Going Beyond the Causal Limit in Acoustic Absorption

Does Nature take something away with one hand and give it back with the other? Yes, in the case of acoustic absorption. The causality principle has been regarded as the reason one needs thick, bulky absorbers for low-frequency noise remediation. By judicious manipulation of the boundary condition on the backside of the absorber, the authors obtain broadband near-total absorption below 100 Hz with a 2-cm-thick sample, when the causal minimum is 15 cm. This complements metamaterial absorbers that are within the causal limit and effective at higher frequencies, yielding a holistic picture of the acoustic absorber design landscape, over the entire frequency range.