The active triple sound insulation structure using planar loudspeaker as the secondary actuator can be easily implemented and has better sound insulation performance in the low frequency range. The key problem encountered when implementing such a control system is to sense the error signal which should be highly correlated with the radiated sound power. In this paper, the theory of sensing the radiation modes of simply supported beam using polyvinylidene fluoride (PVDF) arrays used in one-dimensional case is extended to two-dimensional structure, and then it is used in triple panel structure to optimally design the error sensing strategy. Based on the specific rule of sound energy transmission through triple panel structure, some key problems encountered in realizing the sensing system such as selection of the objective function, optimization of the number of PVDFs and implementing the sensing system, are analyzed thoroughly. The results obtained demonstrate that due to the fact that the majority of vibrating energy of radiated panel is stored in several limited number of panel modes, the amplitude of the first three order radiation modes can be obtained by simply summing the limited number of weighted PVDF film current output. And the weighted coefficient is fixed. The amplitudes of the radiation modes sensed by the proposed method are in good agreement with the theoretical value. The sensing accuracy of the error sensing system can be guaranteed, and this approach highly simplify the implementation of the error sensing system.