| Summary: | 碩士 === 國立臺灣大學 === 應用力學研究所 === 96 === In this thesis, we present a comprehensive analysis of localized defect modes in two-dimensional finite-size phononic crystals by means of the multiple scattering theory (MST). Based on the theory, we calculate the dispersion diagram, the transmission coefficients, the scattering cross sections, and the elastic field distribution. We find that the far-field analysis based on the scattering cross section can show resonance peaks owing to the resonant tunneling through the defect. Further more, the appearance of a flat plateau feature in the scattering spectra coheres well with the band gap phenomenon. Based on the MST analysis, we also propose a directional enhanced acoustic source by introducing a tunable defect mode. A simple geometrical construction gives a very comprehensive guideline to obtain an enhanced emission with angular confinement from the phononic crystals. We demonstrate that the operating frequency of defect modes and the number of surrounding layers of the cavity are the key factors to the directivity and the amplification ratio of the directional enhanced acoustic source.
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