CALIBRATION OF REFERENCE SOUND SOURCES

Sound power measurements on reference sound sources B & K 4204 have be en performed in an intercomparison among eight laboratories in seven h emi-anechoic rooms and in seven reverberation rooms. The project was f unded by the EC (BCR). Personal from the laboratories and the PTB cali brated the sources using prescribed methods. The prescription of the m ethods was the out-come of primary investigations in the first phase o f the project performed by PTB and B & K. The most important features of the hemi-free field prescribed method are meridional scans (quarter circles) using moving microphones and corrections for air attenuation . The prescribed methods for reverberation room measurements are simil ar to the technique described in ISO 3741 and include distribution of six fixed microphones, four source positions and the use of reverberat ion time T-15 instead of T-30. The main results from the intercomparis on are summarized as follows: a) Reproducibility of sound power measur ements can be improved markedly by using the more restrictive methods prescribed instead of allowing the freedom in the methods standardized today. b) Measurements in hemi-free field rooms and in reverberation rooms produce exactly the same results, except in the frequency ranges below 100 Hz and above 10 kHz. c) Below 100 Hz and above 10 kHz discr epancies of up to 2 dB between hemi-free field and reverberation room measurements are found. d) Disturbing influences of non-perfect wall a bsorption in hemi-anechoic rooms are surprisingly small. e) Individual influences of reverberation rooms are noticeable, but can be kept sma ll with certain restrictions concerning volume and shape.