The % times the input watts tells you the watts radiated as sound. At one meter these watts would cover an area of 4/3 PI R^3. The radiated watts are "proportional to" Pressure^2/unit area, with the area of the spher as noted. So multiply the radiated watts by the area of a sphear at 1 meter and you have pressure^2 (within some constants). From this you can get the dB with a log and some more constants. You can look up the constants.