People think this is something we understand well enough to even begin to measure. Why? The answer can only be: because you haven't even tried listening. Soon as you do, all this nonsense about measuring noise goes right out the window. To even ask the question is to show you literally do not have a clue.
Measuring line noise and power conditioners
I recently purchased a Trifield EMI (Dirty Electricity) Line Meter to measure noise coming from my outlets. To my surprise, my $500 power conditioner (name withheld to protect the potentially innocent) appears to not filter any noise per the Trifield readings. In fact, with some of my outlets the measures are higher through the conditioner’s outlets, than the measures coming straight out of the wall. The manufacturer denies anything is wrong with their conditioner, claiming the Trifield is measuring the wrong frequencies. Can anyone explain?