Going Beyond the Causal Limit in Acoustic Absorption

Dictated by the causality constraint, the existence of a minimum absorber thickness has set an almost insurmountable obstacle for the remediation of ultralow-frequency noise in the tens of Hz to 150 Hz regime, owing to the very large absorber thickness required. Here, we show that, by manipulating the boundary condition in a calculated manner on the backside of the absorber, the causal constraint can be circumvented, and broadband near-total absorption of ultralow-frequency acoustic waves may be realized with an absorber thickness that is an order of magnitude less than the causal minimum. Our work delineates a design paradigm for the ultralow-frequency acoustic absorbers.