Study Of Sound Beam Manipulation Based On Acoustic Metamaterials

Acoustics is a classical branch of physics,it mainly focuses on the sound wave's generation,transmission,receiving and other related effects,which have wide and strong intersections and extensibilities.As one of the earliest physical phenomenon studied by human,sound wave has behaved as one of the most important information carriers,just like light or electricity.Recently,due to the development of the nano technology and other related theories,a new concept called metamaterials,which emerged from the field of electromagnetism and optics,is getting more and more attention from academic circles.Like optical metamaterials,acoustic metamaterials can adjust and control the sound wave's propagation in unusal ways,which cannot be realized by traditional methods or natural materials.Acoustic metamaterials with zero or even negative refractive index can offer brand new possibility for acoustic imaging and sound control at subwavelength scale.Combining with transformation acoustics theory and highly anisotropic acoustic metamaterials,some special physical phenomenon,such as acoustic cloaking,can be achieved.Physical properties,boundary conditions and internal structures of the transmission medium are the determining factors for sound wave's propagation,which are the foundation of this thesis' study.The main parts of this article are as follows:1.From the perspective of adjusting the physical properties of the propagation medium,an acoustic blackhole based on temperature gradients is studied.Combining with geometric acoustics theory,a new omnidirectional acoustic absorber scheme with two-dimensional cylindrical structure is presented.The underlying mechanism is a gradual change in the relative refractive index,which dues to continuous change of temperature field in air.This method can have wide working band,since it does not involve any local resonant unit.By optimizing the locations of the heat sources,we also present acoustic focusing and bending acoustic beam.2.From the perspective of adjusting boundary conditions,acoustic radiation pattern based on acoustic metasurface is studied.By changing the locations of the periodic Helmholtz resonators(HRs),we can obtain dipole-like radiation pattern with arbitrary direction in the semi-infinite plane area,the scheme also has wide working band.The cause of such phenomenon is the existence of periodic HRs has changed the effective acoustic impedance at the corresponding spacial position,i.e.,the effective boundary condition has been altered.By virtue of such method,the wave-vector can be tailored at different directions,which contributes to the desired radiation pattern with strong directivity.This method has the ability to compress the side lobes,which improve the usage of sound energy.Besides,owing to the analogy to surface plasmons(SPs),we add periodic acoustic gratings to the rigid surface with HRs and successfully excite the collective acoustic surface wave,which leads to long distance acoustic collimation beam.All these proposals are both feasible under two-dimensional and three-dimensional circumstances.Meanwhile,we also discuss the impact of the size of acoustic gratings on the direction of radiation mode.3.From the perspective of adjusting internal structures,the article also offers a single layer of periodic plates with HRs,which can achieve good sound insulation effect.And,a sound beam split scheme aim at separating acoustic signals with different frequencies is obtained,which contains two HRs arrays with varing sizes.The last design is made to achieve the acoustic rainbow trapping effect,which requires HR array with gradual changing sizes.Most of above mentioned methods are at subwavelength scale.These works will have theoretical guidance on design and development of small scale integrated sound devices.