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© FANTECH
2016
ATTENUATOR SELECTION PROCEDURE
Low Freq
Bias Noise
dB
Overall
Level
Attenuator
Loss dB
After
Attenuator
dB
Overall
Level
63Hz
75
76.8
4
71
72.0
125Hz
70
6
64
125Hz
65
9
56
500Hz
60
10
50
1000Hz
60
15
45
2000Hz
60
13
47
4000HZ
55
14
41
8000HZ
50
12
38
Attenuator Noise Level Reduction
4.9dB
High Freq
Bias Noise
dB
Overall
Level
Attenuator
Loss dB
After
Attenuator
dB
Overall
Level
63Hz
50
76.8
4
46
64.7
125Hz 55
6
49
125Hz 60
9
51
500Hz
60
10
50
1000Hz 60
15
45
2000Hz 65
13
52
4000HZ 70
14
56
8000HZ 75
12
63
Attenuator Noise Level Reduction
12.2dB
In-duct SWL -dB
125
63
250 500 1000 2000 4000 8000
0
20
40
60
80
Octave Frequency, Hz
B
e
f
o
r
e
at
te
nuato
r
A
fte at
r
t
e
n
u
a
t
o
r
Generator
Attenuator
Noise before
attenuator
Noise after
attenuator
76.8
dB
72.0
dB
Attenuator noise
level reduction
4.9dB
To make an attenuator selection that will provide satisfactory performance,
there are several criteria that have to be addressed.
1. Acoustic attenuation/silencing performance
The reduction in noise offered by an acoustic attenuator varies according to the source of the noise being controlled.
Being technically specific, it depends on whether the noise is particularly loud in low frequencies/tones. In the example below,
two different noises could have the same overall decibel levels but because one is from a generator set (low frequency biased),
the performance of the same acoustic attenuator varies to that of a general HVAC system.
0
20
40
60
80
125
63
250 500 1000 2000 4000 8000
Octave Frequency, Hz
In-duct SWL -dB
Before
t
at
e
n
u
ator
Attenuator
Noise after
attenuator
Fan
76.8
dB
12.2dB
Attenuator noise
level reduction
64.7
dB
Noise before
attenuator
General fan noise
(Example fan coil unit)
Low frequency biased noise
(Example generator set)
INTRODUCTION
Assessing whether an attenuator would reduce noise effectively enough for a particular situation usually requires complete
equipment noise data and attenuator insertion loss (SIL) spectrums. Acoustic calculations then have to be performed to assess
the resultant noise levels with the attenuator.
To give the reader a general understanding of the noise reduction performance of different attenuators, the methodology presented
in this section provides attenuator performance as a single noise reduction value against a Low Frequency Biased or General HVAC
noise profile.
The performance of an attenuator in a specific case may vary from these general noise reductions. To assess if this general advice
applies to a specific case, assistance is available through our nearest sales office and through the acoustic analysis tool in the Fans
By Fantech Selection Program and Fantech Website.
NOISE CONTROL PRODUCTS
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