Sound radiation is the most important phenomenon when a structure vibrates, which is strongly dependent on boundary conditions and its shape such as aspect ratio. This paper deals with general aspects of sound radiation from this viewpoint. A particular guided and pinned condition being taken into account here, is that the cross-modal terms can average out for all possible point excitation locations. Then, the average radiation efficiency based on the modal radiation efficiency and corresponding radiation power can numerically be obtained. It is shown that the radiation power of the guided plate is governed by the piston mode as well as the critical frequency. Unlike the pinned plate, no clear corner and edge mode regions below the critical frequency may be found for the guided case. Meanwhile, if the plate damping increases the similar tendency that the radiation efficiency increases below the critical frequency is found, as the vibrational level decreases. For the strip with a large aspect ratio, the radiation power is maximised at frequencies where the wavelength is an integer times the short edge length. This can be seen for both the guided and the pinned boundary conditions.