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)
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:
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.