Building Acoustics

Sound transmission in buildings. Flanking transmission. 343

suspended ceiling as an introduction to the general case where a number of different
transmission paths are participating, including flanking transmission in a strict sense.
We shall then choose a partition that is reasonably “matched to” the suspended
ceiling, which means that there is no point in choosing a very good partition if the
expected transmission by way of the ceiling will be considerably larger than the
transmission directly through the partition. As we wish to apply the measured results
presented above, we choose a partition with sound reduction index as shown in Figure
9.14. This is a lightweight double leaf construction, two layers of 13 mm plasterboard on
common steel studs, the cavity of 70 mm filled with mineral wool.
To calculate the apparent sound reduction index including the transmission path
across the ceiling using measured data from Figures 9.11 and 9.13, we have to decide on
the height of the room, i.e. the height of the partition. We may then refer the sound
reduction index Rcl of the ceiling transmission path to the partition by applying Equation
(9.13), thereafter summing up the pertinent transmission factors. This procedure is
followed, giving the results as shown in Figure 9.14, where we can see a substantial
decrease in sound insulation where there is no absorber present in the plenum. With the
absorber, however, the sound insulation is only slightly poorer than the one offered by
the partition alone.

9.3 Flanking transmission. Apparent sound reduction index

The prediction of the effective sound insulation between rooms in a building, either
airborne sound or impact sound; presupposes a model that includes all types of
transmission path. An important type of transmission involves the flanking constructions
of the wall or floor in question, and as pointed out several times, we shall reserve the
notion of flanking transmission for this type of energy transport. In the model used here,
we shall confine ourselves to neighbouring rooms; two rooms separated by a wall or
floor. We shall also assume that the transmissions involving the different paths are
independent and that all wave fields are diffuse.
We shall furthermore, primarily be treating airborne sound insulation due to the
fact that it will normally represent a greater problem for prediction than the impact sound
insulation. In addition, data used when calculating the apparent sound reduction index,
e.g. vibration reduction index of junctions, may directly be applied to impact sound
problems. For each transmission path of airborne sound we shall, as before, allocate a
transmission factor. Referring these factors to the partition, we may express the apparent
sound transmission index by

df dt it

1

10 lg

with.

R

τ

τ ττ τ τ

′ ⎛⎞

=⋅⎜⎟

⎝⎠′

′=++ +∑ ∑

(9.21)

The indices “d” and “f” indicate the transmission directly through the partition and by
way of the flanking constructions, respectively. To complete the picture, two terms are
added representing the sum of other direct or indirect transmission. The index “dt”
indicates direct transmission through parts of the partition “added to” the basic wall or
floor, such as doors, air vents or leaks (apertures, slits etc.). This theme has already been
treated (see section 9.2) and we may include these factors directly in the transmission
factorτd if appropriate. The last term includes all types of indirect transmission between
the rooms, e.g. transmission by way of a duct system or a suspended ceiling, by way of