Building Acoustics

344 Building acoustics

windows in an outer wall etc. A note of warning: Reduction index product data for small
units such as grills, diffusers etc. is normally referred to a standard area of 1 or 10 m^2 , not
to their actual area.
In the following we shall go into some details on the transmission expressed by the
first two terms in Equation (9.21). Figure 9.15 gives an indication on the different
transmission paths that we shall have to take into account. It should be noted, however,
that the sketch only indicate what we may denote first order flanking paths; paths
involving one element in the sending room, one junction or connection and one element
in the receiving room. Applying the notions from the figure, the transmission factors τd
and τf may be expressed as

dDd Fd

and fDf ,

n

mk

ττ τ

τ ττFf

=+

=+

∑

∑ ∑

(9.22)

where the number of elements n, m and k will normally be four. The main contribution
from the flanking transmission will normally be by paths indicated by “Ff”. For multi
leaf constructions, i.e. a double wall, a floating floor construction etc. other flanking
paths are possible. The same applies for flanking constructions of such types.

D

F f

Fd Ff

Dd

Df

d

Figure 9.15 Sound transmission paths between two rooms. Letters “d” and “f” indicate direct and flanking
transmission, respectively. Capital letters indicate sending room, small letters receiving room. Adapted from EN
12354–1.

The most obvious effect of flanking transmission is seen in cases where the
flanking constructions are lightweight and have no structural breaks at the main partition;
a partition which have good sound insulation properties as a stand-alone element. The
requirement “not lightweight” is constantly underrated by builders. A classical example
is given in Figure 9.16, where the curve shows the measured standardized level
difference DnT between two rooms for music practice. As shown in the insert to the
figure, the 50 mm thick floating concrete slab passes unbroken below the partition. The
situation is not helped by the fact that the partition should be good enough, having a total
of six layers of 13 mm plasterboard. The sound insulation is a total failure at frequencies
above 300 Hz. The result here represents approximately the sound insulation predicted