Civil Engineering Reference
In-Depth Information
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:
CX X
CX X
=
−
and
A,1
w
(6.12)
=
−
,
tr
A,2
w
where
X
A,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,
X
A,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
X
W
stands for the single number calculated
using the reference curve (e.g.
R
W
or
R
'
W
).
For the specification of the sound insulation of façades, in particular against road
traffic noise, a traffic noise sound insulation index
R
A
has been in use in the Nordic
countries. This index is adapted by ISO stating
RRC
Awtr
.
=
+
(6.13)
6.2.1.3
Procedure for calculating the adaptation terms
We shall illustrate the calculation procedure using the adaptation term
C
tr
as an example.
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
L
in
, the corresponding
driving sound pressure level at the façade is
L
out
. We shall define a sound reduction index
for the actual part of the façade, having an area
S
, by the equation
S
⎛⎞
RL
=−+⋅
L
10 lg
⎜
⎝⎠
,
(6.14)
out
in
A
where
A
is the total absorption area in the room. Normalizing the sound reduction index
by setting
SA
≡
, we may express the A-weighted sound pressure levels outside and
inside, respectively, as
/
1
⎡
(
L
)
−Δ
A
⎤
out
j
j
∑
⎢
⎥
(
L
)
=⋅
10 lg
10
10
(6.15)
p
A
out
⎢
⎥
⎣
j
⎦
and
(
L
)
−−Δ
R
A
⎡
⎤
out
j
j
j
∑
⎢
⎥
(
L
)
=⋅
10 lg
10
10
.
(6.16)
p
Ain
⎢
⎥
⎣
j
⎦
