Broadband superabsorption of waterborne acoustic waves by bubble metascreens

Absorption of acoustic or mechanical waves is an important challenge for various applications such as noise insulation, stealth coating, seismic event mitigation and ultrasonic testing. In order to absorb sound, one needs to introduce a medium with sufficient dissipation but without significant reflection of the incoming wave. Ideally, the absorber should also be thin and light, a goal that may be realized through the use of a thin 2-D metamaterial, or metalayer. The conventional way of achieving strong absorption in a thin metamaterial is to exploit low-frequency resonant inclusions. However, most resonant structures have an intrinsically narrowband response, making it difficult to attain broadband absorption in a deeply subwavelength-thick meta-layer, and making it necessary to devise larger structures to increase the bandwidth. For waterborne acoustic waves, an exception to this common situation can be achieved through the fabrication of bubble metascreens, which consist of a single layer of bubble inclusions embedded in a soft solid. In this presentation, we re-visit the optimization of such bubble metascreens and show that, despite being resonance-based, near-perfect absorption is possible over a very wide frequency range even when the metalayer is ultrathin.