Ambient Noise Levels should be considered in Noise Criteria

The Noise Criteria Threshold is too high above ambient Noise Levels. The threshold needs to factor in aircraft’s noise contribution to ambient noise levels.

The EA presents aircraft noise and only considers noises above DNL 45 dB as relevant to the Noise Criteria. The Noise Criteria should include existing neighborhood ambient noise levels as a baseline to compare against the aircraft noise contribution level.  This will demonstrate the amount of noise aircraft contribute above the existing ambient noise.

Most Southern California neighborhoods have  ambient noise levels (without aircraft) below a DNL 45 dB threshold. In these neighborhoods aircraft noise is already louder than other ambient noise prior to the proposed changes.  In some places  ambient noise is so low that even a small increase in air traffic noise could be disturbing to people and become a source of public concern.
The DNL 45 dB threshold was put forth 10 years ago in the Environmental Impact Statement (EIS) for the Expanded East Coast Plan (EECP). This level was recommended on the rationale that “even distant ambient noise sources and natural sounds such as wind in trees can easily exceed this [DNL 45 dB] value.” [1]

Map of natural conditions

Ambient Noise Level in Natural Conditions.

Southern California’s ocean breezes are not as strong as the wind in Chicago and off the Potomac where this standard originated. Our climate lacks the water sources that allows an abundance of vegetation and animals. The palm trees and desert chaparral that rustle in Mediterranean/sub-tropical Southern California are not a significant contributor of noise.

Map of existing conditions

Ambient Noise Level in existing Conditions.(including Aircraft Noise)

A 2015 National Park survey[2] of ambient noise levels recognizes the Southwest United States up to 10 dB quieter than Central and Eastern US – and up to 15 dB quieter after subtracting man-made noise sources.

The Socal Metroplex area’s existing ambient noise is as low as 36 dB in urban areas and 22 dB in non-urban areas. [3]

The EA should be considering the effect that an incremental increase in aircraft noise has on the cumulative impact of aircraft noise “when added to other past, present, and reasonably foreseeable future actions regardless of what agency … or person undertakes such other actions.”[4] Use of existing natural ambient noise levels allows comparing the project to an environmental baseline of natural quiet.

The EA’s Noise Criteria should include levels as low as DNL 25 dB for rural areas. There are communities where aircraft noise significantly exceeds ambient noise levels. They are at, or exceed, an overflight annoyance threshold. A noise analysis that wants to predict community annoyance should include the total difference between non-aircraft and aircraft sources and not just the change in aircraft noise.
DNL is not an adequate measure of annoyance at lower noise levels. Except at the highest noise levels, annoyance is related to the difference between the new noise and the existing ambient noise, not total noise. A more descriptive noise metric would express the difference in noise between the proposed actions and natural ambient, and the number of events above dB levels of 20. Aircraft noise from the proposed action needs to be compared against existing non aircraft ambient noise to predict annoyance.

[1] U.S. Department of Transportation, Federal Aviation Administration, Expanded East Coast Plan – Changes in Aircraft Flight Patterns Over the State of New Jersey, Pp. 5-9. 1995.
[2] National Park Service, Geospatial sound modeling. 2013-2015. Project-2217356
[3] A-weighted hourly L50 sound pressure level dB re 20 uPa on a typical summer day at 270m resolution.
[4] Title 40 C.F.R. § 1508.7- Cumulative Impact.

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