Research On Acoustic Directional Manipulations Based On Acoustic Artificial Materials

Directional manipulation has always been the focus of the research on optical/acoustic wave transmitting and receiving devices.The traditional method to enhance directivity is to build large-size transducers or transducer arrays,which usually lead to bulky,energy-consuming and costly devices.In recent years,the emergence of optical/acoustic artificial materials has provided powerful tools to manipulate waves.The rich physical properties of acoustic artificial materials also provide new ideas for directional manipulation.However,whether using phononic crystals or metamaterials,the existing directional manipulation designs are bulky structure based on resonance units,which inevitably causes losses.Moreover,the complex geometrics of these structures increase the processing difficulty,and their performance are also vulnerable to structural defects.This thesis applies new physical mechanisms to the directional manipulation of sound waves in air and water background.Through the design of zero-index metamaterials,one/two-dimensional acoustic topological materials and three-dimensional acoustic artificial materials,the nonresonant property,anisotropy,miniaturization,omnidirectivity and broadband property of the directional manipulation devices can be improved,laying a foundation for applications such as acoustic communication,noise control and acoustic imaging.The specific works of this thesis are as follows:Firstly,isotropic zero index metamaterials based on gradient space-coiling units and anisotropic zero index metamaterials based on cavity-channel networks are proposed.The zero-index property of the isotropic zero-index metamaterial comes from band folding,so its performance is less affected by loss.The gradient change of the folded channel width increases the design freedom of the material,which can realize negative or zero index with high transmittance.The directional acoustic emissions based on these two zero refractive index metamaterials are numerically studied,and the effectiveness of the directional emission device is verified.Secondly,the double Su-Schrieffer-Heeger model and acoustic topological waveguide are proposed,and the enhanced directional emission of acoustic waves is realized with this waveguide.Compared with the Su-Schrieffer-Heeger model,the positions of the topological nodes in the momentum space of the double Su-Schrieffer-Heeger model can be flexibly adjusted,so the edge states in this topological waveguide can be coupled with the plane acoustic wave in the background medium at specific angles.A leaky-wave waveguide for directional acoustic emission is designed,and the directional acoustic emission based on this waveguide is numerically and experimentally studied.The results show that the radiation resistance of the point source in the waveguide is nearly 100 times that of the point source in the free space.Directional acoustic emission with about 12 times energy enhancement is observed in the experiment.Thirdly,the two-dimensional square lattice acoustic topological insulators for air acoustic and underwater acoustic are proposed.The acoustic topological insulator in air is based on the cavity-channel network,and the underwater acoustic topological insulator is based on the elastic scatterers array.Acoustic directional transmitter with robustness to defects is designed based on the topologically protected edge states between acoustic topological insulators.The half-power-angular-width of the far field intensity of the two directional transmitters is 5° and 3.1°,respectivelyFourthly,a composite face-centered-cubic phononic crystal is proposed,and the three-dimensional refractive index distribution of this phononic crystal is studied.Quasi two-dimensional and three-dimensional underwater acoustic Luneburg lenses are constructed using this phononic crystal.The omnidirectional acoustic focusing performance of the quasi two-dimensional phononic crystal Luneburg lens is studied by using the finite element method,and the propagation of mechanical waves in the lens is analyzed.An underwater acoustic experiment is designed to verify the omnidirectional acoustic convergence function of the three-dimensional phononic crystal Luneburg lens.The results indicate that the three-dimensional Luneburg lens has good acoustic focusing ability at different angles in the frequency range from 30 k Hz to 38 k Hz.