I go 4 fingers down from the collar on the front, and just under the front collar impression on the backside of the shirt. everyones fingers are different so I measured mine and the 4 fingers measure 3.5. I also use my left hand to guage where to put left chest logos.
Grey noise then, has a sound frequency that, if translated to a light frequency, would roughly be the color grey. Grey noise is a random sound that is distinguishable from other types of sound because it sounds the same, to the human ear, at all frequencies.
On the decibel scale, the smallest audible sound (near total silence) is 0 dB. A sound 10 times more powerful is 10 dB. A sound 100 times more powerful than near total silence is 20 dB. A sound 1,000 times more powerful than near total silence is 30 dB.