Asymmetric sound absorption achieved by double-layer piezoelectric metamaterials with tunable shunt circuit

Asymmetric acoustic metamaterials open up a new field for multi-directional sound wave manipulation. The controllability of most asymmetric metamaterials on sound waves is restricted by their fixed structure and material parameters. Here, we propose a double-layer piezoelectric metamaterial that comprises two identical membrane-type piezoelectric acoustic metamaterials and a tunable shunt circuit. For incident waves in a narrow band, one side of the metamaterial matches the air impedance to achieve perfect absorption while the other side mismatches the air impedance to completely reflect the sound waves. The proposed metamaterials can separately manipulate the absorption frequencies and coefficients on both sides of the metamaterial by tuning resistances in the shunt circuit. Both the theory and experiment show that the maximum absorption coefficient can reach over 0.98, and the tunable frequency range has a 60% bandwidth over the center frequency of 1059 Hz.