| Summary: | 碩士 === 朝陽科技大學 === 建築及都市設計研究所 === 99 === This study is designed to estimate the absorptive coefficient of porous material by applying cross-correlation method in an anechoic chamber. In previous study, we applied M-sequence (MLS) to generate the sound source which is served as for the tests and as the basis for calculating cross-correlation between direct sound and the first reflection. However, we could only evaluate the absorptive coefficient through correlation pulse beginning from 250 Hz to the higher frequency. In this study, the absorption coefficient of the porous material in a full frequency range (125 ~ 4000 Hz) can be announced, if we further adopted spectrum analysis to improve the problem of detecting absorptive coefficient at the lower frequencies range (≦ 250 Hz).
The “Cepstrum” analytical method is further adopted to refine the energy generated at lower frequencies of which were calculated by using cross correlation pulse. The spectrum of pulse responses were gathered from four porous materials in an anechoic chamber, which are quickly transformed by IFFT (inverse fast Fourier transform). We found that “Cepstrum” technology is able to calculate the energy of pulse response spectrum at lower frequency range and improve wave form which was easily identified the reflective energy. Inversely, errors exist while we calculated the pulses at higher frequency. Therefore, the “Cepstrum” technology adopted at lower frequency (125Hz, 250Hz and 500Hz) for estimation of the absorptive coefficient using cross correlation in the anechoic chamber have been confirmed as an approach and an effective analysis.
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