My understanding, and I am close but not done with my research, is DNL is a real measurement but almost everyone is too confused, cheap, or lazy to actually do it. Instead, it is modeled and extrapolated without concern or understanding. It is supposed to be a noise measurement at a location for each aircraft operation i.e., each plane in the air, attributable to each landing and takeoff (i.e., airport). It is integrated with known flight data. You could do it in your backyard. But what happens if there is a loud party next door or a truck goes by? It is a year-long measurement process. While the calculation may be complicated, it is based on actual data.
From personal experience, I can vouch for your last section.
NextGen made the traditional use and concept of the DNL obsolete–planes don’t go up from airports and out of audible range quickly anymore. I have not seen DNL measurements, still, in the Denver area where I live and the planes are concentrated. The entire NextGen rollout is deeply flawed in that DNLs in many areas were just assumed to be acceptable… consumers, businesses, parks, historical properties, schools, you get the picture, are still not aware.
In my opinion, be careful when quoting or paraphrasing statements from the FAA as fact. Same goes for DIA or SeaTac for that matter. There are a lot of constituencies in these matters and most do not jibe with the average person.
The first part is remarkable, hard to understand, and hugely important. Logarithms are fairly easy to understand mathematically but hard to grasp as it applies to airplane noise. What it means is loud noises are masked by other loud noises; it takes a louder noise to make a big impact. Sneaky. Airplane noise is a louder noise.
A-weighting, SEL, and adjustments will seemingly increase dB levels recorded on a simple cellphone decibel meter.
A 5 dB increase, which I/we have experienced, is considered major.