Figure 1

(a) Geometry of the proposed acoustic metamaterial with thickness \(H\). Plane incident waves are considered to be incident on the metamaterial. (b) A representative unit of the proposed metamaterial with width \(L\), consisting of a perforated panel (thickness \(t_{1}\), perforation diameter \(d\), perforation ratio \(\sigma = \pi d^{2} /4L^{2}\)) and a space-coiled water channel (channel width \(w\), height \(h\)) with a rubber coating (yellow area, thickness \(t_{2}\)) on the channel wall. The thickness of the walls is \(t_{3}\). (c) Equivalent model of the proposed metamaterial with straight water channel (effective channel length \(L_{eff} = V/w^{2}\), where \(V\) is the volume of the water channel).