Broadband sound absorption at the sub-wavelength scale with noncausal active absorbers

Sound absorption is of vital importance in many applications. In recent years, metamaterial-based absorbers have shown great promise in noise reduction for various setups. However, it has been shown that there is a fundamental tradeoff between the thickness and bandwidth for passive absorbers based on causality. To relax this constraint, here we demonstrate an active absorber that specifically leverages the a priori information for broadband sound absorption. A general relation between the minimum footprint of the absorber and the absorption bandwidth is established under the noncausal limit. Theoretical and experimental results confirm the validity of the proposed approach, in which an active absorber with an adjustable absorption spectrum is demonstrated. The absorber uses a feedforward mechanism to induce a noncausal response and further reduces its thickness compared to passive absorbers. The results showcased a scenario where noncausal information can be important in designing active acoustic elements.