For determination of sound power of sources by the ’’method of enveloping measurement surfaces,’’ the test environment should provide a measurement surface which lies (1) outside the nearfield of the sound source under test and (2) inside a sound field free of undesired sound reflections from room boundaries or reflecting objects near the source. Methods to check the free‐field conditions and to qualify a given measurement surface for an actual source under test are (1) theabsolutecomparisontestusing a (small) calibrated reference sound source, (2) therelativecomparisontestusing a small test sound source which radiates broadband noise that remains essentially constant during the measurement, and (3) thereverberanttest, which requires measurement of reverberation time. Method 3 is only applicable in closed spaces (rooms). Methods 1 and 2 may be used in rooms and outdoors. Methods 1 and 2 require replacing the source under test by the reference sound source or test sound source in the test site. If the source under test cannot be removed, methods 1 and 2 still allow qualification for free‐field conditions, with less accuracy. This paper deals mainly with therelativecomparisontest(method 2) and gives information about the accuracy of the determination of the environmental corrections factorKunder different field conditions.