  Wave absorption metamaterials have been an enduring topic over the past two
decades, propelled not only by novel scientific advances, but also by their
extensive application potential. In this review, we aim to provide some general
insights into the absorption mechanism common to both microwave and acoustic
systems. By establishing a universal model for resonance-based metamaterials,
we present the theoretical conditions for broadband impedance matching and
introduce the fundamental causal limit as an evaluation tool for absorption
performance. Under this integrated framework, we survey the recent advances on
metamaterials absorption in both microwave and acoustic systems, with the focus
on those that pushed the overall performance close to the causal limit. We take
note of some new, emerging metastructures that can circumvent the constraint
imposed by causal limit, thereby opening a new avenue to low-frequency
absorption. This review concludes by discussing the existing challenges with
possible solutions and the broad horizon for future developments.
