| Summary: | 博士 === 國立成功大學 === 機械工程學系碩博士班 === 99 === Sonic crystals (phononic crystals) are periodic elastic composite materials. Such artificial crystals can exhibit acoustic or elastic band gaps in which sound and vibration are all forbidden in any direction, giving rise to prospective applications such as elastic/acoustic filters and noise/vibration isolations. One particularly interesting aspect of sonic crystals is the possibility of creating crystal defects to confine the elastic/acoustic waves in localized modes. Because of locally breaking the periodicity of the structure, the defect modes can be created within the band gaps, which are strongly localized around the local defect. The point defect is created by removing a single rod from the middle of the perfect periodic structure. There exist the defect bands in the absoulate band gap. The acoustic wave can propagate through the sonic crystal, since the defect band acts as a pass band in the band gap. The point defect can also act as the resonant cavity. At the frequency of the defect band, which is the resonant frequency, the acoustic waves should be localized in the resonant cavity and the pressures in the cavity are enhanced.
The plane wave expansion method and supercell calculation are adopted to calculate the band structure of the sonic crystal with a point defect. And, the finite element commerical software is employed to obtain the acoustic pressure in the middle of the cavity and transmission spectra of the sonic crystal. The effects of sizes and filling fractions are investigated, and the quality factor of the cavity is discussed. The transmission spectra and pressures in the defect of the sonic crystal are measured experimentally. The wave propagation of a two-dimensional sonic crystal with a local resonant defect is also studied. The Helmholtz resonantor is placed at the point defect to be a local resonant defect. Band structures are calculated by using the finite element commerical software with periodic boundary condition. Band structures of the sonic crystal with a cavity, circular and Helmholtz resonant defect are discussed and compared. The transmission spectra are measured experimentally. The defect mode characteristics of the sonic crystal with a defect can be use in the implementations of new acoustic filters.
The development of an acoustic energy harvester using the sonic crystal and the piezoelectric material is presented. A point defect is created by removing a rod from a perfect sonic crystal. The point defect in the sonic crystal is acted as a resonant cavity, and the acoustic waves at the resonant frequency of the cavity can be localized in the cavity. The power generation from acoustic energy is based on the effect of the wave localization in the cavity of the sonic crystal and the direct piezoelectric effect of the piezoelectric material. A model for energy harvesting of the piezoelectric curved beam is also developed to predict the output voltage and power of the energy harvesting. The experimental results are compared with the theoretical ones. By using properties of band gaps and wave localizations of the sonic crystal, the noise control and energy harvesting can be achieved simultaneously.
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