Building Acoustics

332 Building acoustics

9.2.2 Sound transmission involving duct systems

Another example where we may assume that the sound transmission takes place by way
of two or more independent surfaces, are when rooms are connected by a duct system.
Two possible situations are depicted in Figure 9.7. In the first situation, the rooms are
connected to the main ventilation duct system by way of the terminal units in the rooms.
An open transmission path between the rooms is thereby established, indicated in the
sketch as open-ended ducts.
The second situation is more complicated; the sound energy transmission between
rooms takes place due to airborne sound transmitted through the duct walls in one room
and a corresponding sound transmission out through the duct walls in the other room.
This problem may arise when a duct passes through a very noisy room and the
transmission may certainly affect areas other than the neighbouring room. One may also
have situations where the neighbouring rooms have terminal units connected to the same
system. It should also be noted, as indicated in b) that added to the airborne sound inside
the duct there may also be flanking transmission by way of the solid duct walls.
The phenomena connected to transmission through duct walls, which is commonly
referred to as acoustical break-in and break-out, will not be theoretically described here.
We shall give a qualitative description only, together with a few examples from the
literature. A fairly recent review paper is given by Cummings (2001).

Sv

S

S

Duct

a)

b)

Figure 9.7 Sound transmission between rooms by way of a common duct system. a) transmission between
terminal units; b) transmission through duct walls (“break-in” and “break-out”).

As for the situation depicted in a) we are in a position to calculate the necessary
attenuation, i.e. in the duct system between the terminal units, so as not to reduce the
overall sound insulation between the rooms. This attenuation, which we shall denote D,
may be defined as 10⋅lg(1/τ) where τ is the transmission factor for this transmission path;