance

We proposed an acoustic metamaterial with deep subwavelength thickness for low-frequency underwater sound absorption. The proposed hybrid metamaterial has a perforated facesheet, a fluid-filled square honeycomb core with inside rubber coating, and a fixed backsheet. A theoretical model is established to predict the sound absorption performance of this perforated honeycomb hybrid metamaterial based on the sound absorption theory of the micro-perforated panel and electro-acoustic analogy. The theoretical model agrees well with our finite element simulation. Results suggest that perfect sound absorption (99.9%) of the metamaterial occurs at 375 Hz, at which the thickness of the metamaterial is only 1/80 of the underwater sound wavelength. According to the simulation, most of the sound energy is consumed by the rubber coating. It can be analyzed that the rubber coating replaces the fluid in the square honeycomb resonant cavity improving the acoustic capacitance and acoustic resistance and triggering a quasi-Helmholtz resonance. This acoustic metamaterial also exhibits a broadband underwater sound absorption performance by parallel design with different perforations, which has a promising potential in engineering applications.