And that is what I'm telling you is not possible, not if measured properly. Sound, regardless of its intensity, produces an atmospheric pressure variation, SPL measures that, doing it so on dB. What it is considered to be the lowest pressure change affecting sound the human ear is able to discern is around 20µPa (which is a pretty low value). Now, that's only an ideal case, and it's like that mainly for sound contained on a frequency range between 2 and 4 khz. For lower or higher frequency ranges, that pressure would have to be higher for the human ear to pick that up, so, even in the case that you wouldn't hear a sound properly, even then, it wouldn't mean that the resultant value in SPL is 0dB. So that you might have a more approximate idea, the sound of a person with a quiet breathing is around 10dB. That's why I'm telling you that a fan moving constantly is certainly going to be picked up, again, if measured properly and with the correct equipment, of course.