Breaking the causality limit for broadband acoustic absorption using a noncausal active absorber

The principle of causality imposes a constraint between the thickness and bandwidth of absorbers. This trade-off applies to any linear, time-invariant, passive system, limiting the development of broadband-absorbing materials that demand a thin profile for sound, light, and radio waves. Here, we demonstrate a strategy to overcome this constraint in acoustics using a noncausal active absorber whose response is controlled over time. A theoretical framework is established, which sets a relation among minimum thickness, bandwidth, and a priori information about the incident signal, representing a relaxed physical bound for noncausal absorbers. We design an absorber based on this principle and experimentally show that its response bandwidth surpasses the conventional limit. Our results showcase an active metamaterial that reduces the footprint of acoustic absorbers and elucidate the role of prior information in enhancing acoustic technologies, offering insights into the design of active acoustic devices.