Structure-borne sound is the vibration energy which transmission in the vibrating engineering structures generated by external dynamic loading. This vibrational energy is radiated eventually into an acoustic medium as noise (Cremer, 2005). Understanding the paths of energy which flow from a vibration source to other parts of a structure helps an engineer to pinpoint and correct vibration problems. Energy flow in a plate structure has been investigated by many researchers. Bernhard (1990) firstly developed a set of equations which govern the space and time averaged energy density in plates to investigate the mechanisms of energy generation, transmission and absorption. Further investigations of the energy model of plates which proved that the vibration conduction equation is valid for the twodimensional vibration field composed of plane wave components (Bouthier,1995; Ichchou,1996) As waves propagate through dynamically loaded mechanical structures, they encounter changes in structural junctions. Cuschieri and McCollum(1996) used a mobility power flow (MPF) approach to study the structural energy flow through the junction between two flat plates coupled in an L-shaped configuration for both in-plane (longitudinal and shear) and out-of-plane (bending) waves" propagation. Li Tianyun et al (1997) also used the mobility approach to study the input vibrational power flow and the transmitted vibrational power flow for an L-shaped stiffened plate excited by a concentrated harmonic force. Wang and Xing (2002) formulated a substructure approach to investigate the power flow characteristics of an L-shaped plate. Kessissoglou (2004) studied the power flow propagation in plates connected by an L-joint in both low and high frequency ranges, and derived an exact solution to describe the flexural, longitudinal and shear wave motions in the plates. Zhou Ping and Zhao Deyou (2008) calculated the vibration energy ratios of a stiffened panel in the mid and high frequency regions based on the dynamic stiffness method.

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