Sound Transmission through Duct Walls

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

L_p = L_N - TL - 10 log(A F / 4 A_d)                                 (1)

where

L_p = sound pressure in room (dB)

L_N = 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)

A_d = 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.