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116 Aircraft Noise: A Toolkit for Managing Community Expectations
Slant Range Distance/Altitude
Another factor frequently cited by the public as a noise issue is the distance from the aircraft
in flight to the listener on the ground (the slant/range distance) or the aircraft's altitude. The
slant/range distance represents the hypotenuse of a triangle formed by the aircraft altitude and
the lateral distance from the ground location to the point of closest approach along an aircraft's
path of flight. Noise complaints recorded by those airports surveyed for this evaluation cited "too
low" as a common source of public concern.
Similarly, the public will often be aware of preferred flight patterns and complain that the air-
craft was "right over my house" and frequently point to a location halfway above the horizon
instead of straight up. The perception of location, coupled with distance, is often difficult to
answer without the use of triangulation. An aircraft 45 degrees above the horizon is as far away
horizontally as it is vertically and farther than either when measured in a straight line distance.
Members of the public often are interested in the number of events that overfly an area at dif-
ferent altitudes and may request that altitudes be raised to abate noise. As the distance between
the noise source and the receiver is increased, the noise level will decrease by six decibels with
every doubling of distance. Consequently, a noise event of 80 dBA of SEL at 3,000 feet overhead
will be decreased to 74 dBA of SEL if the altitude of the aircraft is increased to 6,000 feet. It is
often very difficult to increase aircraft altitudes by significant amounts at any course along a flight
track without adding substantial length to the track prior to passing over, or in some cases, with-
out significantly increasing the power setting used in flight (and the resulting noise level). However,
if the same flight track were relocated laterally by one mile, the noise reduction would be the same
because the distance between the source and the receiver would be increased to 6,000 feet. It is often
much easier to relocate a flight track than to increase altitude enough to achieve a meaningful
reduction in single event noise levels.
Hybrid Metrics
As information has become more available for individual aircraft types, as noise prediction
tools have been improved, and as the public has become more sophisticated in their understand-
ing of the effects and consequences of aircraft development actions, new applications of avail-
able metrics have evolved to be more responsive to public interests. Several supplemental metrics
have evolved in recent years that allow the noise analyst to develop information that may be more
meaningful to members of the public in their understanding of aviation noise. While such sup-
plemental metrics are not required for inclusion in any environmental evaluation, their avail-
ability is well known and often advisable at locations where noise is a contentious issue. These
metrics are typically formed from combinations of simpler single event metrics. Because they are
developed from more detailed computer modeling and post-processing requirements than the
more standard metrics described in earlier sections, they are more costly to produce. They
include Number of Events Above, Time Above, and Audibility.
Number of Events Above (NA)
In recent years, the Number of Events Above metric has become popular as a means of com-
paring the number of operations above various thresholds of loudness associated with opera-
tional alternatives. The INM is capable of computing the SEL or Lmax of every operation that is
included in its input data, at any location selected by the program's user. The availability of this
information will allow the noise analyst to sort output data to provide the numbers of daily
operations that exceed selected noise thresholds. For example, the number of flights exceed-
