以倒頻譜分析改善無響室吸音係數測試法－以多孔性材料為例

Application of Cepstrum Function for the Absorptive Coefficient Measurement of Porous Material in an Anechoic Chamber

陳炯堯 ； 翁漱恆

《中華民國音響學刊》 15期 (2011/06) Pp. 21-40

https://doi.org/10.30049/JASRC.201106.0002

摘要

本研究以半無響室(semi-anechoic chamber)中使用互相關(cross-correlation)脈衝遲延法來推估多孔性建材吸音係數(absorption coefficient)的方法為目的。此法經常被利用於求取特定方向入射於吸音面時之吸音係數。過去，我們以最大長度序列(M-sequence, MLS)製造測試音源，作為互相關脈衝計算之基礎。藉由此法得到的能量脈衝，可了解音波中直達音(direct sound)與反射音(reflection sound)之能量關係，推導出材料之吸音能量，再計算出建材之各頻域(125~4000Hz)的吸音係數。本研究我們進一步使用頻譜分析方法，解決了先前研究中低頻(≦250Hz)吸音係數不易判斷之問題。即使用倒頻譜(Cepstrum)分析法以傅立葉逆轉換(IFFT)將脈衝轉為脈衝頻譜；再針對四種多孔性吸音材量測，將實驗結果與先前研究之互相關法及迴響室法做比對，發現倒頻譜技術可改善先前低頻波形難以辨別的缺陷。但是於高頻之實驗結果與迴響室法有一定的誤差，故本研究確認倒頻譜分析法僅於低頻(125Hz、250Hz、500Hz)吸音係數計算之結果與迴響室法吻合。

關鍵字

互相關 ； 最大長度序列 ； 無響室 ； 吸音係數 ； 倒頻譜

並列摘要

This study is designed to estimate the absorptive coefficient of porous material by applying cross-correlation method in an anechoic chamber. It was usually utilized for estimating absorption of a certain incident angle of the absorptive surface. 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 (125 Hz, 250 Hz and 500 Hz) for estimation of the absorptive coefficient as that gathered in a reverberation room using cross correlation in the anechoic chamber have been confirmed as an approach and an effective analysis.

並列關鍵字

cross-correlation ； M-sequence ； anechoic chamber ； absorption coefficient ； Cepstrum