CONFIDENTIAL
The total NEF at a point is found by logarithmically summing the contributions from all aircraft along all flight tracks:
ΣΕ
NEF =
10 log (10 exp
(NEF1/10), dB
(D-2)
Based on this relationship, NEF varies directly with 10 log N, where N is the weighted number of operations. If the percent of nighttime activity, the aircraft mix and the altitudes are the same for the tracks, a contour with 100 percent operations of NEF value L is the same as a contour with P percent operations of value
=
Lp 10 log (PN) 10
*
log N + 10 log P
L + 10 log P,
d B
(D-3)
Since P is less than 1.0,
is
P
less than L. The computer plotted contour with 100 percent operations which corresponds to an NEF of 25dB for a track with 44 percent operations is then the following contour:
-
L = 25 10 log (0.44) = 28.6 dB
(D-4)
The computer plotted contours were adjusted in this way to reflect the percentages mentioned above. Before divergence, the contours need no adjustment, and well after the turn the contours are simply adjusted as per Equation D-3. In between these two conditions the 25dB and 30dB contour locations are estimated by logarithmically adding intersecting contours lower than these values for the diverging tracks and then drawing a line between points representing values whose sum
sum is 25dB and 30dB. For instance, the point where the 24dB contour of one track intersects the 18dB contour of another track has а total NEF of:
2.2
10 log (10 exp (24/10) + 10 exp (18/10)) = 25.0 dB (D-5)
NIGHTTIME RESTRICTIONS
Noise exposure levels for tracks with other than 10 percent nighttime activity were adjusted as follows. If the actual number of daily operations is A, then the night-weighted number of operations for a flight track with M percent nighttime activity is:
N = ((1-M) + 16.7M) A (1 + 15.7M) A
For 10 percent nighttime operations,
(D-6)
N =
(1 + 15.7 x 0.1) A 2.57 A
the Lp
(D-7)
From equation D-3,
valued contour for a flight track with M percent night activity is then equivalent to a computer-plotted contour (with 10 percent night activity) of value
L = Lp 10 log ((1+ 15.7M)/2.57), dB
(D-8)
9-6
CONFIDENTIAL