Qualification procedures for free-field conditions for sound power determination of sound sources

Abstract

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) the absolute comparison test using a (small) calibrated reference sound source, (2) the relative comparison test using a small test sound source which radiates broad-band noise that remains essentially constant during the measurement, and (3) the reverberant test, 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 the relative comparison test (method 2) and gives information about the accuracy of the determination of the environmental corrections factor K under different field conditions.