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The ability to tune the performance of acoustic metamaterials without structural modifications or complex active control circuits is a remarkable challenge. In this work, we present a square piezoelectric side-branch (PSB) pipe-type structure that consists of the piezoelectric composite sheets (PCSs) on its sidewall and propose the corresponding equivalent electroacoustic circuit model. Compared with the traditional side-branch, theoretical and numerical results show that the thickness of the PSB can be up to λ / 200 , and the tunable phase shift of the transmitted wave in the PSB pipe can achieve nearly 2 π by adjusting the capacitances that are connected to the PCSs. The experiment for a square pipe with one PCS is also given to prove the phase control ability of the PSB model. Furthermore, we show that by merely adjusting the external capacitances of the PCSs, switching the functions between the 45° extraordinary transmission and the acoustic focusing for the same incident wave can be exactly realized. Our work provides a promising potential in active controls and integration designs of acoustic metamaterials and devices. [ABSTRACT FROM AUTHOR]

Due to the potential engineering needs, the passive tunable metasurfaces with a high performance equivalent to the active phased array is worthy of research. Here, a passive ultrathin metasurface unit composed of a piezoelectric composite structure (PCS) connected to an external capacitor, which can modulate the phase of the transmitted acoustic waves at a deep subwavelength scale only by controlling the external capacitor but without changing the structure, is proposed. Then, a tunable acoustic metasurface composed of 20 identical PCSs is introduced to realize three acoustic functions, beam steering, beam focusing, and tweezer-like beam generating, just by changing the external capacitors. The phase-control abilities of the PCS unit and three functions of the designed metasurface are proved both numerically and experimentally. This study provides the possibility to design ultrathin tunable acoustic metasurfaces with the ability of precise control and passive materials. [ABSTRACT FROM AUTHOR]