Experimental data are exhibited for the radiation loading of a piston source enclosed in a circular finite baffle of dimensions of the order of the wavelength. A partial explanation of the data is given in terms of the direct radiated wave from the piston source, the diffracted wave arising at the edge of the baffle, and the wave arising from vibration of the baffle. The effect of flexural vibration of the piston source is shown to increase the radiation loading appreciably. Examination of the theoretical attempts to evaluate the radiation loading shows the lack of suitable expressions for calculation.