Abstract: The results of theoretical analysis for the transmission loss and the acoustic energy flux in a duct with different by-pass length and cross section have been verified by the experiments in the mentioned paper.There exists more noise reduction by using this type of duct with a by-pass for mono-frequency noise, 1/3 octave frequency band noise and white noise. Moreover,the frequency band of noise reduction is much wider.Meanwhile for experimental data processing the "average mothed" presented in the paper is better than the previous "three point method".