Basic Documentation

Duct Attenuation
P/A 63 125 250 500 1,000 2,000 4,000 8,000
Hz Hz Hz Hz Hz Hz Hz Hz
0.95 0.16 0.15 0.32 1.01 1.72 1.35 1.50 1.58
0.86 0.14 0.14 0.30 0.96 1.61 1.25 1.43 1.55
0.83 0.14 0.13 0.29 0.94 1.58 1.22 1.42 1.54
Round Unlined Sheet Metal Ducts
While unlined round ducts provide the least amount of sound attenuation, nevertheless their
attenuating effect should be included when designing a duct system. Table 19 provides dB
per foot values that can be applied to unlined round ducts.
Table 19. Unlined Round Duct Per Foot of Length.
Diamete
r
63 125 250 500 1,000 2,000 4,000 8,000
Inches Hz Hz Hz Hz Hz Hz Hz Hz
6 or Less
0.03 0.03 0.05 0.05 0.10 0.10 0.10 0.11
7 to 10
0.03 0.03 0.04 0.05 0.09 0.09 0.09 0.10
11 to 15
0.03 0.03 0.03 0.05 0.07 0.07 0.07 0.08
16 to 22
0.02 0.02 0.02 0.04 0.06 0.06 0.06 0.07
24 to 30
0.02 0.02 0.02 0.03 0.05 0.05 0.05 0.06
32 to 48
0.01 0.01 0.01 0.02 0.03 0.03 0.03 0.04
50 and
Above
0.01 0.01 0.01 0.02 0.02 0.02 0.02 0.03
Round Acoustically Lined Sheet Metal Ducts
Round spiral wound sheet metal ducts lined with at least one-inch thick fiberglass or similar
material are more effective at attenuating sound, particularly in the mid frequencies. As with
rectangular ducts, the equations to calculate the specific dB attenuation levels for various
lining thickness are very complex and require much effort to work through. Table 20 and
Table 21 provide values that can be used for round ducts with 1-inch and 2-inch internal
acoustical linings.
Table 20. One-Inch Thick Fiberglass Lined Round Duct Attenuation Per Foot of Length.
Diamete
r
63 125 250 500 1,000 2,000 4,000 8,000
Inches Hz Hz Hz Hz Hz Hz Hz Hz
6
0.38 0.59 0.93 1.53 2.17 2.31 2.04 1.26
8
0.32 0.54 0.89 1.50 2.19 2.17 1.83 1.18
10
0.27 0.50 0.85 1.48 2.20 2.04 1.64 1.12
Siemens Building Technologies, Inc. 57