Study Of Sound Propagation Characteristics In PT-symmetric Acoustic Metamaterials

Over the past two decades,the acoustic metamaterials has begun to gradually rise and develop.The research of acoustic metamaterials has attracted wide attention and this new concept has been introduced into various technical areas.Acoustic metamaterials is a kind of composite material with artificial design structure and can produce some unconventional acoustic parameters in nature through appropriate design of the structure of unit,such as negative mass density,negative modulus,zero refractive index,etc.Some novel acoustic phenomena can also be emerged by combined with other acoustic related research fields.For example,the demand for microminiaturization of acoustic devices promotes the development of metasurfaces.The fascinating phenomenon of acoustic cloaking can be realized through introducing the theory of transform acoustics.Recent combination of parity-time(PT)symmetry and acoustic metamaterials or metasurfaces offers a novel approach to extend the real acoustic parameter plane to a complex one and hence providing some new design freedoms in acoustic wave controlling.All mentioned above are developed based on acoustic metamaterials.The article presents the author's researches based on the exotic properties of acoustic metamaterials and acoustic wave propagation is studied by combining with other acoustic areas,such as acoustic metasurfaces,acoustic leaky wave antennas and PT-symmetric acoustics.The dissertation is divided into following sections:In chapter I,we review the background of the researches in acoustic metamaterials,metasurfaces and PT-symmetric metamaterials.In chapter II,the wavefront manipulation based on transmissive acoustic metasurface with plate-type hybrid structure is studied.The gradient metasurface is composed of eight elements,and each element is designed by the plate-type hybrid structure composed of four decorated plate resonators in periodical distribution and a straight pipe with tunable width.The properties of high transmission efficiency and discrete phase shifts over the full 2? range can be achievied simultanelously.The transmissive wavefront manipulations based on metasurface can be obtained as expected from the generalized Snell's law,such as anomalous refraction,acoustic cloak based on flat focusing,acoustic self-bending beam,conversion of propagating wave to surface wave and negative refraction.In chapter III,improving directional radiation quality based on a gradient amplitude acoustic leaky wave antenna is studied.In this work,we show how to modify radiation amplitude with a leaky wave antenna to improve the quality of sound radiation.The gradient amplitude antenna is used to suppress the side lobes and concentrate more radiated sound energy on the main lobe.The main beam of gradient amplitude acoustic leaky wave antenna scans from the backward direction to the forward direction during the operated frequency range with the increase of frequency without band gaps.In chapter IV,the functions of logic gates and amplifier based on passive PT symmetry are studied.The passive PT-symmetric potential is introduced by the waveguide metamaterial with periodically distributed leakage structures.The lossless and loss regions of this passive metamaterial are carefully engineered to approach highly efficient coherent control with a maximum contrast between the absorption and emission states at the coherent perfect absorber(CPA)-emitter point.Therefore,the logic gates(AND,OR,XOR and NOT)and small signal amplifier are realized in a single system by adjusting the phase and amplitude differences between two incoming beams,respectively.In addition,the function of asymmetric sound transmission is realized theoretically and experimentally by the system which is composed of three passive PT-symmetric metamaterials with T-shaped structure.In chapter V,acoustic negative refraction based on PT-symmetric systems is studied.By introducing loss and gain metasurfaces,the unidirectional negative refraction and bidirectional acoustic negative refraction can be observed,respectively.The property of unidirectional negative refraction is related to the exceptional point at the PT breaking phase of the PT-symmetric system.And the property of bidirectional negative refraction is related to the coexistence of amplification and absorption states in the PT-symmetric system at the CPA-emitter point of the PT broken phase.Our results provide a new physical mechanism for realizing the functions of the unidirectional and bidirectional acoustic negative refractions in PT-symmetric systems.The last chapter undergoes summary and discussion of the whole text and gives some prospections in the research field.All in all,with the development and progress of the science and technology,it will promote the interdisciplinary of acoustic metamaterials and acoustic related subjects,which will inspire various fascinating research applications and new principles.