Factoids

Thursday, November 25, 2010

Decibels of Deception

In a March 25, 2009 email to members of the Commissioners Court, James Carpenter indicated that:

“The 65 DNL noise level is the level at which land use is considered potentially affected.” Reading further, Mr. Carpenter indicates that “the 60 DNL level is considered safe for all uses,” and that “only a small area is included just outside the property boundary.”

This is patently untrue because the “noise study” that Carpenter & Associates are trumpeting was done at the Wiley Post Airport outside of Oklahoma City. This airport has been in existence for over fifty-years and the study was done to facilitate an expansion of the airport, as was the Addison airport near Dallas. The 65 DNL is fast becoming obsolete and may be replaced with 60 DNL as uninhabitable, with 50-55 DNL as the new minimum standard for habitability of structures.

Land elevation or “topography” makes a large difference in sound levels, as it will near the Central Texas Airport. The Wiley Post airport is on flat ground, but the CTA lies in the basin of the Colorado River with surrounds averaging 150 feet higher than the runway. A 737 flying at 500 feet above ground level (AGL) produces 115 Db. of sound, while the same aircraft at 650 AGL produces less.

Background noise is also a factor in urban areas, as the higher the background noise, the less apparent difference between a noisy aircraft and the traffic and other background noise. In the suburban areas surrounding the CTA, the background noise levels are much less, so a multi-engine 737 flying at 150’ AGL will seem orders of magnitude louder than in an area with more population.

Mr. Carpenter also plans to change the DNL contours as he states in this same letter, “Our actual noise contours will ultimately be remapped once we have signed up our based aircraft and establish our operating requirements and rules for utilizing CTA runway and airport facilities.”

The recognized standard for aviation noise-level contours in the FAA’s Integrated Noise Modeling program which should be used on this site and should include a complete list of the largest cargo-aircraft that may utilize this facility, and the expected frequency of flights, which Mr. Carpenter has pegged at 250 per day.

Anything less is a deception.

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