Asymmetric absorption in acoustic metamirror based on surface impedance engineering

Asymmetric wave manipulation has attracted growing interest due to its great importance in practical applications. We design and demonstrate a planar acoustic metamirror for realizing asymmetric sound absorption with a controllable retroreflection coefficient, which is absent in conventional lossless metasurfaces. We design the metamirror by realizing the required theoretical surface impedance profile, and then numerically and experimentally demonstrate its asymmetric response. The measured and simulated acoustic fields agree well with each other, which shows that the proposed metamirror can yield strongly asymmetric sound absorption: retroreflection and nearly full absorption for two opposite incident angles. In addition, the measured retroreflection coefficient and retroreflection angle are very close to the design values. We find that the asymmetrical excitation of evanescent waves plays a key role in the realization of asymmetric absorption. The proposed metamirror enriches the functionalities for acoustic wave manipulation and has prospective applications in many fields, such as acoustic antennas, acoustic sensing, and angle-encoded steganography.