Specifies an engineering method for determining the sound power levels of large multisource industrial plants relevant to the evaluation of sound pressure levels in the environment. The method is limited to large industrial plants with multiple noise sources having their main dimensions in the horizontal plane, and which radiate sound substantially uniformly in all horizontal directions. Unweighted sound pressure levels are measured in octave bands. The results obtained are expressed both in octave-band and, if required, in A-weighted sound power levels.