The Investigation Of The Manipulation Of The Property And Boundary Of Acoustic Materials By Acoustic Artificial Structures

Metamaterials is a hot research topic at present. Especially for the recent ten years, the developing of material science and the mature of microfabrication technology provide us a feasible way to design and process materials in micro scale, which leads the birth of metamaterials, a brand new research field. The appearance of metamaterials make people design material cell in very free ways, which induces many exotic phenomena which do not exist in natural materials, such as negative refraction, negative Doppler effect, backward harmonic emission, etc. Metamaterials raise people’s understanding of materials to a higher platform.The rise of acoustic metamaterials follows the development of electromagnetic metamaterials. By analogy, researchers introduce the research method in electromagnetic metamaterials to acoustics, and then the acoustic metamaterials appear. The previous researches on acoustic metamaterials are based on the linear assumption. As the research works goes further, people extend the acoustic metamaterials research to nonlinear field and nonlinear acoustic metamaterials appear. Because the properties of materials and boundary conditions are the two most important factors of the existence of acoustic waves, the researches of material science enlighten people’s thinking of boundary condition, and then metasurface appears. This article presents the author’s researches on acoustic metamaterials and metasurface during his master’s programme, and the research process is very similar to the developing process of metamaterials. Firstly, the research is focused on linear acoustic metamaterials, and then the research is extended to nonlinear acoustic metamaterials. The last part is the research on metasurface. The arrangement of the whole text is as follows:In Chapter Ⅰ, we review the background of the researches in metamaterials field, the development history of this research field and the research status of this hot field.In Chapter Ⅱ, a theory is provided to show that a structure with periodic vibration plates and side holes in an ordinary waveguide can realize an acoustic metamaterial with negative bulk modulus and negative mass density simultaneously. Through effective medium theory and transmission matrix theory, we explain the physical mechanism of the simultaneous negative bulk modulus and negative mass density. These characteristics applied to the frequencies lower than the critical frequency determined by vibration plates fvp shows left-handed properties. Above the critical frequency determined by side holes fsH, the medium is ordinary and shows right-handed properties. When the frequency is in the range fVP<f<fSH, the sound wave is evanescent. We also extend the theory further to the nonlinear case and predict that the backward-traveling second-harmonic can be obtained through the nonlinear propagation of a sound wave in such a metamaterial.In Chapter III, we follow the nonlinear acoustic metamaterials researches in Chapter II. As we know, high efficiency of the second-harmonic, sum-frequency and different-frequency generation can be obtained in optical superlattice by using the conventional quasi-phase-matched method. Although this trick can be played on the acoustic wave, the media with negative nonlinear parameters are not common in acoustics. Furthermore, the quasi-phase-matched method used in acoustic metamaterials has been less studied. In this work, a protocol is provided to realize the quasi-phase-matched method by using nonlinear complementary media in acoustic metamaterials in order to obtain large backward second-harmonic generation. Compared with the conventional method, the method gains a broader bandwidth and can be used in both acoustic and electromagnetic waves.In Chapter IV, we extend the research to metasurface. Radiation pattern control has generated much interest recently due to its potential applications. Here we report the observation of high-efficiency dipole-like radiation of sound with broad bandwidth through a decorated plate with periodical two-dimensional Helmholtz resonators on both sides and a single slit at the center. The decorated plate was optimally designed to adjust the effective impedance of the boundary, and the underlying mechanism of radiation pattern control is attributed to wave vector tailoring. The high radiation efficiency is due to the Fabry-Perot resonances associated with waveguide modes in the center slit. The method to obtain a collimated beam without any sidelobe is also provided. Our findings should have an impact on acoustic applications.In Chapter V, we review the whole text and give some prospections in this research field.All in all, the development of science and technology will promote the researches in acoustics. As a new research field, acoustic metamaterials will develop quickly. More and more exotic phenomena and new findings will appear in this research field.