Getting to Grips with Aircraft Noise

7 - INFLUENCE FACTORS
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7. INFLUENCE FACTORS
CAUTION: all the charts presented hereafter only illustrate examples of the
possible sources of scatter evocated below for a specific case, and
as such must not be used for the evaluation of corrections in
particular situations.
7.1. BACKGROUND
The noise levels computed by the NLC or the NEX are based on the best knowledge we
have of the aircraft (flight tests). This knowledge is translated into an engine acoustic
model.
In addition, both software use models to account for the atmospheric absorption and the
lateral attenuation of the sound.
The noise levels computed for a procedure assume certain atmospheric conditions,
which are likely to differ from the actual conditions (humidity, temperature).
• In that context, it is worth noticing that atmospheric conditions influences on the
noise atmospheric absorption are shown in ICAO Annex 16 Volume 1
Appendix 1 paragraph 8 – Sound attenuation in air.
• The assumed weight of the aircraft is based on the load and trim sheet, and is
likely to differ from the actual one.
• Neither NLC nor NEX account for the actual topography around the airport when
computing the effect of sound waves reflections on the ground.
For all these reasons, a certain difference may exist between NLC computations and
actual levels as monitored during flight operations, though the deviations are small.
Consequently, it is worth mentioning that the confidence to put in these computations
stands in the value of the calculated gain between different procedures, rather than into
the absolute noise level value itself, which must not be considered as guaranteed.
The following charts displays the influence of different parameters on the
measured noise levels 6.5 km from brake release point, considering an A320
straight takeoff from sea level.
Flight Operations Support & Line Assistance
Getting to grips with aircraft noise