The DNL

Analysis

These things are complicated and a major hurdle in understanding overall noise is understanding the measurement of noise in terms of the DNL.

First it is essential to recognize NextGen or the FAA Modernization and Reform Act of 2012. This major rerouting of airplanes allowed them to go lower and much more in the same place; noise levels in some areas may be reduced and those in other more concentrated areas experience significant (e.g., 300% increase, see Phoenix below) increases.

Also, it is important to consider the different objectives of various public or private organizations and those of individuals.

The DNL is designed and used to aid “policy assessments” and I have no qualms with that. It is generally accepted, but not particularly or only for airplane noise annoyance issues. The problems lie with how to use it, when to employ it as an end-all, and issues that occur because of it.

There is one final concept to mention before considering the situation in Evergreen:

Because decibels are logarithmic quantities, combining decibels is unlike common arithmetic. For example, if two sound sources each produce 100 dB operating individually and they are then operated together, they produce 103 dB — not the 200 decibels we might expect. Four equal sources operating simultaneously produce another three decibels of noise, resulting in a total sound pressure level of 106 dB. For every doubling of the number of equal sources, the sound pressure level goes up another three decibels. A tenfold increase in the number of sources makes the sound pressure level go up 10 dB. A hundredfold increase makes the level go up 20 dB, and it takes a thousand equal sources to increase the level 30 dB.

https://www.lawa.org/-/media/lawa-web/noise-management/files/appendix_g.ashx

This is what I meant earlier by “when quieter, it (the DNL) counts for more; when noiser, it counts for more.” In quiet areas you will see large (5dB or more) jumps in dB by planes overhead; in noisy areas such as Evergreen (60 dB) you will see smaller blips from the airplanes–indicating just how loud they are to create those blips. The DNL accounts for this.

This is the part I am still confused about: What happens when you convert dB readings to a DNL calculation? I think it goes up. I tried it here:

https://www.hudexchange.info/programs/environmental-review/dnl-calculator/

The best estimate I have for the DNL at my home in Evergreen, CO is 78. The DNL is higher than my dB estimates. And it seems to jibe with my cellphone dB readings–the background noise including cars on the highway–or ambient dB level is around 55dB.

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