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50 TRG-TRC007-EN
notes
period four
Equipment Sound Rating
In practice, this equation is commonly used to determine how loud a piece of
equipment will be at a given distance. For example, the manufacturer of an air-
cooled chiller lists the sound-pressure level of the chiller as 95 dB at a distance
of 30 ft (9.1 m) from the chiller. This equation can be used to estimate the
sound-pressure level at a lot line, which is 120 ft (36.6 m) from the chiller. In this
example, the sound-pressure level at the lot line is 83 dB.
L
p2
= 95 dB – 20 log
10
[120 ft / 30 ft] = 83 dB
(L
p2
= 95 dB – 20 log
10
[36.6 m / 9.1 m] = 83 dB)
This equation is only valid for sound pressure. It cannot be used to convert
sound power (L
w
) to sound pressure (L
p
). Also, it is only valid in a free-field
environment.
Distance Correction in a Free Field
r
r
2
2
= 120 ft
= 120 ft
(36.6 m)
(36.6 m)
r
r
1
1
= 30 ft
= 30 ft
(9.1 m)
(9.1 m)
L
L
p1
p1
= 95 dB
= 95 dB L
L
p2
p2
= 83 dB
= 83 dB
air
air
-
-
cooled chiller
cooled chiller
lot
lot
line
line
Figure 55