Compact broadband acoustic sink with coherently coupled weak resonances

Broadband sound sink/absorber via a structure with deep sub-wavelength thickness is of great and continuing interest in physics and engineering communities. An intuitive technique extensively used is to combine components (resonators) with quasi-perfect absorption to piece together a broad absorbing band, but the requirement of quasi-perfect absorption substantially places a very strict restriction on the impedance and thickness of the components. Here, we theoretically and experimentally demonstrate that a compact broadband acoustic sink that quasi-perfectly absorbs broadband arriving sound waves can be achieved with coherently coupled "weak resonances" (resonant sound absorbing systems with low absorption peaks). Although each component exhibits rather low absorption peak alone, via manipulating the coherent coupling effect among the components, they collectively provide a remarkably improved performance over a wide frequency range with a significantly compressed thickness. To illustrate the design principle, a hybrid metasurface utilizing the coaction of parallel and cascade couplings is presented, which possesses an average absorption coefficient of 0.957 in the quasi-perfect band (α>0.9) from 870 to 3224 Hz with a thickness of only 3.9 cm. Our results open new avenues for the development of novel and highly efficient acoustic absorbers against low frequency noise, and more essentially, suggest an efficient approach towards on-demand acoustic impedance engineering in broadband.