Sound transmission 213
To a certain extent, the “8 dB rule” is compensated for by the introduction of the so-
called spectral adaptation terms, which is added to the weighted sound reduction index.
These terms, specified by the symbol C, are in this case defined as:
A,1 wtr A,2 wand,CX X
CX X
=−
=−
(6.12)
where XA,1 is the normalized difference in the A-weighted sound pressure level between
the sending and receiving room, the source spectrum being pink noise. Correspondingly,
XA,2 is the normalized difference in the A-weighted sound pressure level between the
sending room (or in the free field in front of a façade) and receiving room, the source
spectrum being road traffic noise. The symbol XW stands for the single number calculated
using the reference curve (e.g. RW or R'W).
For the specification of the sound insulation of façades, in particular against road
traffic noise, a traffic noise sound insulation index RA has been in use in the Nordic
countries. This index is adapted by ISO stating
RRCAwtr= +. (6.13)
6.2.1.3 Procedure for calculating the adaptation terms
We shall illustrate the calculation procedure using the adaptation term Ctr as an example.
We shall calculate the sound reduction index RA according to Equation (6.13) using the
sketches in Figure 6.5 as a basis. We will assume that the source, being road traffic noise,
sets up a diffuse sound field in the room with sound pressure level Lin, the corresponding
driving sound pressure level at the façade is Lout. We shall define a sound reduction index
for the actual part of the façade, having an area S, by the equation
(^) out in 10 lg ,
S
RL L
A
⎛⎞
=−+⋅⎜⎟
⎝⎠
(6.14)
where A is the total absorption area in the room. Normalizing the sound reduction index
by setting SA/1≡ , we may express the A-weighted sound pressure levels outside and
inside, respectively, as
() Aout
( A out) 10 lg 10 10L jj
p
jL
⎡ −Δ ⎤
=⋅⎢ ⎥
⎢ ⎥
⎣ ⎦
∑ (6.15)
and
()out A
( Ain) 10 lg 10 10.
LRjj j
p
j