Generalized causality constraint based on duality symmetry reveals untapped potential of sound absorption.

Causality constraints are known to bind sound absorption to a limit that can only be achieved by optimizing the system bandwidth for a specific material thickness. This limit is defined on the assumption of a one-port system, generally causing duality symmetry to be overlooked. Here, we define a generalized causality constraint of sound absorption by investigating reflection and transmission of a two-port hybrid monopole-dipole resonator. With our theory, we show that the absorption limit is approached by relying on the well-established critical coupling as well as by matching effective compressibility and density. We experimentally show that the designed resonator absorbance follows the duality symmetry condition embodied in the large bandwidth reported, confirming an intrinsic connection between duality symmetry and scattering causality. A comparison with a traditional foam liner and other competitive works further validates our findings. Our results reveal previously untapped absorption potential in broadband acoustic metamaterials.