Building Acoustics

Sound transmission 209

i
t

1

10 lg 10 lg (dB).

W

R

τ W

⎛⎞ ⎛⎞

=⋅ =⋅⎜⎟ ⎜⎟

⎝⎠ ⎝⎠

(6.2)

In the literature one often finds the term transmission loss used for R.
In a traditional laboratory measurement procedure to determine the sound reduction
index of a building element, it is presupposed that all sound energy transmission from the
sending room to the receiver room takes place by way of the actual element (see Figure
6 .2). In practice, there are always limitations in spite of the massive flanking
constructions surrounding the element under test. When the sound insulation gets
sufficiently high there is bound to be additional sound transmission by way of these
flanking constructions in much of the same way as normally encountered in buildings.
This is discussed below.

Figure 6.2 Laboratory set-up to determine the sound reduction index.

We shall assume that the sound field in the sending room, as well as in the
receiving room, is diffuse. The sound intensity at the wall in the sending room will then
be given by

2

S

i

00

,

4

p

I

ρc

=



(6.3)

where pS is the sound pressure in the sending room. The power transmitted through the

building element having the surface area S will be

2

R

tt R

00

,

4

p

WIS A

ρc

=⋅= ⋅



(6.4)

where pR and AR is the sound pressure and the total absorption area, respectively, in the
receiving room. Hence, the transmission factor will be given by

2

t RR

2

i S

.

ISpA

I SSp

τ

⋅

==⋅

⋅





(6.5)

The sound reduction index then becomes

Wi Wt