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# Sound Transmission through Duct Walls

## Sound transmission from ducts to surrounding rooms.

Sound or noise transmitted from the inside of a duct to the surrounding room can be expressed as:

Lp = LN - TL - 10 log (A F / 4 Ad)                                 (1)

where

Lp = sound pressure in room (dB)

LN = sound power level in the duct (dB)

TL = transmission loss through duct wall (dB)

A = room absorption (m2 sabine)

F = cross section area of duct (m2)

Ad = surface area of duct (m2)

Typical sound transmission loss through duct walls are indicated in the table below:

Sound Transmission through Duct Walls - Attenuation - TL - (dB)
Duct sheet
thickness
(mm)
Frequency (Hz)
1252505001000
1.25 17 22 25 29
1.0 15 20 23 27
0.75 14 19 22 26
0.65 12 17 20 24

The acoustic performance of round and oval ducts is superior to rectangular ducts since their curved surfaces allow less breakout noise. Low-frequency sound is better contained in round ducts.

## 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.

## Related Documents

• ### Lined Sheet-Metal Ducts - Sound Attenuation

Noise attenuation in rectangular straight, lined, sheet metal ducts.
• ### Sound Attenuation - Duct split to Terminals

Estimating sound attenuation where ducts splits to terminals.
• ### Sound Attenuation - Main Duct to Branch Ducts

Sound attenuation from main ducts to branch ducts.
• ### Sound Attenuation in Duct Elbows

Noise attenuation in duct elbows by reflected sound.
• ### Sound Attenuation in Ducts due to End Reflection

Low frequency noise transferred from main duct to end terminals is reflected back to main duct.
• ### Sound Transmission through Flanking Paths

Sound transmitted through duct walls, floors and ceilings.
• ### Unlined Sheet-Metal Ducts - Sound Attenuation

Noise attenuation in unlined sheet-metal ducts.
• ### Ventilation Systems - Classification of Noise Reduction Silencers

Noise in a ventilation system can be reduced by reactive, diffuser type, active and/or dissipative silencers.

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## Citation

• The Engineering ToolBox (2003). Sound Transmission through Duct Walls. [online] Available at: https://www.engineeringtoolbox.com/sound-attenuation-ducts-walls-d_79.html [Accessed Day Month Year].

Modify the access date according your visit.

6.3.17

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