Study On Wavefront Manipulation Of Tunable Curved Transmissive Acoustic Metasurfaces

Acoustic metasurface is an artificially designed two-dimensional structure with sub-wavelength thickness.The wavefront manipulation of sound waves can be realized by design of the sub-wavelength structure,thereby different acoustic functions can be achieved such as acoustic focusing,anomalous refraction,sound absorption and source illusion,etc.One of the main reasons for people's interest in acoustic metasurfaces is that they can flexibly manipulate sound waves through these thin and light structures,which promotes the miniaturization of devices and makes new applications possible.Due to the short research history of acoustic metasurfaces,there are still many problems and technical difficulties to be solved.Most of the existing metasurfaces are designed and manufactured for a specific function,which can only achieve a single function within a certain or very narrow working frequency range.This limits the practical application of acoustic metasurfaces.In this paper,curved acoustic metasurfaces with the helical-structured sub-wavelength structure are designed based on the working principle of the "screw-nut" system.By adjusting the screwing depth of the helical unit-cell,the phase can be continuously adjusted in a wide frequency range.Based on the generalized Snell's law,the required phase distribution can be obtained according to the working frequency and specific function to be realized.For other working frequency or acoustic functions,what we need to do is just adjusting the screw-in depths of each unit-cell.In this paper,the finite element software Comsol Multiphysics is used for simulation calculations,and the simulation results are verified by acoustic experiments.The main contents and results include:1.Design of the transmissive helical unit-cell.A reasonable helical unit-cell is designed and the nephograms of the phase shift and transmittance for the unit cell varying with the screw-in depth and frequency are obtained through finite element calculations,which lays the foundation for the design and implementation of the subsequent studies.2.According to the Generalized Snell's theorem,a tunable arc-shaped transmissive acoustic metasurface is designed.The functions of directional propagation of sound waves,beam separation and focusing are realized at different frequencies.At the same time,the corresponding acoustic experiments are carried out.The experimental results agree with those calculated by the finite element software which shows theeffectiveness of the designed metasurface.3.Based on the above study,an annular metasurface is developed.At different frequencies,three-way partial wave function and source illusion functions such as helix of the wave source and virtual movement of the source position are implemented.And corresponding acoustic experiments are carried out.The experimental results are consistent with the results calculated by the finite element software.The curved transmissive acoustic metasurface designed in this paper can continuously adjust the phase of the transmitted wave in a wide frequency range by adjusting the screw-in depth of the spiral unit-cell without changing the shape of the structure.Multifunctions such as directional propagation,beam separation and focusing,and source illusion are realized.The research in this paper provides important theoretical guidance for the manipulation of acoustic waves.At the same time,the study on the curved metasurface helps to promote the development of irregular non-planar conformal acoustic devices.