Basic Documentation

Duct Takeoffs and Divisions
Once the term enclosed within the parenthesis is calculated, Figure 17 provides dB values.
For example, assume that a 36-inch x 18-inch rectangular duct main has an 8-inch round 90
degree takeoff that is followed by a 32-inch x 18-inch continuation of the duct main. What
attenuation would this junction provide for the 8-inch and the reduced sized main?
Attenuation for an 8-inch diameter duct:
= 10 Log [8 in. Duct Area ÷ Total Area Lv. Jct.]
= 10 Log [50.3 in.
2
÷ (50.3 in.
2
32 in. x 18 in.)]
= 10 Log [50.3 in.
2
÷ (50.3 in.
2
+ 576 in.
2
)]
= 10 Log [50.3 in.
2
÷ 626.3 in.
2
]
= 10 Log [0.0803]
= - 11 dB at each octave band
Attenuation for a 32-inch x 18-inch rectangular duct:
= 10 Log [(32 in. x 18 in.) ÷ Total Area Lv. Jct.]
= 10 Log [(576 in.
2
) ÷ (626.3 in.
2
)]
= 10 Log [0.9197]
= - 0.4 dB at each octave band
Table 23. Takeoff/Junction Attenuation.
Branch Duct Area dB Branch Duct Area dB
Total Area Lv. Jct. Attenuated Total Area Lv. Jct. Attenuated
0.95 to 0.91 0.4 0.089 to 0.071 11
0.90 to 0.88 0.5 0.070 to 0.057 12
0.88 to 0.71 1 0.056 to 0.045 13
0.70 to 0.57 2 0.044 to 0.036 14
0.56 to 0.45 3 0.037 to 0.029 15
0.44 to 0.36 4 0.028 to 0.023 16
0.35 to 0.29 5 0.022 to 0.018 17
0.28 to 0.23 6 0.017 to 0.015 18
0.22 to 0.18 7 0.014 to 0.012 19
0.17 to 0.15 8 0.011 to 0.009 20
0.14 to 0.12 9 0.0089 to 0.0071 21
0.11 to 0.09 10 0.0070 to 0.0057 22
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