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Sound Attenuation - Main Duct to Branch Ducts

Sound attenuation from main duct to branch ducts

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When main ducts are split into branches the sound power level - Lw - is reduced according the size of the branch, or amount of air to the branch.

Main duct to branches - Sound allowance

The sound attenuation from a main duct to a branched duct can be calculated as

dLN = 10 log (Ab / Am)                                  (1)


dLN = sound attenuation (dB)

Ab = branch area (m2, ft2)

Am = main duct area (m2, ft2)

The attenuation is indicated in the diagram below:

Main duct to branches - Sound allowance

Example - Sound Attenuation from Main Duct to Branched Duct

In an example 20% of the air goes to the branch duct and 80% of the air goes further on in the main duct. The branch duct area is 20% of the main duct area before the branch and the main duct area after the branch is 80% of the main duct area before the branch.

The sound attenuation from the main duct to the branch duct can be calculated:

dLN_branch = 10 log ((20%) / (100%)) 

     = 7 dB

The sound attenuation in the main duct can be calculated:

dLN_main_duct = 10 log ((80%) / (100%)) 

     = 1 dB

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  • en: acoustics sound ducts hvac allowance main branch
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  • Engineering ToolBox, (2003). Sound Attenuation - Main Duct to Branch Ducts. [online] Available at: [Accessed Day Mo. Year].

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