Study On Abnormal Reflection Of Acoustic Metasurface With Subwavelength Apertures

Acoustic metasurface(AMS)is a kind of artificial material with ultrathin thickness,having the ability of controlling acoustic wave fronts to reflect acoustic waves into an unusual yet controllable direction.According to the reflection characteristics of aperture system composed of a subwavelength aperture grating and a reflection wall,this paper proposes a band-new AMS based on generalized Snell's law.Combined with theoretical analysis,COMSOL simulations and experimental tests for the reflected wave patterns of the AMS in the air,its excellent property of abnormal reflection is verified.In the paper,analytical formulas for the reflection coefficient of the unit cell,consisting of the subwavelength aperture system,are derived based on the classical mode-expansion theory.The reflection characteristics of the system are then emphatically discussed and the mechanism how it works is put forward.More importantly,the influences to the phase shift of the reflected waves generated by grating material,geometrical parameters and incident frequency are also analyzed and the conclusions will provide necessary support to the design of an AMS.The theoretical expression for the reflected field generated by AMS,containing a Fredholm integral equation of second kind with Cauchy core,is obtained from Kirchhoff-Helmholtz integral formula on the grounds of the Green function for perfect rigid impedance interface.A numerical method for the solution of the equation is studied and the theoretical pressure patterns of reflected waves can be determined accordingly.The results show that the propagation direction of the reflected waves can be indeed manipulated through changing the phase distribution along the AMS interface.The design principles for AMS with subwavelength circular holes are summarized,and an AMS with the ability of abnormal reflection is then designed.COMSOL simulations not only demonstrate that both the discontinuous phase shift between different unit cells and the abnormal phenomenon are the direct consequences of Helmholtz resonance,but also prove that the designed AMS could function well in a wide frequency bandwidth and a large range of incident angle.Further application to acoustic focusing is realized and the acoustic energy at focal point is nearly 10 times greater than that of incident waves.An AMS sample with circular holes is designed and then fabricated.Reflected pressure fields are measured in a self-built experimental platform.After MATLAB processing,it could be found that the measured datum for reflected wave patterns are consistent with COMSOL simulations,further verifying the correctness and the validity of the theory obtained and the design about AMS.With simpler configuration,the fabrication for the AMS proposed in the paper could be greatly facilitated.Its total thickness is no more than one tenth of incident wavelength,implying that it should have the potential for the miniaturization and integration of acoustic devices.The fact that reflected wavefronts can be easily controlled by adjusting the phase distribution along the AMS interface demonstrates that this kind of AMS can provide new ideas for noise insulation and reduction.Moreover,the in-depth study about it could be conducive to the deeper understanding for new type of acoustic functional material.