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114 Aircraft Noise: A Toolkit for Managing Community Expectations and 10 p.m. The evening penalty multiplies the sum of the evening energy by 4.77 before adding it to the sums of the daytime and nighttime energy levels. The evening penalty is equivalent to con- sidering each evening operation to have three times the effect of an equivalent daytime operation. The State of California mandates that CNEL noise exposure contours be made available for public land use planning in the same way that DNL contours are used at airports outside the state. Further, at several large airports, the contour patterns are regularly monitored to strive to meet a state requirement that no residences be located within the 65 CNEL contour. Where such incompatible uses remain, the airports must allow variances to the rule and demonstrate their attempts to reduce the overall impact of noise in neighboring communities through noise abate- ment or mitigation. Equivalent Sound Level (Leq) The Leq is the simplest and most flexible of the cumulative metrics. It does not apply penalty factors based on time of day, nor does it require consideration over a 24-hour period. The metric may be used by planners to assess the comparative noise effects of any number of events on peo- ple. For example, the Leq may be computed for 1-hour periods, for the nighttime hours, for school hours, for a peak period of operations, or any other duration desired. It may be used to compare the cumulative contribution of specific aircraft types, users or user groups to the total noise energy. It also may be used to reflect the combination of discrete aircraft events having different noise characteristics. The analytical use of the Leq metric may be widely varied in considering various noise abate- ment operational techniques. Leq levels allow a more in-depth assessment of the specific costs or benefits of flight actions by comparing, for example, the noise energy from aircraft flying along one departure track as compared to another. When mapped, the two resulting patterns may guide noise management programs to maximize the reduction of impacts on underlying populations. Single Event Noise Metrics Cumulative aircraft noise contours often are challenged by airport neighbors as not represent- ing what can be heard and measured every time an aircraft flies over their home. Long duration measurements and computer technology may indicate the contour patterns are accurate for the community, but they fail to capture the discrete nature of the single events that people actually identify and complain about. As louder Stage 2 aircraft were removed from the commercial operating fleet during the 1990s, cumulative noise contours shrank significantly from earlier sizes. Although the con- tour reduction could be attributed largely to the reduction of noise from individual aircraft, the number of actual operations has generally increased. As a consequence of this change, the public has become more vocal in demanding that the number and noise levels of single events be assessed in environmental evaluations. Several metrics are available to respond to this demand. Sound Exposure Level (SEL) The SEL is a mathematical expression of the noise energy present during an event or a period of time, normalized to a single second. Consequently it is always larger than any cumulative noise measurement of the same event that lasts longer than one second. It provides the noise analyst the ability to directly compare the acoustic energy generated by two separate events,