| Summary: | 碩士 === 國立臺灣大學 === 機械工程學研究所 === 105 === A microphone can transduce sounds into analyzable signals, however, the major products like condenser microphones and dynamic microphones cannot be used under EMI and RFI environments due to its operating principle of transmitting signal by charges. Optoelectronic microphones are immune to these harsh environments as the signal is transmitted by light. This thesis is to design and develop an optoelectronic microphone based on the astigmatic optical pickup head, which has many advantages including a small and compact volume, a high sensitivity, and high displacement resolution.
To understand how diaphragm parameters affect the resonant frequency and displacement, we used Ansys to simulate the variation trend of resonant frequency and displacement of diaphragm to the variation of length, thickness, and tension. Base on the simulation, we designed the microphone structure which is highly compatible to the diaphragm. The astigmatic pickup head measures the S-curve and the linear response region of the microphone, which can then be used to obtain the relationship between focus error signal and displacement.
The two rectangular diaphragms with 6 mm width and 0.02 mm thickness are made of polyethylene terephthalate (PET) and steel and used as the mediums for performance measurements. We carried out a series of experiments for the sensitivity, the signal noise ratio, the frequency response and the total harmonic distortion of the optoelectronic microphone. The results show that the optoelectronic microphones have high sensitivity. Given the length of 10mm and the tension of 50 N/mm2 for both diaphragms, the sensitivities of the PET and steel diaphragm are -26.91 dB and -27.89 dB respectively, and the signal to noise ratios are 21.02 dB and 19.03 dB respectively.
The frequency response of PET diaphragm induces the first resonance frequency of 7240 Hz and the working frequency of 198 - 6209 Hz. The steel diaphragm has the first resonance frequency of 4318 Hz and the working frequency of 136 - 4598Hz. In which distortion is achieved less than 10 % under both working frequencies. Both diaphragms have the ability to measure supersonic waves at frequency of 21 kHz with low distortion.
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