The acoustic noise is measured in an semianechoic chamber by means of B & K precision integrating sound level meter with back ground noise level below 20dBA.
The fan is running in non-resistant air with microphone at a distance of one meter from the fan intake.
Sound pressure level (SPL) which is environmentally dependent and sound power level (PWL) are defined as SPL = 20 log10 P/Pref
SPL = 20 log10 P/Pref
PWL = 10 log10 W/Wref
Pref= A reference pressure
W =Acoustic power of the source
Wref =An acoustic reference power
Fan noise data is usually plotted as Sound Power Level against the octave frequency bands.
The measurement standard is according to : CNS 8753(which is comparable to DIN 45365)