Gives a direct and a comparison method to determine the sound power level that would be produced by a source operating in an environment at standard meterological conditions corresponding to a characteristic impedance of pc = 400 N-s/m3. Specifies test room requirements, source location and general rules for operating conditions. Suitable for steady noise with broad-band, narrow-band and discrete-frequency components. Applies to noise sources with volumes which are preferably not more than 2% of the volumes of the reverberation room which is used for the test.