| Summary: | 碩士 === 國立臺灣大學 === 機械工程學研究所 === 102 === Condenser microphones and dynamic microphones are the major products on the market. However, these microphones cannot be used under EMI and RFI environments. Optical microphones are immune to these harsh environments as the signal is transmitted by light. This thesis is to design and develop an optical microphone based on the holographic optical element (HOE) pickup head, which have many advantages including a small and compact volume, a small focusing spot size, and high displacement resolution.
Using optical simulation software and acoustic theory, this study analyzes the light path and the membrane characteristics of the microphones. We find the suitable lens system through simulation of optical analysis. Base on the reasons of simulation, we design a microphone cavity and light path modules that are high compatible. The HOE 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. By the analysis of COMSOL, the different vibration modes of the thin film are derived and used to confirm their deformation.
Integrating the optical measure system and acoustic sensors, the circular polyethylene terephthalate (PET) membrane with 4mm and 6mm diameter are used as the mediums for performance measurements. We carry out a serious of experiments for the sensitivity, the signal noise ratio, the frequency response and the frequency distortion of the holographic optical microphone. The results show that the optical microphones have high sensitivity. The sensitivities of the membranes are -41.47dB and -31.09 dB respectively. Both have the ability to measure supersonic waves with frequency as high as 30kHz. The noise ratios are 33.91 dB and 34.37 dB respectively. The frequency response of 4mm film induces the first resonance frequency of 10823.1 Hz, the second resonance frequency of 19279 Hz. The 6 mm film has the first resonance frequency of 6015 Hz, the second resonance frequency of 13339 Hz. In which distortion is achieved less than 5% under 10kHz.
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