From Whence Came Ldn / DNL 65?

04.01.2010 | HMMH |

by Nick Miller

With FAA developing a research roadmap, and fears (hopes?) expressed by many in our airport noise community that the compatibility guideline might change, I became “curiouser and curiouser” about the real origin of 65 DNL as various claims were made about its origin, its immutability, its arbitrariness, and its scientific basis or lack thereof.  What follows is my take on what seems to have happened.  Please forgive oversights, it’s the best I could do with what materials I could quickly dig up.

This is, indeed, a topic with a tortuous and uncertain history.  Apparently, many efforts from the 1950’s on to the late 1970’s were underway to determine levels that could be identified as the threshold between compatibility and incompatibility with noise.  Efforts were pursued in many countries.

In the U.S., it appears that a “west coast”, an “east coast”, and a U.S. EPA effort were simultaneously underway.  The various engaged personalities were surely aware of each others’ work, and it is likely that most of them attended the legendary 1973 International Conference on “Noise as a Public Health Problem” at Dubrovnik, Yugoslavia, but produced separate reports.  (Legend has it that during that conference in a taxi cab, Ken Eldred and Liz Quadra derived the relationship between population density and Ldn.  Another legend is that Henning von Gierke finished drafting the “Levels Document” in his hotel room at 5 in the morning.)

On the west coast, Wyle Laboratories provided a report Supporting Information for the Adopted Noise Regulations for California Airports, WCR 70-3(R), January 29, 1971.  This report documents the science behind the California airport noise criteria that were adopted in November 1970.  The criteria limited airport noise in residential communities to 65 CNEL.  The report shows that behind selection of this level, however, was a review of considerable research on the effects of noise on people.  Data on speech and sleep interference, hearing loss, physiological stress and health effects, annoyance and community reaction were all reviewed.  In the end, using community reaction data, 65 CNEL was chosen as the apparent “threshold of complaints,” suggesting that complaints are a reasonable indicator of annoyance. Of note, and generally suffering from benign neglect, is the report’s clear recommendation that: “The CNEL limit should be periodically reviewed by the State with a view to the possible necessity of reducing the limit in light of any new human factors research which may become available,” and that the review should be every five years, at maximum.

On the east coast, Ted Schultz at Bolt Beranek and Newman was hard at work assisting HUD develop compatibility guidelines.  In BBN Report No. 2005 R, Technical Background for Noise Abatement in HUD’s Operating Programs, 8 November 1971 he reviewed noise ratings (dBA, loudness, NNI, etc.), made comparisons across noise ratings, compared noise ratings with subjective judgments, criteria of acceptability, including social surveys and existing noise exposures, and criteria in different countries.  (His work investigating surveys appeared in the “Synthesis of social surveys on noise annoyance,” in JASA, vol. 64, No. 2, August 1978.)  Ted developed criteria for non-aircraft noise that were identified as “clearly acceptable,” “normally acceptable,” “normally unacceptable” and “clearly unacceptable.”  These were indicated on simple graphics that showed areas of level versus percent of time exceeded, over which a measured distribution could be traced or laid and acceptability determined.  However, these were probably too complicated for practical use, since sound distributions were at the time almost impossible to predict – how would a proposed project be judged?  The distributions were also stated first in terms of L33, then as NEF values in 1971, then finally as Ldn values in 1978.

For our purposes, i.e., for aircraft, in Report No. 2005R, he identified “about NEF 30” (~DNL 65) as the criterion of acceptable exposure in the U.S.  This criterion appears to be a synthesis of what other countries were doing.  But note what he says about this criterion: “It should be emphasized that criteria in the NEF 30 range must be regarded as provisional.  In each of the national studies in which these limits were developed, these levels of noise showed up as ‘maximum tolerable’ and were regarded as turning points above which annoyance increased very rapidly; but sizable portions of the population were seriously disturbed at much lower levels.  These turning points, however, were seized by the authorities and treated as acceptable levels such that special precautions and noise abatement measures are required only for more severe exposure.” [Ted’s emphasis] “The situation is even more extreme in the U.S., since the criteria are based on overt action in terms of complaints or legal action.  It is well known that serious public annoyance is prevalent long before official complaints are lodged.  It is therefore obvious that these criteria are not adequate for aircraft noise abatement in the long run, since they are deliberately permissive.”

Schultz’ “Synthesis” JASA article, after long analyses and descriptions, provides a way to choose a “community noise level suitable for a living environment….”  He does this in a graph (Figure 23 in the article) that plots, as a function of Ldn, % U.S. populations exposed to values of Ldn or higher, and % of people experiencing different types of effects at a given Ldn – high annoyance, sleep or speech interference.  What he tries to offer decision-makers is information that balances what is desirable with what is feasible.  We have forgotten, or never knew, that high annoyance was only one of the effects he proposed minimizing or limiting in selecting a level for a suitable environment.

Meanwhile, the U.S. EPA’s Task Group III, led by Henning von Gierke, in responding to the Noise Control Act of 1972, recommended Ldn 60 as the limit of compatibility, and based this conclusion on minimizing annoyance, complaints and community reaction, and speech interference both outdoors and indoors. (See the EPA report Impact Characterization of Noise Including Implications of Identifying and Achieving Levels of Cumulative Noise Exposure, PB224408, 27 July 1973.)

Finally, we also know that for the Maryland Department of Transportation Maryland Aviation Administration in 1975, Schultz recommended Ldn 65 as the residential standard, to be reduced to Ldn 60 when “the U.S. fleet noise level is reduced 5 dB below 1 July 1975 levels,” Maryland Department of Transportation State Aviation Administration, Selection of Airport Noise Analysis Method and Exposure Limits, January, 1975.

My conclusions? These folks at the beginning tried to account for all the effects they were aware of and had confidence in, and balance what might be desirable with what would be feasible.  And they all suspected or decided that 65 CNEL / Ldn was likely too high as a long-term goal.

History is not as simple as we’d like it to be.  Sorry.

Author’s notes: For people younger than about 55, the people mentioned are: Ken Eldred, now living somewhere in Maine, was a chief participant at Wyle and then at BBN in development of the background for metrics and effects of noise.  Liz Quadra, now lost to us in acoustics, was a major force in the U.S. EPA’s Office of Noise Abatement and Control (ONAC – defunded in 1981).  Henning von Gierke, deceased, was a scientist the U.S. managed to get from Germany after World War II and who lead research on the effects of noise and vibration on people at the U.S. Air Force research laboratories at Wright Patterson Air Force Base.  Everyone reading this should know Ted Schultz.  If not, check some of the above links.