We’ve often made the claim that DNL65 is not really a unit of measure. And when we say that it comes off as both condescending and patronizing. Okeedokee…
If you want to understand DNL65, you should probably start by going to the regulations governing Airport Noise Compatibility and Planning aka Part 150.
Got it? Great. Now, you’re not the kind of person who has all day to read through a bunch of legalistic mumbo jumbo. You want to get to the calculation. We understand. So let’s jump to Appendix A.
OK, there’s some talk about choosing the right microphone; what, where, when to measure, that sounds good. There is a discussion of the day vs. night penalties. It’s boring, tedious but it seems like you’re on the right track. Until…
Sec. A150.103 Use of computer prediction model.
Well that took a left turn. All of a sudden there is a list of requirements to be input into some form of computer modeling program, including the INM database. And scrolling a bit further down to subsection 105, you start to see (horrors!) a short section filled with those strange symbols you can’t recall from Calc 101.
AEDT
So, you take a deep breath, Google INM and find out that… the FAA hasn’t used that modeling system for many years now. In fact, it’s been superseded by something called the Aviation Environmental Design Tool, which is a far more advanced software program that contains the current database.
We link to the totally action INM 1976 database report for a couple of reasons
1. It’s free. Obtaining a current AEDT license will run you about $1,200 and the whole thing is proprietary so we can’t share it.
2. The ancient INM is much more understandable because, well, it was a simpler time, when men were men and… there were far fewer variables.
But the principles are exactly the same. You start with data on engines, topography, glide slopes, flight paths, schedules, etc., for a given airport, plug them into the program and that gives you your noise boundary maps and DNL65 calculations.
It’s not an equation
In other words, DNL65 is not an equation one can ‘calculate’. DNL65 is to airports what the 1040 Schedule C form 165 (depreciation) is for business owners. Which means that it is not a ‘formula’ or a ‘measurement’. It’s one element of a compliance system which is only manipulated by specialist noise consultants–in the same way that only Certified Public Accountants prepare tax returns for large organizations because they are the only ones who know how to work ‘the system’. (For example, the government charges for access to AEDT for the same reason hospitals charge doctors for access to operating rooms. It’s a tool meant only for professionals.)
The process does consist of taking some measurements, with, you know, microphones; but only to build the initial model (and then perhaps to do occasional checks to confirm that the readings are still relevant.)
The measurements are just inputs to the system. It’s the modeling which is the real information used to make the decisions. Let that sink in for a minute.
The AEDT system is an order of magnitude more complicated than the old INM database. Just like the US Tax Code, it’s grown and now consists of a bajillion data points–every commercial jet engine, air frame, hundreds of topographic coordinates around every airport, wind factors, weather factors, seasonal factors, population densities, landing routes, glide slopes, and on and on. it’s like SIM City for airports. That is what determines the DNL65 for any given area, a computer model.
So, until the regulations (not law, regulations) change, it is pointless to:
- Try to figure out how DNL65 is calculated. It’s calculated by a machine and a noise specialist (usually a 1private contractor hired by the airport owner who is inaccessible to the public.)
- Go outside with a microphone and measure noise levels. Not because the readings will likely be much higher than ’65’, but because ’65dBa’ in DNL-land does not, Not, NOT have anything to do with 65 decibels (A weighted) in the physical world.
The goal was laudable. A way was sought to quantify ‘dosage’ and ‘annoyance’, ie. the effects on human psychology and physiology of something very hard to quantify–not one single sound, but bursts of sound, occurring hundreds of times a day in a semi-random fashion, which cause greater or lesser harm depending on many factors (if you’re asleep, for example.) Trying to put all that into one ‘number’ is asking a lot.
However, whoever chose the term DNL65 and used the unit of measure ‘dBa’ was either an idiot or a sadist. It’s like how we ended up using the unit ‘degrees’ for both heat and compass directions. But in that case, no one gets confused because the contexts are so different. The ’65’ in ‘DNL65’ has (almost) nothing to do with the ’65’ in a decibel meter.
Instead of worrying about how it’s calculated, we recommend really reading the FAA’s series of articles on noise. They are both maddening and very useful. Maddening because they insist on always putting a positive spin on a positively unspinnable issue (aircraft are so much quieter now!) Useful in that they provide a precise vocabulary we can use to help get to a better solution.
We can’t do anything about DNL65 unless we understand the problem it was trying to solve.
One last thing: We pointed out that DNL65 is a regulation, not a law. Well, sorta. The basic concept was outlined in the 1990 law ANCA. However, it’s very vague and left all the heavy lifting of what it means to the regulations, in Part 150.
Theoretically, the FAA Administrator could wave his magic wand and create a better system. However, the likelihood of that is about nil for two reasons:
- Congress was vague on some things, but the overall will of ANCA is unambiguous: promote air travel. The whole ‘noise mitigation’ thing was (and is) a tertiary concern. ANCA was meant to stop community law suits, not make them easier.
- The basis of FAA’s broad regulatory authority rests on a relatively new legal framework referred to as Chevron Deference which is now being called into question by the US Supreme Court. In previous eras, Congress wrote very detailed laws and the Executive Branch (of which the Department of Transportation/FAA are but one sliver) were the… well… the people who executed those laws. But as the government has gotten so ginormous, agencies like the FAA began to exert more and more autonomy. Congress deferred to those agencies because it simply did not have enough time to write detailed rules for frickin’ department in. Recently, the Supreme Court has, in several decisions, signalled that this is overreach, especially in areas of environmental and consumer-related law. So it may be the case that as any significant element of FAA regulations come up for review, they will be subject to legal challenge. In other words, the ‘fairness’ of DNL65 may not be an issue that any Administrator will be able to decide; the issue may come down to whether or not the FAA has the constitutional authority to create such regulations in the first place, without the explicit permission of Congress.
1In the case of Sea-Tac Airport, for about 25 years that person has been Vince Mestre.