Tunable Steering Of Transmitted Acoustic Waves By The Reconfigurable Metasurface With Rotational Units

The manipulation of acoustic waves has always been a research hotspot in the field of acoustics.Acoustic metasurfaces with subwavelength thickness can achieve wave-front control of acoustic waves by introducing phase gradients,and they have attracted wide attention because of their thin structure and flexible control ability.Usually,only a single acoustic function at a fixed frequency can be realized by the traditional acoustic metasurfaces which have fixed geometric structure.And most researches are limited to two-dimensional situations.These problems limit the application of metasurfaces to a large extent.In this paper,two kinds of reconfigurable metasurfaces with rotational unit are designed.By simple rotation operation of the unit cell,the geometric reconstruction of the metasurface can be realized,and then the switch between different operating frequencies and different acoustic functions can be realized,which can improve the flexibility of the metasurface.At the same time,this paper expands the research and application of the metasurfaces to the three-dimensional field.The generalized Snell's law is derived in three-dimensional space.Steering of the two-dimensional and three-dimensional transmitted acoustic waves are implemented by the finite element software COMSOL Multiphysics.The switch of various acoustic functions such as abnormal refraction,focusing,Bessel beam and self-bending beam at different operating frequencies,are realized.Corresponding experiments are also implemented to verify the effectiveness of the metasurface.The main contents and results include:1.The generalized Snell's laws for regulating the transmission of acoustic waves are deduced in two-dimensional and three-dimensional fields.And then according to prospective acoustic functions,the phase distribution functions of metasurfaces are derived based on this theory.2.The unit cell consisting of rotational resonant cylinder is designed.The opening angles of the unit cell can be changed just by rotating the inner cylinder,and then the phase and transmission coefficient of the transmitted acoustic waves can be modulated.Resonant metasurfaces with unidirectional and bidirectional gradients are designed by using this rotational unit cell,and the acoustic functions such as abnormal refraction,focusing,and self-bending beam of acoustic waves at different frequencies are realized numerically in two-dimensional and three-dimensional situations,respectively.At the same time,the acoustic performance of the resonant unit cell and some functions of the unidirectional gradient metasurface are studied experimentally.The experimental results and the numerical simulation results jointly verify the effectiveness of the metasurface.3.Based on the working principle of the "screw-nut" system,a double-blade helical unit cell is designed.By changing the screw-in depth of the helix,the propagation path of acoustic waves is adjusted,and then the phase of transmitted acoustic waves is changed.A reconfigurable bidirectional gradient coiling-up space metasurface is designed based on the helical unit cell.The functions of abnormal refraction,focusing,point source to plane wave,Bessel beam and self-bending beam in three-dimensional space are realized in the wide frequency range.Corresponding acoustic experiments are carried out for the focusing function,and the results are in good agreement with the finite element calculation results.The reconfigurable transmission acoustic metasurfaces with rotational unit cells designed in this paper can realize the continuous phase adjustment by tuning the opening angles of the resonant cylinder unit cell or screw-in depths of the helical unit cell though the simple rotation operation.The flexible switch between different operating frequencies and different acoustic functions is realized without reprocessing of the metasurface.The research in this paper is of great significance for the design of tunable acoustic metasurface and acoustic wave manipulation in three dimensional situations.The studies will provide a theoretical basis for the design of tunable transmissive acoustic elements and devices.