It is hard to conduct proper acoustics measurements on a PSU since you have to isolate it from the loaders; they are way too noisy and will interfere with the readings. However, if you move the loaders far away from the test PSU, you need long connection cables, which incur increased resistance. Power losses become significant in that case.
Thankfully, we found a way around these issues, and with the help of a fully featured hemi-anechoic chamber, we are able to conduct accurate sound measurements using a Class 1 Sound Analyzer.
We measure the fan's noise from one meter away. The noise measurements are conducted with a Class 1 Bruel & Kjaer 2250-L G4 Sound Analyzer, equipped with a type 4955-A low-noise and free-field microphone which can measure down to 5 dB(A). We also have a type 4189 microphone that features a 16.6-140 dBA-weighted dynamic range. The sound analyzer is installed into a hemi-anechoic chamber which allows for lower than 6 dB(A) ambient noise. A Bruel & Kjaer Type 4231 is used before every noise measurement, to calibrate the sound analyzer.