Broadband low-frequency sound absorbing metastructures based on impedance matching coiled-up cavity

We report a kind of sound absorbing metastructure based on micro-perforated panel (MPP) and impedance matching coiled-up cavity (IMCC), which can broaden the low-frequency working bandwidth tremendously using single unit (e.g., 3.13 times compared to conventional designs) within a deep sub-wavelength thickness (e.g., ∼λ/20 at 322 Hz). Physically, impedance matching stemming from the dramatic cross-sectional area contrast of coiled-up channels reduces the acoustic reactance variation with respect to frequency, leading to broader low-frequency bandwidth. Theoretical predictions of sound absorption coefficients of the metastructure coincide well with the numerical and experimental results. The band broadening characteristics of the proposed IMCC are confirmed in relatively wide range of MPP parameters and partition panel positions. This work contributes to expanding the designing degrees of freedom for coiled-up metastructures to manipulate low-frequency waves in broadband.