Sound transmission 211
Figure 6.3 Measurement of sound insulation in a building. Examples of transmission paths between rooms.
It should be noted that the notion flanking transmission only implies that the
transmission takes place in the manner of setting the flanking constructions into
vibration, a part of which is transferred to constructions on the receiving side that are
capable of radiating sound energy. We shall use the common term transmission path
when referring to all the other mechanisms for transmission of sound between rooms.
The requirements for airborne sound insulation in buildings are in most countries
given by certain limiting values of the sound reduction index. The same measurement
procedure and calculation method as for the laboratory is applied. However, as one does
not quantify the various contributions to the sound pressure level in the receiving room,
we will write instead of Equation (6.6):
R'10lg,S
RD
A
=+⋅ (6.10)
where D again is the difference in sound pressure level between rooms, but now the
sound reduction index R' is called the apparent sound reduction index of the partition. A
popular way of expressing this is to say that the partition takes the blame when other
transmission paths contribute significantly to the level in the receiving room. There are,
however, other measures that may be used to specify the sound insulation. As stated in
ISO 140 Part 4, which concerns airborne sound insulation measurements in the field, we
may also use the sound pressure level difference referred to a given reverberation time
T 0. Here we shall write
(^) n
0
T 10 lg ,
T
DD
T
⎛⎞
=+⋅⎜⎟
⎝⎠
(6.11)
where the quantity DnT is denoted standardized level difference with T 0 set equal to 0.5
seconds for dwellings. Some countries are using this quantity in their requirements
concerning airborne sound insulation, and it could be argued that this quantity is more in
line with the actual sound insulation experienced by the users than the R'-value.
6.2.1.2 Single number ratings and weighted sound reduction index
When specifying the sound insulation capability of constructions, in particular when
connected to acoustical requirements in building codes, it is sensible to use a single
number instead of the whole frequency curve. Normally, the latter is composed of data in
one-third-frequency bands from 100 to 3150 Hz or in an extended range going from 50