The Investigation Of Acoustic Manipulations By Acoustic Artificial Structures

In recent years, there is a growing interest in the studies of acoustic or elastic wave propagating in acoustic/phononic crystals and metamaterials, which are composite materials consisting of artificial structures. Acoustic/phononic crystals are composite materials, which are formed by periodical heterogeneous materials, and the geometrical parameters of the structure are comparable with the acoustic wavelength. Because of the existence of frequency band gaps, the acoustic or elastic waves in this kind of structure show some special properties such that the acoustic/phononic crystals are expected to be utilized as sound filter and waveguide materials, etc. The geometrical parameters of the units in metamaterials are much less than the acoustic wavelength. Through introducing different kinds of units, the acoustic metamaterials can be effectively regarded as effective medium with some unavailable parameter in nature, such as anisotropic mass density, negative mass density, negative modulus, etc. Some fascinating phenomena, such as acoustic cloaking, acoustic super lens, etc., can be realized through these parameters Actually, the structure, which can be utilized to realize some novel acoustic phenomenon, can also be regarded as metamaterials, such as unidirectional acoustic transmission, etc. In this point of view, the acoustic/phononic crystals can also be considered to be some special acoustic metamaterials.As mentioned above, the investigations of acoustic properties of acoustic/phononic crystals and metamaterials are of fundamental scientific significance, which has great application value in practice. 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/phononic crystals and metamaterials are reviewed that serve for the background of the research and the progress of the investigations on these topics is introduced. The main contents of the present study are briefly described, such as lamb wave, surface acoustic wave in phononic crystals, unidirectional acoustic transmission and acoustic gradient-index lens.In Chapter II, we reviewed the related theories on the crystals in solid states physics, and the acoustic/elastic waves equations. More attention is paid to the discussion of numerical calculations and simulations of artificial structures, such as the plane wave expansion method, finite element method and the retrieving of effective parameters of metamaterials with complex structure.In Chapter III, we reviewed the background of unidirectional acoustic transmission and present some problems in the previous works, such as the large size stemming from the employment of acoustic crystals and the disorder of the transmitting wavefront. In order to overcome this problem, we present two systems. One is double-layered structure consisting of periodic grating and uniform plate. The one-way transmission can be realized based on the reconstruction of plane wave source. It is found that the size of the one-way device can be modulated to the wavelength scale and the angles of transmitting beams can be tuned effectively by appropriate selection the geometrical parameters. The other is a prism with near zero refractive index. Unidirectional acoustic transmission can be achieved due to the highly angular selectively of near zero index materials. The transmitted wave in this system is still plane wave and the wavefront of the transmitting wave are almost consistent with the incident plane wave.In Chapter IV, we firstly reviewed the background of the gradient-index lens. By appropriate selection of the size of the coiling structure along the direction perpendicular to the lens axis, the gradient-index lens is constructed to realize the acoustic focusing effect. Due to the high refractive index providing by the coiling structure, the lens can be designed with thinner thickness. Moreover, the more effective focusing effect can be observed due to the lower impedance mismatch.In Chapter V, we reviewed the background of the surface acoustic waves (SAWs) in phononic crystals, and find that due to the existence of uniform substrate, the SAWs modes can only be modulated below the sound cone of the substrate. To overcome the problem, we present a system consisting of a finite thickness phononic crystal plate stubbed with uniform/composite layer. Numerical results show that SAWs modes can be obtained below the sound cone and in the band gaps of the bulk waves. By introducing the uniform/composite layer, the SAWs modes both below the sound cone and in the band gaps can be tuned effectively. The attenuation depth of SAWs modes is introduced to interpret the relationship between the variation of the band structures and the geometrical parametersFinally, the main conclusions of the present study and the prospect for the future work are drawn in Chapter VI.