Research On Novel Acoustic Devices Based On Metamaterials

As a hot topic in acoustics, the realization and application of acoustic metamaterial have attracted wide attentions. Acoustic metamaterial is a composite or structured material that exhibits extraordinary transmission properties not found in naturally occurring materials or compounds. Its transmission properties mainly depend on the modulation effect of special microstructured units in subwavelength scale, which further leads to the rich physical phenomenon such as negative refraction, reverse Doppler shift, etc. These novel properties greatly advance the conventional principles of acoustic theory. Therefore, we can freely control the propagation of sound wave and design novel acoustic functional devices with the assistant of revolutionary design concepts, such as subwavelength imaging, acoustic cloaking, sound beaming, etc. In this dissertation, some novel acoustic devices are proposed based on metamaterial and the acoustic transmission properties of these devices are studied with theoretical and numerical methods systematically. We focus on a negative refraction induced acoustic cloaking and its effects of concentration, imaging, and mirage, acoustic cloaking realized by layered effective medium, acoustic omnidirectional superabsorber with arbitrary contour, acoustic transmission in zero-index metamaterials with defects. The dissertation consists of six chapters:In chapter one, we give an introduction for the related experimental and theoretical background, the research progress on metamaterial, and a brief outline of the dissertation.In chapter two, we propose a three-dimensional acoustic cloaking based on transformation acoustics. The finite element method (FEM) simulations, geometrical acoustics, and rigorous scattering theory are used to investigate the proposal. The coherent numerical and theoretical results show that the proposal has perfect acoustic cloaking effect, which is ascribed to coordinate transformation effect induced by the double negative acoustic parameters. The cloaking can be regarded as an acoustic concentrator with much higher than that of traditional concentrator made by positive-index materials with the same size. Besides, the cloaking shell can also function as an acoustic spherical magnifying superlens, which produces perfect same-shaped image with bigger geometric and acoustic parameters located at a shifted position. Based on the imaging effect, we further propose an intriguing acoustic transformer which can transform the sound scattering pattern of one object into another object arbitrarily.In chapter three, we propose an acoustic cloaking made of concentric alternating layered structure with homogeneous isotropic materials. The FEM simulations and rigorous scattering theory are used to investigate the pressure field distribution and far-field scattering patterns. The influences of wavefront shapes, layer thickness, wavelength, and properties of cloaked object on cloaking performance are discussed in detail. We find that the proposal works efficiently in a wide bandwidth in which the cloaking efficiency decreases with increasing frequency. The working frequency of the cloaking depends on the thickness of the layered shells. The acoustic wave can pass through the cloaking shell with unchanged wavefront shape, which endues the cloaked object with duplex communication ability.In chapter four, we propose an arbitrarily-contoured acoustic omnidirectional superabsorber (AOSA) based on transformation acoustics. The guiding shell with negative-index endues AOSA with expanded absorption cross section. Square AOSA with anisotropic parameters and circular AOSA with isotropic parameters are demonstrated. The consistent results by numerical simulations and theoretical derivations show that the AOSA can completely absorb the incident acoustic waves with arbitrary incident angle in wide bandwidth. A ventilated sound insulation window, as an interesting application of the superabsorber, is further discussed.In chapter five, the acoustic wave propagation is investigated in the zero-index metamaterial (ZIM) waveguide embedded with defects, in which the acoustic total transmission, total reflection, and partial transmission are obtained. The transmission coefficients of two types of ZIMs with various defects are derived. Theoretical derivations show that the transmission coefficients are determined by the frequency, the acoustic parameters of the host medium, and the acoustic and geometric parameters of the ZIM and the defects, suggesting arbitrary control of acoustic transmission by properly tailoring these parameters. The FEM simulation is also used to investigate the transmissions, the results agree well with theoretical derivations. Moreover, acoustic super-reflection and cloaking are obtained in the proposal. The underlying physics of these interesting effects are determined by the intrinsic properties of ZIMs, i.e., in steady state the acoustic pressure field in the ZIM is static without phase variation in space.In chapter six, a summary of this dissertation and some outlook for future investigation are presented.In summary, we propose some novel acoustic devices based on metamaterials and systematically investigate the acoustic propagation properties in these devices by theoretical and numerical methods. We combine the research on the acoustic metamaterials and the design of novel functional devices, which are meaningful in perspective of both theoretical exploration and practical application.