The Research Of Manipulating The Propagations Of Acoustic Waves By Employing Metamaterials And Boundary Conditions

In the past decade,metamaterials have drawn extensive attention.Metamaterials,which breaks through the limitation of medium in nature,could be applied to manipulate the acoustic wave in a flexible way.The appearance of metamaterials induce many exotic phenomena,which do not exist in natural materials,such as negative Doppler effect,negative refraction,etc.Here,we apply two ways to manipulate the propagation of sound.One is that the sound propagates in the waveguide system with acoustic metamaterials embedded with defects.The other is that the propagation of sound is manipulated by acoustic boundary.In Chapter I,we review the background of the researches in metamaterials field,and the development history of this research field,and then we present a brief outline of the paper.In Chapter ?,we theoretically investigates the reflective properties of splitting and squeezing devices based on a zero-index metamaterial(ZIM)waveguide embedded with defects.This structure can guarantee no reflections,making it possible to build reflectionless splitters and squeezers.Moreover,the proposed theory has been used to couple input and output waveguides with different cross sections and to achieve an arbitrary angle of exotic transmission.The proposed defect structure can offer potential advances in cloaking,splitting,and squeezing technologies without restricting the object's shape.The control of exotic transmission can occur by incorporating tunable parameters into the defects and the ZIM.In Chapter ?,we know that total reflection or transmission of waves has been explored extensively in different types of zero-index metamaterials(ZIMs).Almost all previous studies have shown that only monopole mode is excited inside the defects if they are cylindrical.However,the underlying physics for excited modes inside defects is wrongly ignored.In this work,we reveal that there is not only a monopole mode but also additional higher modes excited inside cylindrical defects by a system of two-dimensional waveguides.Actually,the total acoustic transmission and additional higher modes can occur simultaneously in a ZIM waveguide embedded with defects.We reveal the physical mechanism of excited higher modes,which will perfect the current perception of the excited modes in the acoustic metamaterials inside defects.In Chapter IV,we report a finite array of dipole sources,which is used to enhance the directivity and radiation gains of sound in the meantime,it is demonstrated that this idea can be realized as several sub-wavelength slits in a plate with periodic Helmholtz resonators.Moreover,the array gain of this structure can be further improved by adjusting the effective boundary impedance.The directivity of the dipole array is compared with that of a point array,and the advantages of the dipole array are revealed.Prospective applications of this mechanism include achieving directional radiation in high-gain loudspeakers and ultrasonic medical instrumentation.In Chapter V,we review the whole text and give some prospections in this research field.