Measuring at 15cm is problematic - that is smaller than the fan opening on many computers and the presence of the microphone itself could easily add noise because it is interfering with the air flow. 15cm from *what*? The center of the opening, or the edge, or what?
Agreed. In fact, the entire set of noise measurements on various PCs listed there is more misleading than useful. Not even their "relative" noise levels can be taken at face value. OK, let's be frank: It's pathetic, worse than useless data.
I'm far more comfortable with Apple's own claim -- "Typical acoustical performance, sound pressure level (operator position): •12 dBA at idle
" Operator position clearly refers to the ISO 7779 "seated operator position", which is a little over 0.5m away with the PC atop a table in an anechoic chamber. Since I've measured PCs with multiple fans and HDDs in our lab that measure that low, I have no trouble believing this to be the case in a system with just one fan and no other noise sources.