Specifies a method for measuring the component of sound intensity normal to a measurement surface which is chosen so as to enclose the noise source(s) of which the sound power level is to be determined. The one-octave, one-third-octave or band-limited weighted sound power level is calculated from the measured value. The method is applicable in situ or in special purpose test environments to any source for which a physically stationary measurement surface can be defined, and on which the noise generated by the source is stationary in time.