The Investigation Of Sound Field Regulation Mechanism Based On Acoustic Metasurface

Acoustic metasurface is an artificial composite material,which is constructed by subwavelength microstructures arranged in a specific way.By adjusting the geometric parameters of the meta-atoms,sound waves could be regulated efficiently and accurately with the help of acoustic metasurface.Due to its characteristics of ultrathin,flexible and simple structures,the acoustic metasurface has provided a new idea for the design of acoustic devices,and played an important role in energy storage,nondestructive testing and low-frequency absorption,etc.On the other hand,it is reliable to construct a non-Hermitian acoustic system based on metasurface.By precisely adjusting the lossy parameters introduced by lossy materials,special acoustic phenomena in the non-Hermitian scattering system can be observed,which have not been found in classical lossless systems.However,there are several problems in traditional acoustic metasurfaces that need to be solved.Firstly,most of the meta-atoms were constructed with resonant structures,which were responsible for a narrow frequency band.And it was obvious that the performance of the device would be greatly restrained if the working frequency deviating from the resonant one.Secondly,metasurfaces were designed mainly relying on Generalized Snell’s Laws(GSLs).It required that each meta-atom in one period to provide phase compensation independently.This procedure ignored the coupling effect between meta-atoms.At the same time,the complex internal design also made it difficult to avoid the viscous loss in metasurfaces,leading to a sharp decline in efficiency at high operation frequencies and in diffraction with large angles.In addition,the transverse size of traditional gradient metasurface was always too large due to the periodic arrangement of abundant meta-atoms,which limited the application in complex environments.To solve the above problems,the author intends to design an artificial meta-atom working on broadband,and to propose the tunable acoustic coding metasurface in this dissertation.The latter is constructed with the coding arrangement of meta-atoms,which could realize a variety of acoustic regulation functionalities.Then,a metagrating will be proposed to make the best use of the coupling effects between meta-atoms.This non-local metamaterial would demonstrate its ability to control multichannel acoustic waves,which will break the limitation of GSLs,and the sizes of meta-atoms would no longer follow the requirements from gradient metasurfaces.Finally,non-Hermitian acoustic systems will be constructed based on acoustic metasurfaces.The author will explore the intriguing physical phenomena accompanied with the degeneracy of eigenstates in scattering matrix.The physical mechanism corresponding to each order Exceptional point(EP)in non-Hermitian acoustic system will also be explored in this dissertation.The main contents are as follows.1.Establishing the theoretical model to clarify the interaction between incident waves and the coding metasurface.The compact coding metasurface can be designed according to the acoustic field regulation in demand.Pipes with gradient index will be introduced on both sides of the helical meta-atoms to realize the impedance matching with background medium(air).So that the broadband meta-atoms can be constructed to widen the working bandwidth of metasurface.Adjusting the pitch of helical metaatoms to provide phase compensation of and /2 respectively,so as to correspond to two logical elements “1” and “1/2”.Then the finite element simulation of the logical elements will be put on the agenda,and the transmittance and phase distribution diagram can be obtained.With the help of modular combination of logical elements,the compact acoustic coding metasurfaces with different functions will be built,which could simplify the design procedure of metasurface.The theoretical model can be demonstrated through full-wave numerical simulation and experiments.Acoustic splitting and bending will also be presented to show the acoustic regulation ability of this coding metasurface.Besides,its broadband response can be proved from the perspective of refraction angles.2.Based on the classical diffraction theory and coupled mode theory(CMT),the physical model of non-local metagrating would be provided.Then the author will calculate the amplitudes of diffraction modes in each reflection or transmission order.The mathematical equations between acoustic responses and the meta-structure will be established,and the reverse design procedure of non-local metagrating can be provided.According to the specific requirements of sound field regulation,the appropriate structural parameters can be found from the broad parameter space with the help of genetic algorithm(GA).This optimization algorithm is relied on the objective function and constraint conditions.The theoretical,numerical simulation and experimental results will be proved to be consistent with each other.So the accuracy and efficiency of this reverse design procedure can be demonstrated.In addition,simple structures such as straight tubes and grooves are adopted as metaatoms,which will avoid the viscous losses and other unnecessary frictional effects.It should be noted that,this design will make full use of the higher-order waveguide modes to regulate the transverse energy transport.So that it will break down the limitation from the generalized Snell’s laws and overcome the key problem that the diffraction efficiency drops sharply at large refraction angles and high frequencies in traditional gradient metasurfaces.This would provide a new idea for the manufacture of high-performance acoustic devices.3.The high-order scattering system based on the non-local metagrating will be provided in this dissertation.It demonstrates the flexible effects from metagrating in multi-channel sound field control.The loss will be introduced into metasurfaces to construct non-Hermitian acoustic systems with different orders,and the generalized condition for eigenstate degeneracy in scattering system will be established.According to the objective function in optimization,the structural parameters corresponding to the Exceptional points(EPs)with any orders can be obtained,and the macroscopic physical phenomena on EPs could also be presented.What’s more,the sensitivity evolution in non-Hermitian acoustic systems at EPs would be demonstrated,and then the influence of eigenvectors’ degeneracy on sensitivity will also be explored.In conclusion,the acoustic field regulation based on metasurfaces will be systematically investigated in this dissertation.The proposed compact coding metasurface presents an idea for the modular assembly of acoustic metamaterials.On the other hand,the acoustic regulation mechanism hided in non-local metagrating is completely different from the traditional gradient metamaterials.Thus,an ideal platform will be established for the precise regulation in multi-channel sound fields and for the exploration on non-Hermitian systems.The relevant study in this dissertation will also provide guidance for the design of multifunctional acoustic devices.