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Sound Transmission through Duct Walls

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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)


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

Frequency (Hz)
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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Related Topics


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.

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