Abstract: Active Acoustic Structure(AAS)proposed in recent years has been viewed as an encouraging approach to actively control sound radiation from vibrating structures.Analyzing the physical mechanism of AAS can offer some guidance to key issues e.g.design optimization,the arrangements of secondary sources and error sensors,and selecting objective function.In this paper,under minimization of the total sound power output,the physical mechanism of noise reduction is investigated by analyzing the sound power output change of primary and secondary structures and the distribution of sound intensity.The results show that there are three mechanisms in active control,which are energy restraint,energy absorption and energy un-absorption.For near field sound intensity distribution,the effect of active control is revealed by amplitude restraint and direction adjusting for sound intensity.Partial acoustic energy which is transferred into far field before control will flow to sound sources after control.