Abstract: The active triple-layer sound insulation structure with planar source as the secondary actuator can be easily implemented and has better sound insulation capability in the low frequency range.In order to further explore the potential of noise reduction and also optimize the system design,it is necessary to investigate the physical mechanisms of such an active control system.Firstly,the theoretical model is established using vibro-acoustic coupling approach,and the response of the coupled system is also resolved.Then,the rule of sound energy transmission through triple-panel system is analyzed.Under the condition of minimizing the radiated sound power of the radiating plate,the physical mechanisms of sound insulation is investigated by analyzing the change in behaviors of energy transmission in controlled and uncontrolled conditions.Simulations demonstrate that there exist four different energy transmission paths for four different types of panel mode group in which the sound energy transmits independently.The role of the planar source and two cavities is very similar to a band-pass filter whose pass-band is different for different transmission paths.The physical mechanism lies in the fact that the energy transmission is suppressed in the pass-band after control so that the sound insulation capability of the triple-layer structure is improved greatly.This finally leads to the sound propagation being blocked.