Getting to Grips with Aircraft Noise

Flight Operations Support & Line Assistance
Getting to grips with aircraft noise
9 - A BIT OF THEORY
9.3.3. ADDING SOUND PRESSURE LEVELS – NOTION OF MASKING
EFFECT
In the case of several independent sound sources, it is not possible to add arithmetically
the decibel values, as they are logarithmically defined.
Let us consider two sound sources, each of them emitting a pure tone at different
frequencies ω1 and ω2 (corresponding characteristic periods are T1 and T2)
Source 1: ω 1
The resulting sound pressure at a given point is then:
p ( t)
= p1(
t)
+ p2
( t)
(9.3.3-1)
p1max
p2
max
p ( t)
= cos(
ωωωω 1(
t − r1
/ c)
) + cos(
ωωωω 2 ( t − r2
/ c)
)
r
r
(9.3.3-2)
1
The corresponding effective sound pressure is (considering a period T that is the lowest
common multiple of T1 and T2:
p
r 1
2
e
r 2
Source 2: ω 2
2
T
1 ⎡ p1max
p2
max
⎤
= ∫ ⎢⎣
cos(
ωωωω 1 ( t − r1
/ c)
) + cos(
ωωωω 2 ( t − r2
/ c)
) ⎥ dt
T r
r
0 1
2
⎦
(9.3.3-3)
2
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