Civil Engineering Reference
In-Depth Information
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2
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1
ReZ - Measurements
ReZ - Prediction
ImZ - Measurements
Im Z - Predi
c
tion
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0
0.5
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1.5
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2.5
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3.5
Frequncy(Hz)
Figure 11.8
Surface impedance
Z
at normal incidence of the layered material repre-
sented in Figure 11.6. Measurements reproduced from Rebillard
et al
. (1992).
The second example of a material with a porous screen is represented in Figure 11.10.
Between foam 3 and the blanket, there is a thin layer of high flow resistance created by the
gluing of the blanket on to the foam 3. The parameters for the different porous layers are
indicated in Table 11.4. As for the first material, the bulk modulus of the screen is large
and, due to its small thickness, the screen can be considered to be a rigid framed material.
The Young's modulus of the screen has not been measured, but has been set sufficiently
large so that the screen behaves as a rigid framed material at normal incidence. The three
other layers are modeled as porous-elastic layers. The Johnson - Champoux - Allard model
is used for all the layers of the material. The surface impedance at normal incidence is
presented in Figure 11.11. The measurement is reproduced from Allard
et al
. (1987).
The predicted surface impedance of the material without the screen is also represented
in Figure 11.11. There is no measurement related to this prediction, because the screen
is inside the material, and the material is damaged when the screen is removed. For this
material, the density of the screen is smaller and its flow resistivity larger compared with
the screen of the first material, and the screen is not immobilized by the inertial forces and
the stiffness of the frame over the range of frequencies for which the measurements were
performed. Moreover, the screen is set between two porous layers and its effect on surface
impedance is not as simple as for the previous material. The difference between the two
predicted impedances cannot be calculated by the simple model which was valid for the
case of the first stratified material. The absorption coefficients of the second material
