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Noise Metrics and Community Response 113 or CNEL in California and Leq, as described in the following sections. Other cumulative metrics such as the Day-Evening-Night Level (Lden) used in Europe, the Australian Noise Exposure Fore- cast (ANEF), the Weighted Equivalent Continuous Perceived Noise Level (WECPNL) used pre- dominantly in Asia, and the Noise Exposure Forecast (NEF) used in Canada, will occasionally be raised by the public in discussions of noise issues, but are not accepted as primary metrics in the United States. Occasionally these metrics may be used in complex studies for supplemental analyses. They are described in the toolkit. 7-1 FAA evaluations of the environmental effect of airport development or air traffic modifica- tions require an assessment of the change of noise level that will occur. Consequently, the best examples of metric usage often are maps displaying areas of expected change. Throughout this chapter, examples of the key metrics and ways that change may be graphically illustrated for pub- lic consumption will be provided. Day-Night Average Sound Level (DNL or Ldn) DNL considers all the noise energy that occurs during an average day of operation. The sum of the noise energy present between the hours of 10 p.m. and 7 a.m. is multiplied by 10 then added to the sum of the noise energy present between 7 a.m. and 10 p.m. (this equates logarith- mically to multiplying each nighttime operation by 10 before the energy of each noise event is summed). This penalty is applied in recognition of the increased sensitivity of people to opera- tions during the nighttime hours. The total is divided by the number of seconds in a 24-hour day (86,400) to obtain a 1-second average energy level. The logarithm of the average is multiplied by 10 to obtain the DNL level (see the glossary for formulae) 7-2 . The metric is complex, the mathematics are obscure to the general public, and the measure cannot be directly heard by neighbors. Many members of the public complain because the DNL averages energy across every second of the day, rather than only those seconds when aircraft noise occurs. The FAA has adopted the DNL as the only noise metric required for consideration in airport projects, although it may be supplemented by other metrics to better explain characteristics of the noise exposure pattern. As the metric of choice, the 14 CFR Part 150 land use compatibility guidelines are based on thresholds of DNL. A DNL of 65 dBA has been identified as the thresh- old of significant noise impact above which efforts should be taken to mitigate noise levels. Computer-generated contours connecting locations of equal noise exposure are typically used in the selection of land use development criteria and plans in airport environs. Example of Metric Display for Public Information: Change in Day-Night Average Sound Level (DNL) Combined Alternatives (Boston Overflight Noise Study, Phase 1) (155) 7-3 Comparative DNL Contours for Baseline and Alternative Conditions at Boston Logan Inter- national Airport (Boston Overflight Noise Study, Phase 1) (156) 7-4 Example of DNL computation Video: (prepared for ACRP 02-05 project) 7-5 Community Noise Equivalent Level (CNEL) CNEL is similar to DNL and used for essentially the same purposes. Required by state law in California, the FAA has approved CNEL as an acceptable substitute for DNL in federally funded airport noise analyses there. In addition to the penalty applied to nighttime operations by DNL, CNEL also applies a penalty to operations that occur during the evening hours between 7 p.m.