We measured the amount of noise produced at a distance of 10 cm at two different loads, 120W and 350W. 30 dB(A) is the lowest we are able to measure, and values in this vicinity mean that a PSU is inaudibly silent. Everything below 35 dB(A) is extremely silent, and above this level you will start to hear the power supply. At 40 dB(A) and higher the PSU is clearly audible.
Our meter did not pick up any sound above the normal ambient sound levels of 30 dB(A). The Be Quiet Dark Power Pro 10 550W is completely quiet in other words.