Broadband low-frequency sound absorbing metastructures composed of impedance matching coiled-up cavity and porous materials

In this work, we propose a sound absorbing metastructure based on impedance matching coiled-up cavity embedded with porous materials (IMCCP), which demonstrates tremendous broadband (e.g., 4.55 times compared to conventional designs) low-frequency absorption capability with deep subwavelength thickness (e.g., ∼λ/10.96 at 602 Hz), without considering multiple units coupling. By tuning the geometric parameters of the non-coupled unit, relative bandwidths ranging from 111.07 % to 142.55 % are achieved. In particular, two notable perfect absorption peaks are obtained in the low- to mid-frequency range by changing the number of coiled-up channels. Theoretical predictions, numerical simulations and experimental measurements are conducted to investigate the acoustic characteristics of IMCCPs, and the results agree well with each other. Physically, the bandwidth broadening stems from the impedance matching effect which increases the degree of smoothness of sound wave propagation inside coiled-up space with strong inherent dissipation. The bandwidth broadening effect is generally confirmed for diverse geometrical and transport parameters of IMCCPs. This work contributes to understanding the sound wave propagation inside porous coiled-up space and designing broadband porous metastructures.