Abstract: The noise reduction in ducts with flow is a very useful engineering problem. In this paper the method of noise reduction in ducts without flow using bypass is extended to the ducts with flow by the linear acoustic theory. The numerical calculation and experimental research were completed by the author. The numerical work includes formulating acoustic relations and joint conditions in the joint points of the duct and calculating spectrums of trasmission loss, i.e. TL, for various noise sources. Then the influences of main parameters on anti-sound are discussed. The experimental research was carried out for different noise sources and different flow velocities in the duct with single bypass and outlet absorbing wedge. The results of measurement are in good agreement with numerical calculation.