Study On Manipulation With Acoustic Near-zero Refractive Index Materials

In recent years,the manipulation of acoustic waves by sonic crystals and acoustic metamaterials has gradually become a research hotspot.Owing to its high transmission and high acoustic velocity,acoustic waves can pass through the near-zero refractive index material with tunnel effect.The near-zero refractive index materials have strong robustness,and the transmitted acoustic waveform and energy remain unchanged even with rigid scatterers embedded in them.Besides,owing to the selective property of acoustic directions,we can realize different novel acoustic characteristics by changing the propagation directions of acoustic waves.The design of the acoustic devices with the near-zero refractive index materials has potential applications in medical ultrasound,architectural acoustics and military technology.In this work,based on the near-zero refractive index acoustic metamaterials and sonic crystals,several acoustic devices have been designed.First chapter is the introduction and seventh chapter includes the summary and the outlook.The main research contents are from second chapter to sixth chapter:(1)Theory and calculation method of sonic crystals and acoustic metamaterials.(2)Manipulations of acoustic wavefronts with near-zero refractive index metamaterials.(3)Acoustic focusing lenses with near-zero refractive index metamaterials.(4)Extraordinary acoustic transmission with near-zero refractive index sonic crystals.(5)Acoustic one-way transmission and one-way focusing with near-zero refractive index sonic crystals.In second chapter,based on the basic equations of acoustic waves,we introduce the theory of the band structure of the sonic crystals and the calculation method of the equivalent acoustic metamaterials,and provide basic theory and calculation method to study the characteristics of near-zero refractive index sonic crystals and acoustic metamaterials.In third chapter,based on the selection mechanism of the acoustic directions in the near-zero refractive index materials,the acoustic wavefront lenses are designed by using coiling-up space units with the near-zero refractive index property and the acoustic wavefront can be manipulated.When both sides of the lens are vertical and inclined,respectively,the wavefront of the plane acoustic wave can be manipulated.When both sides of the lens are circular and inclined,respectively,the wavefront of the cylindrical acoustic wave can be manipulated.Based on the tunneling characteristics of the near-zero refractive index metamaterials,acoustic bending propagation and acoustic stealth effect can be realized,and therefore the plane acoustic wave keeps the waveform characteristics when bypasses the U-channel structure or the stealth region with a rigid boundary.Compared with other acoustic devices,the proposed acoustic wavefront lenses realized by the near-zero refractive index metamaterials have advantages of single-unit structure,high transmittance,and high robustness,which provides the theoretical solutions and design concepts for the development of acoustic devices with near-zero refractive index.In fourth chapter,based on the selection mechanism of the acoustic directions in the near-zero refractive index materials,the acoustic focusing lenses can be realized by the coiling-up space units with near-zero refractive index.So that plane acoustic wave focusing,cylindrical acoustic wave focusing,and line focusing of plane acoustic wave are obtained,respectively.When both surfaces of the lens are plane and circular,respectively,the focusing effect of the plane acoustic wave can be achieved.When both surfaces of the lens are circular,the focusing effect of the cylindrical acoustic wave can be achieved.When the left surface of the lens is plane and the right surface is composed of two planes with specific angles,the line focusing effect of the plane acoustic wave can be achieved.Besides,the rigid scatterers inside the three kinds of lenses have little effects on the focusing performance.Compared with other acoustic devices,the proposed acoustic focusing lenses realized by the near-zero refractive index metamaterials have the advantages of single-unit structure,high focusing performance and high robustness,which provides theoretical basis and experimental reference for designing acoustic focusing devices.In fifth chapter,two near-zero refractive index rectangular sonic crystals with same size are composed of cylindrical units.The two sonic crystals are placed on both two sides of the horizontal slits immersed in water.The extraordinary acoustic transmission effect can be realized,and the transmitted waveform remains unchanged.When there is only one sonic crystal on the left side of the slit or there are no sonic crystals on both sides of the slit,the extraordinary acoustic transmission disappears.Besides,the transmitted acoustic energy reduces greatly,and the plane waveform is changed to the cylindrical waveform.In addition,the rigid scatterers inside the near-zero refractive index sonic crystals and the width of the slit almost have no effects on the extraordinary acoustic transmission.Therefore,the extraordinary acoustic transmission based on the near-zero refractive index sonic crystals has the advantages of unchanged transmitted waveform and high robustness.The results provide new concepts and methods for designing acoustic devices with extraordinary acoustic transmission.In sixth chapter,based on the selection mechanism of the acoustic directions in the near-zero refractive index materials,the near-zero refractive index sonic crystals with specific incident and exit surfaces are composed of the cylindrical units.High performance acoustic one-way transmission and acoustic one-way focusing can be realized with this sonic crystal.The parallelogram sonic crystals are embedded in a curved waveguide structure immersed in water.Acoustic one-way transmission device,bidirectional acoustic conduction device,bidirectional acoustic insulation device and multi-channel acoustic one-way transmission device are realized by adjusting the channel direction of the waveguide.Furthermore,we propose an acoustic one-way focusing lens with a plane surface and a circular surface based on the proposed sonic crystals.When the acoustic waves are incident from the left side,the acoustic focusing with high performance can be achieved.When the acoustic waves are incident from the right side,the acoustic waves are reflected and cannot pass through the lens.High-performance acoustic one-way transmission and one-way focusing effects can be simultaneously realized in the lens.In addition,the focal length can be adjusted by changing the radius of curvature of the lens,and the rigid scatterers inside the lens almost have no effect on the one-way acoustic transmission and focusing performance.The results provide theoretical solutions and experimental references for designing acoustic devices with one-way transmission and one-way focusing.