DNL65 For Dummies

There is an area around an airport where the FAA says the sound level measures at 65 decibels ‘DNL’. That area is referred to as the Noise Boundary or just ‘DNL65’ and all big airports have a noise boundary map.

Now DNL65 isn’t a true unit of measure like a decibel. It’s actually a formula that attempts to put all the flights day and night someone might experience over their head into one number representing  ‘annoyance’. A among the many unfortunate aspects of the formula, is the fact that using ‘decibels’ creates the entirely false impression that it is a measurement. it is not a measurement.

First off, the formula  gives special weight to flights at night as being more annoying than those during the day. But on the other hand, it also allows for individual flights (referred to as a SEL) to be much louder than 65db. Again, ’65’ doesn’t represent any measurable unit per se. You just have to assume that if the number over your house is ’71’ it’s somehow more annoying than the guy down two blocks over where the number is calculated as ’57’.

As with all things government, the whole point of DNL65 was money. So if you’re inside that boundary (because the spot you live in is calculated as ’71’ using the formula), your home may be eligible for noise insulation money from the FAA. That is how Port Packages at Sea-Tac were funded. Everyone inside the DNL65 boundary (with a number 65 or higher) got sound insulation. Those outside (with lower numbers–thus in less noisy spots), were left, well… outside.

Now according to the FAA, the size of that boundary keeps decreasing every year because:

  1. Aircraft keep getting quieter and
  2. The flight paths keep getting tighter due to NextGen

Both things are true. And in fact, the FAA will tell you that the ‘average’ annoyance has actually decreased by almost 2/3rds in the past 30 years. Hooray for technology.

OK, so why does that feel like a cruel joke if you live near an airport?

Some technical reasons why DNL65 sucks…

  1. That 65 was always —way— too high. It was set that high many years ago because, frankly, it was impossible to sound insulate homes to anything much better. The real number should probably be less than 45 in terms of human health.
  2. The formula is -calculated-, not based on measurements. IOW: no one goes out with a sound meter and actually -tests- the noise levels around the airport. They look in text books that have a particular number for each aircraft type, engine and a bunch of cockamamie stuff involving topology, geography and wind patterns. It’s 100% -contrived- and often wildly inaccurate compared to real measurements.
  3. The formula does not take into account -frequency- when it comes to annoyance. When a human encounters 700 flights a day—even if they are ‘quieter’ than in past years, the cumulative effect on the body is MUCH worse than say 100 ‘noisier’ flights from 30 years ago. The formula assumes that we are all less ‘annoyed’ now because each plane is quieter, while completely ignoring the massive increase in -quantity- of exposure.

Now the bad news: physiology

It is remarkable what human beings can become acclimated to.

When a human hears a loud noise, it creates a state of physical arousal. It’s a survival mechanism. It’s automatic and affects you even if you absolutely love jet airplanes and have heard them fly over 100,000 times. Many people living near airports get so used to it they may not even notice it consciously, but again that physical response occurs every time to some degree. The blood pressure elevates, stress hormones are generated. Whether you notice it or not, when you get into that state of excitement it takes a few minutes to return to normal.

Now when flights are at least a few minutes apart, the body has at least some rest period to return to normal. But when they are coming almost continuously, the body never has a chance to calm down. It creates an almost constant low-level state of arousal at a physical level and this has, what scientists refer to as a ‘weathering effect’. It basically wears the body out prematurely from a constant low level of stress. Again, many people get so used to it that they are not even consciously aware that it’s going on.

Conversely, even one or two flights at night can have the same arousal effect. When we’re asleep, many of us are deeply affected when a flight goes overhead. And sleep is such an essential function that, even if it happens every night, our bodies never become acclimated–it keeps making us jump every night at 3:05AM.

The tricky part has been in getting the government to recognize that noise is a serious health issue. For example, the term of art the government uses to describe the effects of noise is ‘annoyance’. It’s not mere semantics to say that the term itself discounts the importance of the issue. Calling these health effects ‘annoying’ automatically creates the impression in decison-makers minds that the sufferers are simply ‘complaining’ and that the problem is not serious enough to warrant action.

2 Replies to “DNL65 For Dummies”

  1. 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 an annual 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.

    1. Thank you for your very thoughtful comment. I did not “show my work” and that was intentional. I have spoken with the consultants in charge of noise measurements at Sea-Tac Airport, as well as their noise team and I can assure readers that sampling (using microphones) are not a part of the process in determining the noise boundary maps. The odd thing is that the Port and FAA do perform sampling, using very expensive equipment, but -not- for the actual boundary map calculations–rather as a ‘check’. As to the modeling process itself, that’s an article we need to write. The only reason we haven’t done it thus far is because the footnotes alone will be ridonculous. It is such a Rube Goldberg process I am not sure the public will believe it.

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