The Investigation Of Functional Devices By Acoustic Artificial Structures

In recent years, the acoustic manipulation based on the artificial structures has been a hot research topic. By utilizing the unique properties of the acoustic artificial structures, more and more functional devices have been designed, and supposed to have broad and attractive application prospects, such as acoustic negative refraction, acoustic unidirectional transmission, acoustic cloaking, etc. Acoustic artificial structure is a kind of composite materials with designed units and can be mainly classified into two types: acoustic/phononic crystals and metamaterials. Acoustic/phononic crystals, which is an analogy to the photonic crystals, are formed by periodical heterogeneous materials and the main feature is that the geometrical parameters of the structure are comparable with the acoustic wavelength, which can bring rich band gaps. Acoustic metamaterial is another type of artificial materials, which has subwavelength size and can be regarded as effective medium by using effective medium theory. By introducing different kinds of units, the acoustic metamaterials can have unavailable parameters which can be hardly achieved by conventional materials, such as negative modulus, negative mass density, etc.As mentioned above, the investigation of acoustic functional devices based on the acoustic artificial structures are of fundamental and practical significances. This dissertation gives systematic studies on acoustic wave propagation in these artificial structures. The dissertation is divided into following sections:In Chapter Ⅰ, the previous theoretical and experimental works on acoustic artificial structures are reviewed that serve for the background of the research and the progress of the investigations on these topics is introduced. Also, for a better understanding of this dissertation, the related theories and calculation methods have been briefly described.In Chapter Ⅱ, we firstly reviewed the background of acoustic omnidirectional absorber and present two problems in the previous works, the complicated acoustic parameters and the problem that unable to be tailored. In order to overcome these problems, we propose a new structure to realize acoustic omnidirectional absorber. Based on the anisotropic acoustic parameter distribution, we design fin-like structures which are arranged periodically to guide the incident beams to the center of the model and absorber the acoustic energy effectively. Thus, we can expand the absorption cross-section of the sound-absorbing material with small size. Compared to the previous designs, this strategy has a simpler acoustic parameter distribution and can be fabricated by simple process. Meanwhile, by tailoring the size of the model or cascading more layers, this design may find applications in many situations.In Chapter Ⅲ, we firstly introduced the physical concept of diffusion and the significance of the diffusion phenomenon in different fields. Then, we take the optical case as an example to give a detailed description of diffusion and present the problem why the diffusion can be hardly achieved in acoustics. According to the previous work in optics, we proposed an ultrathin planar metamaterial to achieve the acoustic diffusion. Based on this strategy, we can scatter the incident beams into all directions to suppress the scattered lobes and achieve the acoustic energy redistribution. Besides, a practical implementation by labryinthine-like metamaterial with designed phase shift profile is demonstrated. The numerical results which include pressure distribution and directivities show that our designed planar surface could be used to mimic a rugged surface as expected.In Chapter IV, we briefly reviewed the background of acoustic unidirectional transmission and present some representative papers in this field. By allowing the acoustic waves to propagate in one direction and blocking in the reverse direction, the acoustic unidirectional transmission may have potential applications in nondestructive test and ultrasound imagine. Differentiated by the implementation means, the previous approaches can be divided into two categories:nonlinear method and linear method. In the cutoff direction, both these two method could have a minimal transmission, however, in the conducting direction, both these two method have limitations and cannot recovery the incident waves. To solve these problems, we design and experimentally demonstrate a broadband yet compact acoustic diode by using an acoustic nonlinear material and a pair of gain and loss materials. Another work in this Chapter is acoustic unidirectional reflection. We have designed a one-way acoustic mirror comprising anisotropic zero- index media. For acoustic beam incident at a particular angle, the designed structure behaves like a high-efficient mirror that redirects almost all the incident energy into another direction predicted by the Snell’s law, while becoming virtually transparent to beams propagating reversely along this output path. Furthermore, the mirror can be tailored to work at arbitrary incident angle by simply adjusting its geometry.Finally, the main conclusions of the present study and the prospect for the future work are drawn in Chapter V.