| Summary: | 博士 === 國立臺灣科技大學 === 機械工程系 === 105 === To investigate the resonance modes and sound pressure of an ITO (indium tin oxide) push-pull electrostatic speaker (60-mm diameter) and free-edge-like piezoelectric speakers (73-mm diameter), we calculated the sound pressure of an electrostatic circular membrane and the eigenfrequencies of piezoelectric circular plates, as well as employed four evaluation methods. In this study, lumped parameter measurement (LPM), distributed parameter measurement (DPM), amplitude-fluctuation electronic speckle pattern interferometry (ESPI) and acoustic measurement (AM) were employed from 20 Hz to 20 kHz. Measurements of displacement amplitude and vibrating mode were used to predict sound pressure levels (SPLs) based on LPM and DPM results. Comparisons with measured SPL values of AM were used to verify our predictions. The experimental results demonstrate the electrostatic and piezoelectric speakers, without additional acoustic mesh damping, both produced numerous resonant frequencies. The LPM, DPM and AM produced the SPL values of each mode that were in good agreement. Under linear operations, the DPM and ESPI techniques proved effective in determining the visualization of axisymmetric mode shapes. The piezoelectric speakers produced jagged SPL curves with peaks steeper than those of the electrostatic speaker. At mode (0,1), the electrostatic speaker was affected by acoustic resistance, which resulted in the following quality factor (Q) values: Q = 9.8 without mesh and Q = 1.8 using a mesh with specific acoustic resistance of 145 [rayl]. By contrast, the piezoelectric speakers at modes (0,1)–(0,4) were unaffected by acoustic resistance, which resulted in Q values of 10–11, regardless of whether mesh was applied. These characteristics make electrostatic speakers suitable for headphones and piezoelectric speakers suitable for audio signaling devices.
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