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.
The best white noise machines mask external noise to keep your environment quiet while the best sound machines have non-repetitive sounds that are soothing and lull you to sleep. Whether you prefer a sound machine or a white noise generator, the best ones create an environment favorable for sleep.