Investigations On Broadband Characteristics And Applications Of Helical-like Acoustic Metamaterials

The transmitting characteristic of the sound wave in fluid such as air and water is influenced by the two constitutive parameters,i.e.mass density and the bulk modulus.In order to realize arbitrary manipulation of acoustic waves,it is greatly important to obtain acoustic materials with various exotic constitutive parameters.Due to the physical properties that go well beyond those of natural materials and great advantages in size,metamaterials have become one of the research focuses in the field of new materials in recent two decades.The properties of the metamaterials are governed by the parameters of the meta-atom structures such as shape,geometry,size rather than their base materials.By carefully designing and engineering these parameters,various exotic constitutive parameters such as negative mass density,negative bulk modulus,double negative and near-zero material parameters,can be realized,enabling fascinating phenomena such as acoustic super-resolution imaging based on metamaterials with negative parameters and perfect absorption of sound waves and acoustic cloaking based on anisotropic metamaterials.As mentioned above,the investigation of acoustic characteristics of wave propagation in acoustic materials keeps great theoretic significance and application value which not only enriches the form of manipulation of sound waves but also establishes a new material platform to build new devices.However,most realized acoustic devices based on metamaterials rely on resonant "atoms" and therefore suffer narrow-band and strong dispersion problem.Also too much variety of "atoms" adds to the difficulties in design and production of acoustic devices.These shortcomings of above-mentioned metamaterials limit greatly their application potential in reality.In order to overcome these disadvantages of metamaterials,in this dissertation,a type of helical-like acoustic metamaterials with spatially varying pitch is presented and the non-dispersive nature of helical-like metamaterials is verified by experimental and numerical results.Based on above-mentioned helical-like metamaterials,some kinds of acoustic devices have been fabricated and its applications in the manipulation of acoustic wavefront are studied.The dissertation is divided into following sections:In Chapter ?,several common acoustic metamaterials and the progresses of the investigation and the challenges on its application are introduced.The related theories and methods for calculating the effective parameters of some simple metamaterials have been briefly described.In Chapter ?,we present the concept of acoustic metacoupler for broadband impedance match.The background of impedance match materials and current problems in development are reviewed firstly.Then we propose to design an acoustic metacoupler for broadband sound energy transfer between two media with large mismatch in both acoustic impedance and cross section.A helical structure with spatially varying structural parameters to mimic an inhomogeneous effective medium is designed.Numerical results verify the effectiveness of our design in achieving near-unity coupling efficiency in a broad band,showing that our scheme allows for effective coupling between two media with larger impedance mismatch.In Chapter ?,the dispersionless manipulation of sound waves based on helical-like metamaterials with spatially varying pitch is studied.We firstly reviewed the background of the manipulation of acoustic wavefront briefly and present the some problems in previous works such as narrow operation bandwidth and strong dispersion.Then we present a novel mechanism for realizing near-dispersionless manipulation of transmitted acoustic wave.To achieve this goal,a nearly linear phase delay response is needed based on analytical derivation of the desired phase response for eliminating the frequency dependence.We demonstrate the effectiveness of our proposed device both numerically and experimentally via two distinctive examples of generating anomalous refraction and convergence of acoustic energy.In Chapter IV,the manipulation of transmitted wavefront for airborne sound based on helical-like coding metamaterials is studied.We firstly reviewed the background of coding metamaterials briefly and point out that acoustic devices based on coding metamaterials have many advantages such as simple design and convenient manufacture and processing.Even so,the present coding metamaterials still have some disadvantages such as narrow operation bandwidth.To solve this problem,we design eight different coding units which have constant phase differentials based on helical-like metamaterials.Based on these coding units,several transmission-type coding acoustic devices in a certain bandwidth which is capable of focusing,refracting normally incident sound waves and generating acoustic vortex beams are demonstrated.In Chapter ?,the main conclusions of the present study and the prospect for the future work are drawn.The principal contribution of the present study is summarized as below:1.We present the scheme of designing acoustic broadband metacouplers with sub-length size.Only two kinds of acoustic materials with large impedance mismatch are needed to resolve the impedance mismatch problem caused by different material parameters and cross sections and the technical difficulty in experimental realization is largely reduced.Also the proposed cylindrical metacoupler has a regular contour and sub-wavelength scale that helps to increase the potential for integration.2.We propose a scheme for achieving dispersionless manipulation of transmitted wavefront for airborne sound.The focal lengths and refraction angles of the transmitted acoustic waves almost keep constant as the working frequency varies within an ultra-broad frequency range.The proposed scheme also bears advantages of planar profile,lightweight and compact configuration,high transmission efficiency.Also the horizontal size of the unit cells is about 0.6 of the minimum operating wavelength and this indicates that our scheme has a higher upper frequency limit.3.Anew strategy for realizing manipulation of transmitted acoustic waves is proposed based on helical-like coding metamaterials.By using coding metamaterials,the processes of designing and fabricating acoustic devices can be greatly simplified.Compared with the previous coding metamaterials,the proposed coding units have smaller horizontal size and wider working bands.