A new numerical method, referred to as the energy finite element method, had been previously developed to predict the high-frequency response of built-up structural acoustic systems consisting of subsystems such as rods, beams, plates, and acoustic spaces. The methodology for prediction of behavior in the subsystems is based on a diffuse energy field approximation. Subsystems are coupled together using net energy flow and energy superposition principles. In this paper, the structural acoustic coupling relationship for energy flow analysis of coupled plates and acoustical spaces is shown. The coupling approach chosen for structural acoustic radiation uses plate radiation efficiency. The approach has been implemented into an energy finite element model. The energy finite element predictions will be compared to measured results for an acoustical enclosure with one flexible wall.