Studies On Novel Acoustic Functional Devices Based On Artificial Metamaterials

Acoustic metamaterials,based on electronic band gap theory and electromagnetic metamaterials have already become one of the hottest research directions in acoustics.Acoustic metamaterials exhibit novel properties and ability to manipulate acoustic waves discretionarily.Due to artificial design and elaborate assembly,despite acoustic metamaterials are made of natural materials,they usually exhibit unique and novel properties which cannot be found in nature,such as negative or zero index parameters.Various novel phenomenon and functional devices have been realized,e.g.,negative refraction,acoustic focusing,acoustic cloaking,subwavelength imaging,which demonstrate huge application potential in telecommunication,biomedicine,industry,military and many other fields.However,the researches on acoustic metamaterials are still in progress and there might remain certain limitations and disadvantages.This dissertation mainly concerns about two problems,one is the anti-stealth technique of unknown acoustic cloak and the other is the near field limitation and working band of the acoustic subwavelength imaging device.Systematic researches and solutions are put forward by utilizing theoretical analyses and finite element numerical calculations.The dissertation comprises four chapters and described as follows:In chapter 1,we introduce the history and background of acoustic metamaterials.The domestic and overseas research achievements and application potentials are further presented.Then we brief the main research contents of this dissertation.In chapter 2,we propose a reconstructed imaging method on geometric information of acoustic cloak via time reversal and time lapse techniques,which can detect unknown cloak precisely and accurately.Scattering study on acoustic cloak demonstrates its disadvantage of narrow working band.Time reversal invariance of acoustic wave equation demonstrates that under the irradiation of broadband pulse,fairly strong scattered signals from the cloak can be used to detect the precise position,shape,size and other cloak’s geometric information after time reversal and time lapse operations.Finite element simulations study the reconstructed imaging,i.e.anti-stealth effect,of the proposed method and major influence factors on the imaging quality including number and distribution pattern of the transducers.Numerical results correspond with theoretical analyses and both show good anti-stealth effect of the method.This proposal may have potential values in military fields and other confidential occasions which require anti-stealth.In chapter 3,we propose a subsurface imaging method to lift the near field limit of traditional acoustic subwavelength imaging devices.First,by means of rigorous theoretical derivations,the holey-structured sub wavelength imaging devices can magnify the incident evanescent waves drastically due to the structure’s strong anisotropic character.The magnifying effect leads to high contrast,high resolution image of subsurface object at the center of the structure.Numerical calculations verify the proposed method and agree well with theoretical conclusions.It is demonstrated that the method allows object to be imaged can be placed at a certain distance away from the structure,which overcomes the disadvantage of near field limit of traditional imaging methods.The method can also enhance imaging contrast dramatically which traditional methods cannot obtain.Therefore,this method with high resolution,high contrast and subsurface objects are of great significance in biomedical imaging,industrial non-destructive testing and many other fields.In chapter 4,in order to overcome the narrow working band of current acoustic subwavelength imaging devices,two broadband tunable imaging methods are proposed.One method is introducing flowing background medium as an additional tunable physical field.Theoretical analyses show flowing background medium can adjust the imaging structure’s working frequency via controlling the wavelength and sound speed in the holes.Numerical calculations correspond well with theoretical results and both of them demonstrate that the working frequency red shifts as the flow speed of background medium increases.Another method is adjusting the ambient temperature.The relation between structure’s working frequency and ambient temperature is derived theoretically.Numerical results agree with theoretical derivation and demonstrate that the working frequency blue shifts as the ambient temperature increases.Since these two methods can adjust the working frequency of the structure,widening the working band and broadband subwavelength imaging can be realized.Hence,the results provide theoretical support for the application of the holey-structured acoustic subwavelength imaging metamaterials.In chapter 5,a brief summary of the dissertation and some prospects for future researches are presented.In conclusion,via theoretical analyses and numerical calculations,the dissertation mainly studies the limitations and disadvantages of two novel acoustic devices and presents several methods to deal with the problems,which are of great significance and value in both theoretical researches and practical applications for acoustic metamaterials.