Environmental Engineering Reference
In-Depth Information
To determine the sound pressure level from multiple identical sources, use the fol-
lowing equation:
SPL
Ti
=
SPL
Ti
+ 10 log(
n
)
(2.14)
where
SPL
Ti
= Total sound pressure level (dB).
SPL
Ti
= Individual sound pressure level (dB).
n
= Number of identical sources.
Noise Levels in a Free Field
SPL
=
L
w
- 20 log(
r
) - 0.5
(2.15)
where
SPL
= Sound pressure (reference 0.00002 N/m
2
).
L
w
= Sound power (reference 10
-12
watts).
r
= Distance (feet).
Noise Levels with Directional Characteristics
SPL
=
L
w
- 20 log(
r
) - 0.5 + log(
Q
)
(2.16)
where
SPL
= Sound pressure (reference 0.00002 N/m
2
).
L
w
= Sound power (reference 10
-12
watts).
r
= Distance (feet).
Q
= Directivity factor.
= 2 for one reflecting plane.
= 4 for two reflecting planes.
= 8 for three reflecting planes.
Noise Level at a New Distance from the Noise Source
d
d
1
2
SPL PL
=
+
20 log
(2.17)
1
where
SPL
= Sound pressure level at new distance (
d
2
).
SPL
1
= Sound pressure level at
d
1
.
d
n
= Distance from source.
Daily Noise Dose
The following formula combines the effects of different sound pressure levels and
allowable exposure times:
Daily noise dose =
++++
+++
CC C C
TT TT
1
2
3
n
(2.18)
1
2
3
n
where
C
Ti
= Number of hours exposed at given
SPL
Ti
.
T
Ti
= Number of hours exposed at given
SPL
Ti
.
