Sound transmission in buildings. Flanking transmission. 353
Here we shall present results from a couple of special cases where the task is to
increase the vibration reduction index by adding elastic layers to the joints. This will also
give a very good illustration as to the effect of flanking transmission on the apparent
sound reduction index.
The first example is taken from experiments on a small test “building”, two rooms
with floor area of 10 m^2 , built upon one another. Walls and floors were aerated concrete
slabs of density 600 kg/m^3 , of thickness 125 and 150 mm, respectively. The vibration
velocity differences between wall elements were measured both with direct contact
between wall and floor elements and with elastic layers placed in the joints (see results
and insert in Figure 9.21). The elastic layers were foam rubber bands, 7 mm thick and of
120 mm width.
Simultaneous results on Kij cannot be presented as the structural reverberation times
were not measured in these early experiments. Using the expression for a T-junction in
EN 12354–1 we arrive at the value 6.9 dB for Kij. What we see here is that Dv,ij is
frequency dependent having a minimum value of 11 dB. The elastic layers give quite a
dramatic increase in the velocity level difference, which, as presented in the next section,
nearly completely offset the effect of the flanking transmission on the sound insulation
between the rooms.
In a NORDTEST project (see Brøsted Pedersen (1993)), aiming to work out a
standard method for in situ determination of transmission properties of structural joints,
similar results were obtained. The work included a number of laboratory measurements
but measurements performed in a two-storey dwelling are maybe of special interest,
where elastic layers were introduced on both sides of the floor separating two apartments
(see results with a sketch of the situation in Figure 9.22). The floor was concrete of
density 1750 kg/m^3 , the walls being lightweight concrete of density 650 kg/m^3. The
elastic layers are 4 mm thick polyurethane with cement (Sylomer P).
Concerning the addition of elastic layers to a joint, a point worth mentioning is that
it could affect the energy losses from the floor to the connected structures. The total loss
factor will decrease, which may affect e.g. the impact sound pressure level. This adverse
effect may probably be of less importance than that which is gained by the decreased
flanking transmission but the effect should certainly be considered.
9.3.3 Complete model for calculating the sound reduction index
Being now in the position to calculate the vibration reduction index, we shall return to
the model for predicting the sound insulation between two rooms given by Equation
(9.22). Computer software based on this model is commercially available, e.g. Bastian®,
which include prediction of airborne and impact sound insulation as well as airborne
sound insulation against outdoor noise. For airborne and impact sound insulation the
prediction models found in EN 12354 Parts 1 and 2 are implemented. We shall conclude
this chapter by presenting a few results of the airborne sound insulation based on this
software, intended to take into account all transmission paths as sketched in Figure 9.15.
For a simple illustration of the principles behind these calculations, we shall set out
to find the apparent sound reduction index R ́ in a case where only transmission paths of
type Ff are contributing in addition to the direct transmission path. We may then use
Equation (9.23), which we repeat here:
d f
10 lg 10^101010.R R
R