Sound Attenuation - Main Duct to Branch Ducts

Calculate sound attenuation when main ducts are split into branches. The sound power level reduction is based on the branch area relative to the main duct area using the formula: dLN = 10 log(Ab / Am)

Sound Attenuation - Main Duct to Branch Ducts

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

Formula

dLN = 10 log(Ab / Am)

Where:

dLN = Sound attenuation (dB)
Ab = Branch area (m², ft²)
Am = Main duct area (m², ft²)

How it Works

The attenuation depends on the ratio of the branch duct area to the main duct area. A smaller branch receives more attenuation (reduction in sound power level).

Calculator

Inputs

Branch Area (Ab) (m²)

Main Duct Area (Am) (m²)

Results

Sound Attenuation (dLN)-- dB

Description	Branch Area Ratio	Sound Attenuation (dB)
Branch receives 20% of air	20%	−7.0
Branch receives 50% of air	50%	−3.0
Branch receives 80% of air	80%	−0.97
Main duct continues at 80%	80%	−0.97
Main duct continues at 50%	50%	−3.0
Main duct continues at 20%	20%	−7.0

Example

In a typical scenario where 20% of the air goes to the branch duct and 80% continues in the main duct:

Branch duct area = 20% of main duct area
Main duct area (after branch) = 80% of main duct area

Sound attenuation in branch:

dLN_branch = 10 log(0.20 / 1.0) = −7 dB

Sound attenuation in main duct:

dLN_main = 10 log(0.80 / 1.0) = −0.97 dB

The branch duct experiences greater attenuation due to its smaller area.

Sound Attenuation - Main Duct to Branch Ducts | MEPKit Engineering Tools | MEPKit