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# Sound Attenuation - Main Duct to Branch Ducts

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.

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

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

where

dLN = sound attenuation (dB)

Ab = branch area (m2, ft2)

Am = main duct area (m2, ft2)

.

The attenuation is indicated in the diagram below:

### 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

## Related Topics

### • Acoustics

Room acoustics and acoustic properties. decibel A, B and C calculations. Noise Rating (NR) curves. Sound transmission through walls. Calculate sound pressure, sound intensity and sound attenuation.

### • Noise and Attenuation

Noise is usually defined as unwanted sound - noise, noise generation, silencers and attenuation in HVAC systems.

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### Sound Transmission through Duct Walls

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### Sound Transmission through Flanking Paths

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### Ventilation Systems - Acoustic Calculation Procedure

Acoustic noise calculation procedure HVAC systems.

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