Broadband acoustic absorbers based on double split-ring resonators at deep subwavelength scale

We report both experimentally and numerically that a low-frequency acoustic absorber is realized by double split-ring resonators backed with a rigid wall. This absorber leverages the impedance matching and dissipation effect, which arises due to the thermal-viscous loss within the dual channels. As a result, this absorber achieves near-perfect sound absorption (the absorption coefficient alpha = 0.99) at a subwavelength thickness of around lambda/23. By assembling six unit cells with distinct structure parameters to form a supercell, the fractional bandwidth (the ratio of the bandwidth to the center frequency) is increased to 40% with an average alpha of 0.86. Acoustic experiment results validate this exceptional performance, which is also in agreement with the simulation results. Moreover, by employing the supercell, we create an anechoic room demonstrating broadband sound absorption in a wide range of incident angles while occupying significantly less space than traditional sound-absorbing porous materials. Our double split-ring composite design paves the way for broadband acoustic absorbers at the deep subwavelength scale