Civil Engineering Reference
In-Depth Information
having centre frequency
j
. The frequency bandwidth may be octave or one-third-octave,
whichever is appropriate.
L
(dB)
L
out
L
in
L
in
S
R
A
L
out
After A-weighting
Frequency
Figure 6.5
Sound transmission through a window. Sketch showing sound pressure levels outside and inside
before and after A-weighting.
The quantity of interest, the difference in the A-weighted sound pressure levels, is
given by
Δ=
L
(
L
)
−
(
L
)
,
(6.17)
p
A
p
A
out
p
A
in
a quantity that we will be able to calculate knowing the spectrum of the outside noise.
Here one uses a standard spectrum for road traffic noise. When expressing this spectrum
in frequency band values in such a way that a summation after A-weighting gives zero
dB, i.e.
−Δ
⎡
(
L
)
A
⎤
out
j
j
∑
⎢
⎥
≡
10
10
1
or
(
L
)
≡
0,
p
Aout
⎢
⎥
⎣
j
⎦
we arrive at the sound reduction index for traffic noise
LR
−
⎛
⎞
j
j
∑
(
L
)
=
0
⎜
⎟
=Δ
⎯⎯⎯⎯ →−
=− ⋅
⎜
10
(6.18)
R
(
L
)
p
A
out
(
L
)
10 lg
10
.
A
p
A
p
A
in
⎟
⎝
j
⎠
The spectrum level values
L
j
is given by
( )
LL
=
−Δ
A.
(6.19)
j
out
j
j
These values are tabled in ISO 717 Part 1 for one-third-octave bands as well as for
octave bands. We may then readily calculate
R
A
if we have laboratory measurement data
1
The A-weighting curve is specified in the IEC standard for sound level metres; see references to Chapter 1.
