Investigations Of The Properties Of The Double-negative Acoustic Metamaterials And Metasurfaces

Metamaterials are artificial composite structures,which normally consist of periodically arranged structural units.The dimension of the unit is much smaller than the working wavelength.If the geometrical structures of the units are elaborately designed,the constructed metamaterial will macroscopically exhibit effective properties that cannot be realized by natural materials.The research of metamaterials originated from the electromagnetic field,and the presented electromagnetic metamaterials induced many extraordinary properties,such as negative refraction,inverse Doppler effect,subwavelength imaging and cloaking et al.Since metamaterials have such distinct functions,the international top magazine Science has twice listed it into the ten scientific and technological progress of the year.Thus it can be seen that metamaterials have become a research focus in the early 21 st century.As the acoustic wave shares most of the characteristics with the electromagnetic wave,the study of metamaterials is also introduced into the acoustic field;and great progress of the acoustic metamaterials have been made during the past ten years.In this dissertation,on the basis of two previous ‘meta-atom' models,we creatively proposed a double-negative acoustic metamaterial by using a ‘meta-molecule' model.Furthermore,we designed a ‘meta-molecule' cluster set,and realized broadband double-negative characteristics with both experiments and simulations.In addition,on the basis of elastic membrane and cavity structure,we designed ultra-thin transmission type and reflection type metasurfaces,and further presented an ultra-thin skin cloak.The main results of this dissertation are listed as follows:1.An acoustic ‘meta-molecule' model with double-negative material parameters was proposed,and the relationships between the double-negative frequency range of this ‘meta-molecue',the tube length and the the side hole aperture are studiedOn the basis of previously proposed two acoustic single-negative ‘meta-atom' models,we integrated them into a ‘meta-molecule',and creatively realized a double-negative metamaterial with only one structure.Besides,we designed a systematic and universal test method in free space,which breaks the limit that the acoustic property can only be obtained by using an impedance tube.The method of retrieving effective parameters was used to obtain the refraction,mass density and bulk modulus.These three parameters were demonstrated to be simultaneously negative from 5.38 kHz to 5.94 kHz.We further experimentally verified the planar focusing and negative refraction effects of the proposed metamaterial.By using the simulation software(COMSOL Multiphysics 4.3b)and the constructed experimental platform,we studied the shift of the double-negative frequency region with the tube length and the side hole aperture of the ‘meta-molecule'.We found that the double-negative frequency region moves to lower frequency when the tube length gets longer or the side hole aperture gets smaller.Furthermore,we tried to combine the ‘meta-molecules' with different dimensions to achieve multiband double-negative metamaterials,and laid a good foundation for the design of broadband double-negative metamaterials.2.A kind of acoustic ‘meta-molecule' cluster set was presentedThis model consists of two clusters which are respectively composed of seven ‘meta-molecule' with different dimensions.On the basis of this model,we experimentally fabricated a ‘meta-molecule' cluster set sample.The measured and simulated results demonstrated that the effective refraction,mass density and bulky modulus of this model are simultaneously negative in an ultra-broad frequency range.Furthermore,we experimentally verified the inverse Doppler effect of this model from 1.186 kHz to 6.534 kHz.3.A kind of 2D acoustic ultrathin transmission-type metasurface was presentedBy combining the elastic membrane and cavity structure,we designed a ‘drum-like' subunit cell.The physical principle of this subunit to control transmission phase is theoretically explained.The simulations also verified that the width change of the membrane is sufficient to shift the transmission phase from 0 to 2?.Metasurfaces constructed with this subunit show efficient performance in altering the travelling direction of the sound wave.The working frequency of the proposed metasurface is from 3.45 kHz to 3.7 k Hz.As the incident angle changed from-90° to 90°,the metasurface shows many abnormal phenomena which cannot be realized from natural materials but can be predicted by the generalized Snell's law.By appropriately optimizing the configurations of subunits,the steering of transmitted wave trajectory is demonstrated and some extraordinary ultrathin planar acoustic devices are realized at 3.5 k Hz,such as planar acoustic axicon and acoustic lens,both of which show good stability oblique incident waves with the angle smaller than 40°;the conversion from spherical waves to plane waves;the transformation from propagating waves to surface waves;and remarkably,we realized focusing sound waves after they bypass an obstacle that is 6? larger than the wavelength.4.A kind of acoustic 2D ultrathin reflection-type acoustic metasurface was presentedWe designed a kind of ultrathin reflection-type acoustic metasurface.The subunit of this metasurface is constructed by a cavity coupled with a membrane.We did the feasibility study on this model to construct metasurfaces by theoretical derivation;the influence of physical parameters and geometrical parameters on the acoustic properties of this model was also studied theoretically and verified by simulations.The relationship between the abnormal reflected angle,phase gradient and incident angle is detailed described.The working frequency of constructed metasurface is from 3.3 kHz to 4.0 kHz.We also realized the planar focusing effects by elaborately arranging the subunits on the metasurfaces,including an axicon and lenses.5.A kind of acoustic ultrathin skin cloak was presentedOn the basis of the presented reflection-type metasurface,we designed an ultrathin skin cloak and realized to cloak objects with nearly arbitrary 2D shapes.This cloak can completely compensate the wave front discrepancy generated by the scattering of the hidden object because the subunits are capable of arbitrarily modulating the reflected amplitude and phase.Therefore,the hidden object would be invisible to detectors that are sensitive to both the wave amplitude and the wave phase.The operating frequency ranges from 3.54 kHz to 3.93 kHz.Besides,the cloak's thickness is only approximately ?/10,so that the constructed cloak can be put on an object in almost any shape without obviously enlarging the size of the whole system.