Abstract The acoustic radiation from circular cylindrical shells is of fundamental and applied interest primarily because cylindrical shells are widely used in industry, and because their acoustic behaviour is different from that of beams and plates due to curvature effects. In previous studies of the subject, cylindrical shells have been categorized into acoustically thin and acoustically thick shells in terms of the ratio between the ring frequency frand the critical frequency fC, i.e., fr/fC<1 for acoustically thin shells, and fr/fC>1 for acoustically thick shells. For acoustically thin shells, it has been found by statistical methods that the radiation efficiency has a peak at the ring frequency. Above the ring frequency, the shells behave like flat plates. For acoustically thick shells, especially with finite length, however, the behavior is not so clear. From the analysis in the wavenumber domain, a formula for calculating the modal radiation efficiency of finite length circular cylindrical shells (immersed in light fluid) under mechanical excitation is obtained analytically. Based on this method, the modal-averaged sound radiation efficiencies of acoustically thick circular cylindrical shells are calculated. It is found that unlike acoustically thin shells, the radiation efficiencies of acoustically thick cylindrical shells very much depend on the acoustic behaviour of each individual vibration mode, and thus on the geometries and the boundary conditions. Results obtained by acoustic boundary element calculations and experiments verify these conclusions.