The Characteristics Of Asymmetric Wave Propagation In Acoustic Artificial Materials

Recently, the investigation of acoustic artificial materials has been given more attention. Acoustic artificial materials are usually artificial composite structure or composite materials, which can be categorized into phononic crystal and acoustic metamaterials. The difference between them is that, the variation scale of embedded structure or material in phononic crystal is comparable with the acoustic wavelength, while the variation scale in metamaterials is much less than the acoustic wavelength. Henceforth, phononic crystal and acoustic metamaterials have their own characteristics in acoustic properties. The study of acoustic wave in phononic crystal is mainly focused on the propagation of body wave, surface acoustic wave and Lamb wave, also including the acoustic forbidden band gaps. For Lamb wave phononic crystal, incident Lamb waves are strongly affectted by Bragg scattering, which may lead to total reflection when the carrying frequency is located in the band gap. The property of wave blocking in Lamb wave phononic crystal has great potential application in designing high performance microstructure acoustic filter and resonating cavity. For acoustic metamaterials, it can be regarded as a bulk of "effective medium". By introducing various microstructure units, acoustic metamaterials can render many novel physical properties, such as anisotropic density, single negative acoustic parameter (locally resonant material) and double negative acoustic paramenter (left-handed material). The material with anisotropic density can be employed to realize arbitrary acoustic waveguide and acoustic cloak; locally resonant material can be used to block "low frequency" sound in a small scale; left-handed material can be exploited to realize evanescent wave magnification, acoustic super-lenses and magic acoustic illusions. As mentioned above, the investigation of unique characteristics of wave propagation in acoustic artificial materials is of fundamental learning significance, which has great application value in practice without saying. This dissertation gives a systematic and indepth theoretical study on asymmetric wave propagation in acoustic artificial materials.In Chapterâ… , the previous theoretical and experimental works on acoustic artificial materials are reviewed that serve for the background of the research. The progresses of the investigation on the acoustic wave propagation in phononic crystal and acoustic metamaterials are introduced respectively. The development of transformation acoustics is briefly described.In Chapter II, the related theories on the acoustic wave propagation in acoustic artificial materials are introduced. More attention is paid to the discussion of numerical calculation of band structure, transmission spectrum and displacement field in Lamb wave phononic crystal, as well as the fundamentals of left-handed material, anisotropic density and transformation acoustics.In Chapter III, the influence of topological structure on band structure in periodical Lamb wave phononic crystal is studied. First, the phenomenon of "acoustic band pinning" is introduced, in which the dispersive curves are pinned in pairs at the boundary of Brillouin zone. Further study shows that, acoustic band pinning can lead to something more different, such as quasi-Rayleigh wave, which exhibits very plentiful physical contents. In the end, the effect of strong inter-mode conversion is discussed qualitatively, which is exploited in designing the linear "one-way plate".In Chapter IV, the development in the domain of acoustic cloaking is briefly introduced. Then we discuss in depth how to exploit Super-lenses and normal lenses to realize the "non-double-blinded" acoustic cloaking.Finally, the main conclusions of the present study and the prospect for the future work are drawn in Chapter V.The principal contribution of the present study is summarized as below:1. A new design of "acoustic cloak" is proposed, which requires only the "single negative" materials to effectively realize the "non-double-blinded" acoustic cloaking. Since the parameter of cloak is absolutely independent with the parameter of surrounding environment as well as the cloaked object, the technical difficulty in experimental realization is largely reduced. With the "non-blinded" cloak, the object can undistortedly observe the surrounding environment without being targeted.2. A new general cloaking strategy is proposed, in which the material parameter is positive with no singularity and a constant anisotropic density. Since there is no resonanting unit, i.e. negative index material, it is possible to realize the broad-band acoustic cloaking in experiment.3. A one-way Lamb wave transmission can be obtained by using the maximum mode converting effect due to the "band pinning" phenomenon in the anti-symmetric phononic crystal plate. This one-way model can realize the one-way tunnel for the elastic wave under linear condition, which can be integrated on-chip and employed to realize the logic and arithmetic processing with simple structure and competitive advantage in converting efficiency.