Abstract: Conditions of generation of acoustic shock waves (ASW) and effects of ASW on noise reduction are numerically studied in this paper. Effects of intensity and frequency of noise sources, duct configuration, throat flow Mach number and mixed noise sources on noise reduction by ASW are discussed in detail. Meanwhile, a group of experimental data of additional noise pressure level and fluid pressure loss generated by flow in a duct is presented to explain the readability of using ASW silencers, because an additional Laval nozzle with high speed flow in it is necessary to reduce noise by ASW, which results in generation of the additional flow noise and pressure loss.