This provides more detail:
4. Q: Who decided that 65 DNL was the right threshold?
FAA Response: DNL has been widely accepted as the best available method to describe aircraft noise exposure and is the noise descriptor required by the FAA for use in aircraft noise exposure analyses and noise compatibility planning. The DNL has also been identified by the U.S. Environmental Protection Agency (USEPA) as the principal metric for airport noise analysis. Day-Night Average Sound Level (DNL) is a 24-hour equivalent sound level. DNL is expressed as an average noise level on the basis of annual aircraft operations for a calendar year. To calculate the DNL at a specific location, Sound Exposure Levels (SELs) (the total sound energy of a single sound event) for that particular location are determined for each aircraft operation (landing or takeoff). The SEL for each operation is then adjusted to reflect the duration of the operation and arrive at a “partial” DNL for the operation. The partial DNLs are then added logarithmically — with the appropriate penalty for those operations occurring during the nighttime hours — to determine total noise exposure levels for the average day of the year.
As directed by the U.S. Congress in the Aviation Safety and Noise Abatement Act (ASNA) of 1979, the FAA and other branches of the federal government have established guidelines for noise compatibility based on annoyance. FAA Order 1050.1E, Environmental Impacts: Policies and Procedures, Appendix A, paragraph 14.3, page A-61, defines the threshold of significance for noise impacts as follows. “A significant noise impact would occur if analysis shows that the proposed action will cause noise sensitive areas to experience an increase in noise of DNL 1.5 dB or more at or above DNL 65 dB noise exposure when compared to the no action alternative for the same timeframe”.
Now we are getting somewhere, but the first sentence and last sentence are entirely wrong. Anyway, we’re after the calculation.
Everyone can learn a lot from Mr. Putnam, and I would like to stop for a minute and consider the woman who asked the question about electrocution. Suppose someone receives several really serious electric shocks but those are averaged-out over a longer period to make it look as if they are harmless? This is a repeat from the above:
To calculate the DNL at a specific location, Sound Exposure Levels (SELs) (the total sound energy of a single sound event) for that particular location are determined for each aircraft operation (landing or takeoff).”
The DNL really is a sophisticated calculation. It is not a 24/7 noise calculation at all. It is a noise reading attributable to every single landing and takeoff. It is a noise reading at the time of airplane noise, everywhere, attributable to that landing or takeoff. When quieter, it counts for more; when noiser, it counts for more. It is not an average. It accounts for spikes and duration. It is weighted for what humans can hear. The day/night aspect of the calculation is a factor in terms of the penalties and different ambient noise levels.
These kinds of manipulations are generally dangerous when it comes to survey research or statistical data; compressed or weighted data are not the same as the real thing. The point is, the DNL is a calculated figure that is intended to be simple and usable.
Mr. Putnam, a super-knowledgeable man, answered every question he was asked but he paused on this one and ultimately he did not answer it. The answer is, the DNL is a measure of noise at the time of each airplane event in each specific location. It is a generalizeable–and general–measure and it includes airplanes in measuring noise.
14 C.F.R. Part 150 participation is voluntary and this (the link above) is a response relating to the agreement of an airport located in a different county. It confirms that the noise levels are linked-to actual landings and takeoffs and are monitored year-round, 24/7. The Denver link shows a computer model for a very specific use to produce one chart. It is designed to make simple, recurring projections. It is not a “noise study” and it provides nothing more than what we see.
This particular application of the DNL principle distorts it, but it shows how it can be calculated.