Civil Engineering Reference
In-Depth Information
from a theoretical point of view, then becomes of less interest in practice. Furthermore,
judging from
Figure 8.3
, there has to be large variations in the flow resistivity to really
affect the reduction index, much larger variations than between the products normally
used in lightweight double walls. The big difference lies in the amount of filling, either
just a part or completely. There may be differences in the reduction index of 4-5 dB from
a percentage of filling being 30-50% as compared with 90-100%.
8.2.1.1
Lightly damped cavity
Double walls, where the cavity is not efficiently damped by a porous absorber, will
necessarily give a sound reduction index somewhere between the one found for an empty
cavity and a completely filled one (see
Figure 8.4)
. A case where one will benefit from a
filling of the cavity, but for obvious reasons cannot fill it completely, is by window
constructions applying a lining inside the window frame. In practice, these are cases
having a reasonably large cavity depth, at least more than 50 mm, excluding common
compact double (or triple) glazed units. For a double construction equipped with a frame
absorber one could use Equation
(8.3)
as a first approximation, writing
A
α
Ud
RR R
=++⋅
10 lg
=++⋅
R R
10 lg
,
(8.10)
1
2
1
2
S
S
where α is the absorption factor for the absorber along the frame, and
U
is the
circumference or total length of the frame. Most correctly, one should use the absorption
factor for normal incidence. (Why is that?)
12 mm chipboard
200 mm
100 mm mineral wool
Figure 8.5
Arrangement for measurements on a double leaf construction (2.25 x 1.25 m) with no structural
connection between the leaves. Adapted from Brekke (1979).
Even without an absorber in the cavity there will be a certain surface absorption
due to viscous effects but the magnitude is difficult to estimate. In a relatively early
phase of applying SEA models on problems in building acoustics the cases of an empty
cavity and a cavity with a frame absorber were treated (see e.g. Crocker et al. (1971);
Brekke (1979)). Both also treated the case when the leaves are structurally connected, a
case recently taken up by Craik and Smith (2000), also within the framework of SEA. In
parallel with the SEA type of modelling, analytical models for incorporating the effect of
structural connections as studs or other types of mechanical links have been developed.
A number of these efforts are mentioned by Wang et al. (2005), who themselves are
