Civil Engineering Reference
In-Depth Information
⎛
⎞
D
4
2
tot
θ
pWc
=
ρ
+
⎟
.
(4.48)
⎜
⎝
0
0
2
A
4
π
r
⎠
Expressed by the corresponding levels using standardized reference values for sound
pressure and sound power, we may write
⎛
⎞
D
4
θ
π
LL
=+⋅
10 lg
+
⎟
.
(4.49)
⎜
⎝
p
W
2
A
4
r
⎠
For simplicity, we have given the characteristic impedance ρ
0
c
0
the value 400 Pa⋅s/m.
The difference between the sound pressure level and the sound power level is shown in
the absorption area
A
. The dashed curve indicates the relative level of the direct field.
10
5
A
= 5 m
2
0
10
-5
20
50
-10
100
-15
Free field
200
-20
0.1
1.0
10.0
100.0
Relative distance from source,
r
/(
D
θ
)
1/2
(m)
Figure 4.9
Sound pressure level as a function of relative distance from a source of sound power level
L
W
.
The
parameter is the room total absorption area
A
. The dashed line indicates the free field level.
The distance
r
H
from the source, where the contributions from the direct field and
the diffuse field are equal, is called the
hall
radius
or also
room radius
for the case where
the directivity factor is equal to 1.0:
A
55.26
V
r
=
=
⋅
.
(4.50)
H
16
π
16
π
cT
0
