Study On Manipulation Of Acoustic Waves With Artificial Structures

In recent years, acoustic artificial structures have been attracting increasing attention due to their unique acoustic features and potential applications. According to the ratio of structural periodicity to the operating wavelength, acoustic artificial structures can be generally divided into two categories:phononic crystal and acoustic metamaterials. In general, both phononic crystal and acoustic metamaterials are composite material with periodic spatial distributions. The structural periodicity of phononic crystal is comparable with its operating wavelength, while acoustic metamaterials are usually constructed with sub-wavelength structures (viz., the dimension of the unit cells is far shorter than the corresponding wavelength). Phononic crystals get the researchers’attention first. Based on the band theory and analog with photonic crystal, the concept of phononic crystal is proposed. In the study of the phononic crystal band structure, phononic band gap, negative refraction and related phenomenon are discovered. Functional devices including sonic isolation, acoustic focusing, wave filter and waveguide are invented based on such novel phenomenon. When the operating signal moves to the low frequency region, the corresponding wavelength become far larger than the unit structure. According to effective media theory, the composite material can be treated as homogeneous medium, and the concept of metamaterial turns up. Such effective medium is often designed with unique features not found in nature, such as anisotropic mass density tensor, or negative acoustic parameters after introducing resonant components. In the same time, transformation acoustic is proposed as effective design method, acoustic metamaterials can be used to fabricate many new acoustic devices including acoustic cloak and superlens. As mentioned above, the study on the propagation behavior of acoustic waves in phononic crystals and metamaterials are of fundamental scientific significance and great application value in practice. This dissertation gives a systematic study on manipulation of acoustic waves using artificial structures from the aspect of diffraction, transmission and scattering. This dissertation is divided into following chapters.In Chapter Ⅰ, the previous theoretical and experimental works on phononic crystal and acoustic metamaterials are reviewed that serve for the background of the research. The progress of the investigation on these two topics is introduced.In Chapter Ⅱ, start from the basic wave equation, the related theories and methods on the acoustic wave propagation in artificial materials are introduced, including band theory of phononic crystal, effective media theory, theoretical basis of transformation acoustics, plane wave expansion method and multiple scattering theory.In Chapter III, from the aspect of wave diffraction, the topic of using artificial structure to manipulate acoustic waves and produce a nondiffracting frozen sound field is investigated. For a two-dimensional phononic crystal without defects, the phenomenon of three-dimensional localization of acoustic waves is discovered. By studying the wave packet assumed to consist of Bloch modes at the isofrequency surface, we predict the existence of stationary X-shaped waves at either a local top point or a saddle point of a band. Due to the bidispersive behavior of the phononic crystal, the X-wave will emerge when diffraction in one direction is counteracted by negative diffraction in another direction. The property of the wave packet with the characteristic X-shaped profile has been numerically exemplified for a particular phononic crystal with a square lattice.In Chapter IV, from the aspect of wave transmission, the topic of using artificial structure to control the propagating direction of signals and rectify acoustic energy high efficiently is investigated. An acoustic model comprising a nonlinear cavity and superlattices is proposed and the acoustic transmission properties are numerically studied. The resonance in the high-Q cavity significantly enhances the transmission efficiency of acoustic rectification, and leads to abnormal dependences of efficiency on the incident wave amplitude and the nonlinearity parameters. The results show that the transmission efficiency can be promoted by three orders of magnitude as compared with a conventional acoustic diode of the same size and, moreover, reach its maximum when both the incident wave and the nonlinearity are weak. The proposed design helps to remarkably improve the sensitivity and reduce the size for acoustic diodes, and may benefit the potential application of the resulting devices.In Chapter V, from the aspect of wave scattering, the topic of using artificial structure to camouflage one special object by transforming the scattering wave field is investigated. An "illusion cloak", that makes an arbitrary object appear to be like some other one, is proposed to manipulate the acoustic scattering field of an object located near a curved boundary, and the first experimental demonstration of acoustic illusion is presented. This prototype of illusion cloak has a simple structure comprising anisotropic materials with positive index, for which the material parameters are non-singular, homogeneous and, moreover, independent of the properties of either the original object or the boundary. Both the numerical and experimental results demonstrate that the proposed structure can generate an acoustic illusion by changing the scattering field of an object to mimic that of a particular "illusion object", or, become an "invisibility cloak" as this illusion object is chosen as a bulk of background medium. And then the scheme is extended to three-dimensional (3D). The three-dimensional broadband acoustic cloak, capable of rendering invisibility effect for objects near arbitrary curved surfaces and applicable for various practical scenarios, is designed and experimental demonstrated. With the flexibility of applying to arbitrary boundaries and detecting signals, our finding may take major a step toward the application of acoustic cloaks in reality and open the avenue to build other acoustic devices with versatile functionalities.Finally, in Chapter VI, the main conclusions of the dissertation and the prospect of the future work are presented.