A broadband acoustic waveguide cloak based on the gradient impedance boundaries

Invisibility cloaking is a longstanding research topic due to its significant applications in various human activities like communication, manufacturing and aerospace, etc. In this paper, we theoretically design and experimentally demonstrated a broadband acoustic waveguide cloak for acoustic waves in air, which is composed of a straight waveguide coated with two gradient impedance boundaries (GIBs). When the plane wave (PW) incidents into the waveguide, it exercises a mode conversion of PW-surface wave (SW)-PW. As SWs are confined by GIBs, obstacles in the waveguide between the confined SWs become invisible to the incident PW with negligible reflection. Thus, an acoustic waveguide cloak is obtained. The experiments demonstrate the negligible reflection (<5 %) in the waveguide coated with GIBs, i.e., in our proposed acoustic waveguide cloak from 2.62 kHz to 3.74 kHz. However, in the same frequencies, the incident PW is almost totally reflected (>98 %) in the waveguide without GIBs. It agrees well with the simulation results and the theoretical predictions. Thus, a broadband acoustic waveguide cloak is obtained. In addition, the acoustic waveguide cloak is also demonstrated in the waveguides with sharp bends and corners, such as the L-shaped and the U-shaped waveguides. Our proposal may provide a strong candidate for acoustic wave manipulation and acoustic functional devices.