This paper is concerned with the understanding of vibration characteristics of a box-type structure using the finite element method as a tool. We found that mode shapes of the box structure can be classified according to their symmetrical properties, which are defined by the relative motion between the six plate panels constituting the box. The infinite number of modes of the box-type structure is divided into six groups. Each group has common features of symmetry and similar coupling mechanisms between component panels and distribution of out-of-plane and in-plane components of vibration. Local and net-volume displacements associated with each mode can be correlated with the characteristics of the box as sound sources. Large volume displacement modes resembling the simple sound sources (e.g., monopole and dipole) are identified among the low frequency modes. The distributions of in-plane and out-of-plane (including translational and rotational vibrations at the box edges) vibration components in the modes of the box are also investigated to illustrate the energy transmission mechanism between the box panels.