Ultrasparse Acoustic Absorbers Enabling Fluid Flow and Visible-Light Controls

Manipulation of acoustic waves, in particular for perfect absorption through a subwavelength medium, is of great importance in scientific and practical aspects. Conventional absorbers (e.g., thick foams or dense media) can sufficiently absorb acoustic waves, but at the same time, block fluid flow and visible light. Although sparsely arranging acoustic resonators will permit fluid flow and light propagation, this sparse arrangement leads to high acoustic transmission because of acoustic radiation symmetry. We analytically and numerically demonstrate that a sparsely distributed resonator array consisting of pairs of lossy and lossless resonators has perfect absorption at resonance with ultrahigh sparsity (a volume-filling ratio of <5%) owing to the symmetry breaking induced by the asymmetric loss. We find that the maximum period for ultrahigh sparsity is bounded by four times the absorption cross-section limit of a single-resonance subwavelength resonator, independent of its physical dimensions. Therefore, one can, in principle, obtain arbitrarily high sparsity. We experimentally demonstrate that our sparse absorber array exceeds 90% absorption at resonance with a volume-filling ratio of approximately 26%. In addition, we show that the ultrasparse absorber enables air-flow-direction control and optical cloaking. These results open up an area of multidisciplinary, multifunctional acoustic metamaterials.