Abstract: The noise reduction in ducts with.flow is a very useful engineering problem. Sometimes the noise is so strong that the acoustical problem becomes nonlinear. In this paper, a nonlinear method with numerical samples is presented to numerically simulate the noise reduction using bypass in ducts with now for strong noise sources. The key problems are formulating and solving the now relations at joint points. Numerical results show that the present method is suitable for both non-linear and linear acoustical problems of noise reduction by anti-sound using bypass in ducts with flow.