Civil Engineering Reference
In-Depth Information
50
50
SBIL
ABIL
SBIL
ABIL
45
45
40
40
35
35
Bare panel damping:
η
= 0.07
Bare panel damping:
η
= 0.3
30
30
25
25
20
20
15
10
5
0
−
5
15
10
5
0
−
5
−
10
−
10
10
2
10
3
10
2
10
3
Frequency(HZ)
Frequency(HZ)
Figure 12.15
Airborne (ABIL)versus, structure-borne (SBIL) of a plate - foam - plate
system for two damping values of the bare panel.
calculated from the force and velocity at the excitation point. It is important to use the
real part of the complex product which stands for the input energy (effective power).
This definition of the SBIL is similar to the classical ABIL obtained by subtracting the
transmission loss (TL) of the bare panel from the TL of the same panel covered with the
trim material
ABIL
=
TL
with NCT
−
TL
bare
(12.59)
While airborne insertion loss is based solely on acoustic excitations (diffuse acoustic
field), the structure-borne insertion loss is based on point loads excitation randomly
positioned on the panel.
Figure 12.15 shows that the SBIL depicts the same behaviour as the ABIL. It is higher
than the ABIL for the lightly damped panel and similar for the highly damped panel.
The difference between the two is in particular important at low frequencies and near
the double wall resonance of the system. This difference diminishes with the damping
of the main structure (including damping added by the sound package). This is in line
with the experimentally observed similarity between ABIL and SBIL for damped systems
(Nelisse
et al
. 2003). This also justifies to some extent the current SEA practice which
uses ABIL to correct both the resonant and nonresonant transmission paths in a panel
with an attached sound package under various excitations.
12.5
Point source excitation
Using the methods of Chapters 7 and 8, the application of the TMM in the case of a
point source excitation is uncomplicated. First, the Sommerfeld representation is used to
decompose the incident pressure into plane waves (Equation 7.2 and Figure 7.1)
∞
∞
=
−
j
2
π
exp[
−
j(ξ
1
x
+
ξ
2
y
+
µ
|
z
2
−
z
1
|
)
]
p(R)
d
ξ
1
d
ξ
2
µ
−∞
−∞
(12.60)
k
0
−
ξ
1
−
ξ
2
,
Im
µ
µ
=
≤
0
,
Re
µ
≥
0
