Abstract: There are in total three energy mechanisms for active noise control:acoustical energy absorption,spatial reflection and reactive storage respectively.In this paper,the acoustical energy distribution and the radiation of primary and'secondary sources in a duct with active noise attenuator of monopole secondary source are analyzed and discussed theoretically.The results show that when the distance between primary source and the secondary changes from very long to very short,the predominant mechanism for such a duct system also varies from the reflection form to the storage one,by which it is demonstrated that the energy storage mechanism of multipole sources always accompanies either of the other two.