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Sound Transmission through Duct Walls

Estimating sound transmission from ducts to surrounding rooms

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Sound transmission through duct walls

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:

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.

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