Along with efficiency we are also able to calculate a PSU's overall output noise, by averaging the noise measurements that we take from our test results.

Noise data averaging process isn’t straightforward, since decibels (dB) are based on logarithmic scales. For example, if you have a PSU producing 40 dB, a doubling of the acoustic power sound intensity would not be 80 dB but 43 dB (and 50 dB for the volume loudness). This means that we must convert first the dB values to sound pressure units (Pa), before we average them, and then convert the result to dB again, to make it familiar to most of you. We call our noise measurements program Lambda, deriving from the Greek letter “Λ”. The same conditions with efficiency testing also apply to our noise measurements. 
Noise Levels (115V Input)Noise Requirements (1)
A++<20 dB(A)
A+≥20 dB(A) & <25 dB(A)
A≥25 dB(A) & <30 dB(A)
B≥30 dB(A) & <35 dB(A)
C≥35 dB(A) & <40 dB(A)
D≥40 dB(A) & <45 dB(A)
E≥45 dB(A)

(1) In case the DUT is close to the lower limits, with a maximum margin of 0.25 dB(A), we will provide a second chance to the manufacturer which will have to provide us with another sample. In case the second sample registers a major overall difference compared to the first one [>2 dB(A)], we will ask for a third one and the average noise measurements of all three samples will deliver the final result.