Study On Acoustic Energy Regulation And Control Based On Acoustic Metamaterial

In recent decades, the realization and application of novel acoustic artificial composite materials have attracted considerable attention. The acoustic artificial composite materials generally exhibit unusual acoustic properties not found in nature due to their specially designed structures, which provides many new ideas for the study of acoustic materials. In the field of acoustic artificial composite material, phononic crystal and acoustic metamaterial are two representative concepts, which differ greatly in their structural scales and physical mechanisms. The studies of phononic crystal concentrate on the multi-scale analyses of the propagation of acoustic waves in it. The phononic crystal shows great potential in acoustic filtering and vibration isolation due to the existence of the phononic band gap in it. In contrast, the studies of acoustic metamaterial concentrate on the effective acoustic parameters of the artificial structures in macroscopic scale. With the assistance of the specially designed micro-structures, unusual properties unavailable in nature, such as zero index, anisotropic mass density, negative mass density and negative bulk modulus, can be realized. These extraordinary properties show great value for academic studies and practical applications including acoustic cloaking and sub-wavelength imaging. In this dissertation, through theoretical calculations and numerical simulations, two ways of acoustic energy regulation and control based on the acoustic artificial composite material, including phononic crystal and acoustic metamaterial, are studied in detail. Specifically, the study mainly involves enhanced transmission of acoustic energy based on solid-fluid superlattice and flow manipulation of acoustic energy based on inhomogeneous anisotropic density-near-zero metamaterial.In the first chapter, the study background and related progress on the acoustic artificial composite material are reviewed briefly and then, a brief outline of this dissertation is presented.As a preliminary study of the solid-fluid superlative, the second chapter utilizes the transfer matrix method to theoretically derive the transfer matrix of the solid-fluid superlattice under omnidirectional incidence. Based on the derivation, the band structure and the transmittance for corresponding superlattice structures are calculated. The results show that an acoustic band fracture appears in the low frequency transmission zone for a certain incident angle.In the third chapter, the enhanced transmission of acoustic energy based on solid-fluid superlattice structure is studied. By utilizing the Green’s function approach and the transfer matrix method, the dispersions of the surface mode and the energy transmittance for the solid-fluid superlattice structure are calculated respectively. The results show that the enhanced transmission occurs when the wave vector and the frequency of the incidence wave coincide with the dispersion relations of the surface mode for the solid-fluid superlattice. The enhanced energy transmittance is substantially larger than ordinary transmission in the bulk bands, and further calculated displacement fields confirm the existence of the acoustic resonant state due to the interaction of the incident waves with the interface vibrations near the entrance surface of the finite superlattice. Based on the transmission enhancement, tunable coupler can be designed for two fluids with large impedance mismatch. By adjusting the interspacing between the solid layers of the solid-fluid superlattice, active control of the acoustic tunneling can be easily achieved.In the fourth chapter, inhomogeneous anisotropic density-near-zero metamaterials (IADNZMs), with only one component of the mass density tensor near zero, have been proposed and used to manipulate the flow of the acoustic energy in designed paths. The effective mass density and wavelength of the IADNZM under normal incidence are derived theoretically. Further, a strong averaging effect on the non-zero component of the mass density tensor is theoretically and numerically demonstrated in the IADNZM. Based on this effect, the acoustic intensity vector, which represents the average direction and magnitude of the acoustic energy flow, can be manipulated by simply designing the spatial profile of the non-zero mass density component. This method provides new possibilities in controlling the acoustic intensity in almost an arbitrary way.The last chapter presents the summary of this dissertation and some prospects for future study.