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OCR for page 114
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,
