Abstract: It is shown from both the numerical calculation and experiments research in the flow duct with outlet absorbing wedge that there does exist considerable noise reduction using single bypass added to the main duct, but the value and the spectrum of noise reduction sometimes are not satisfactory in the engineering practice. Furthermore, the outlet of practical duct is always connected with atmosphere directly, so there is not any absorbing wedge in the outlet. In order to resolve these two problems, a method for predicting the noise reduction in the practical duct using multiple bypasses and numerical samples are presented in the paper. It is proved correct by a site test, and there is noise reduction of 5 dB(A), even though the site test is not under the optimum situation.