Handbook for Sound Engineers

Acoustical Noise Control 87

4.3.11 Transmission Loss of a Compound Barrier

We are using the term compound to refer to those parti-

tions that are not homogeneous—e.g. those partitions

that include areas with differing TL ratings. For

example, when an observation window having one TL

is set in a wall having another TL, the overall TL is

obviously something else, but what is it? It most

certainly cannot be obtained by simple manipulation of

TLs or STC values. The problem must be referred to as

the basics of sound power transmission. Fig. 4-41 illus-

trates the case of a 4.4 ft × 6.4 ft window set in a

10 ft × 15 ft partition between control room and studio.

The way the transmission loss of the window and the

wall affect each other is given by the expression:18,19

(4-4)

where,

TL is the overall transmission loss,

S 1 is the fractional wall surface,

TL 1 is the wall transmission loss in decibels,

S 2 is the fractional window surface,

TL 2 is the window transmission loss in decibels.

As an example let us say that for a given frequency

the wall TL 1 = 50 dB and the window TL 2 = 40 dB.

From Fig. 4-40 we see that S 1 = 0.812 and S 2 = 0.188.

The overall TL is

The 40 dB window has reduced the 50 dB wall to a

45.7 dB overall effectiveness as a barrier. This is for a

given frequency. Fig. 4-42 solves Eq. 4-4 in a graphical

form using the following steps:

1. Figure the ratio of glass area to total wall area, and
   find the number on the X axis.


2. Subtract window TL from wall TL, and find the
   intersection of this value with the area ratio on the
   X axis.


3. From the intersection, find the reduction of the wall
   TL from the left scale.


4. Subtract this figure from the original wall TL.

Using the graph of Fig. 4-42, find the effect of the

window on the compound wall. The ratio of the window

area to the wall area is 0.23. Locate 0.23 along the

bottom axis. The difference in TL between the two is

10 dB. Find the intersection between the 10 dB line and

the ratio of the areas. A reduction of slightly less than

5 dB is read off the left scale. Subtracting 5 dB from the

Figure 4-38. Construction details for practical observation
windows set in a partition between control room and
studio.

(^1) / 2 " gap
(^3) / 4 " glass
plate
Studio
1' plaster
Control room
(^1) / 2 " glass plate
2" glass fiber – 3 lb/ft^3
All glass plates set
in soft rubber gaskets
Bead of acoustical
sealant
Concrete block
8" × 8" × 16"
Building insulation
A. A window suitable for a high TL wall.
(^3) / 8 " glass plate
Studio
Control room
Double^5 / 8 "
gypsum board
(^5) / 8 " gypsum board
outer layer on
resilent strips
B. A window suitable for a more modest frame wall.
Building insulation
(^1) / 2 " glass plate
2" glass fiber
3 lb/ft^3
All glass plates set
in soft rubber gaskets
Bead of acoustical
sealant
TL 10–
S 1
10
TL 1
10

©
̈
̈
̈
§
= log
S 2
10
TL 2
--------- 10
+-------------
¹
̧
̧
̧
·
TL 10–
S 1
10
TL 1
--------- 10

©
̈
̈
̈
§
log
S 2
10
TL 2
--------- 10

¹
̧
̧
̧
·
= +
=45.7dB