| Summary: | 碩士 === 國立臺灣大學 === 應用力學研究所 === 94 === In this thesis, we propose a design of a highly amplified directional acoustic source. The calculations in our work are based on the finite difference time domain (FDTD) method. A parallelized computation program with a message passing interface (MPI) is written and executed on a PC cluster system with 8 CPUs. The program is then adopted to calculate the dispersion relations, the transmission coefficients and the elastic field distribution throughout this thesis. We find that the order of the cavity resonant mode and the reflection coefficient of the phononic crystal slab are the key factors in designing a directional acoustic amplifier. In the design, the first order resonant mode of the cavity is highly recommended for obtaining a much higher amplification ratio. To obtain directional acoustic source, the first order resonant mode has to be tuned so as to be located in the complete band gap. Beyond that, the first resonant frequency is required to match with the highest reflection coefficient of the phononic crystal slab to obtain the highest amplification ratio. On the other hand, we demonstrate that a highly directive radiation source operates at the band edge of phononic crystals without requiring defect modes. The radiation pattern of a point source embedded inside phononic crystals strongly depends on the frequency and the crystal size. The findings of our study may be employed to improve the performance of certain devices such as sonars.
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