65 DNL is the administrative construct the FAA uses as a general indicator of excessive noise. More specifically:
“The FAA uses the DNL metric and the 65dB noise threshold to make policy assessments.” (https://www.faa.gov/regulations_policies/policy_guidance/noise/history/)
These things are incredibly complicated at first glance. Once you understand them they are quite simple.
In this case, as it applies to Evergreen, Colorado, the NextGen FAA rollout is illegal and must be reversed. For various other reasons the City and County of Denver is liable as is, potentially, Jefferson County, Colorado, but those are subjects for another time.
For now the topic is 65 DNL. It is a day/night measurement of noise and it is only partially related to airplanes:
The calculation for DNL considers the time of day an aircraft operated and applies a 10-decibel penalty on aircraft arriving or departing between 10 p.m. and 7 a.m.; the output is a numeric value in decibels that represents a 24-hour average noise exposure value. The current federally-established threshold of significance is 65 dB DNL.https://www.macnoise.com/faq/what-dnl-terms-aircraft-noise
That is a simple description of the DNL and not accurate in several ways. What it shows is that it is a calculation, seemingly linked with actual aviation or airport data, with a day/night component and a penalty calculation component.
Remember, the definition says 65 DNL and 65db noise threshold.