Civil Engineering Reference
In-Depth Information
CHAPTER 9
Sound transmission in buildings. Flanking
sound transmission
9.1
INTRODUCTION
With some exceptions, we have up to now treated sound transmission through a specific
building element. A sound reduction index or an impact sound level is then ideally an
element specification but as pointed out a number of times; the boundary conditions of
an element may have considerable influence on the result. The type and properties of the
connections to adjoining constructions are important factors when specifying the
transmission properties of a given element. An example is the contribution to the total
loss factor of an element by the vibration energy “leaked” to adjoining structures, making
it advisable for laboratories following ISO 140 to determine the total loss factor of their
test specimens. Another example of the importance of the couplings was presented in
Chapter 8 when dealing with heavy double walls.
In this chapter we will deal with the interplay of building elements with the
objective of predicting the airborne and impact sound transmission in real buildings, in
which there are normally a number of transmission paths between the source and
receiver. We shall look for models enabling us to predict the acoustic performance of
buildings based on the acoustic performance of each element making up the complete
structure. To prepare such models has been an important task for the European Standards
Organization CEN and sound transmission inside buildings are covered by the standards
EN 12354 Part 1 and Part 2. We have referred to these standards before concerning
predictions of element performance. Here we shall show some examples of the full
model.
The prediction accuracy of such models is obviously dependent on the types of
element taking part and the complexity of the boundary conditions. Dealing with simple
heavy constructions such as concrete, the accuracy will be good whereas combinations
involving lightweight, multilayered elements are always difficult to handle. This should
not prevent the use of these models in practical design cases, a use that contributes to the
gathering of a larger information base for these standards.
By way of introduction, we shall refer back to Figure 6.3, which sketches a number
of possible transmission paths between two rooms, one room being excited by a sound
source. As shown, in addition to the airborne sound transmission through the partition
wall, there will be energy transmitted by way of the flanking walls, through cracks and
crannies, by way of the windows or a common ventilation or cable duct etc. We also
pointed out that we shall reserve the concept of flanking transmission for the energy
transport in the following way: the source excites the flanking constructions on the
source side into vibration, thereby causing a part of this vibration energy to be
transmitted to flanking constructions on the receiver side, which in turn radiates sound.
Another important transmission path, which is not shown in Figure 6.3, is the
