Handbook for Sound Engineers

Acoustical Noise Control 85

each graph. Although the heavy ¾ inch plate attains a
TL of 40 dB or more above 2 kHz. It is inappropriate
for use in an STC-50 or STC-55 wall. Considering this
general lack of sufficient TL and the complication of the
coincidence dip, the single-glass approach is insuffi-
cient for most observation window needs. Laminated
glass is more of a limp mass than glass plate of the same
thickness and, hence, has certain acoustical advantages
in observation windows. The characteristics of ¼ inch,
½ inch, and ¾ inch laminated single-glass plates are
shown in Fig. 4-36.

Transmission Loss of Spaced Glass Plates. Fig. 4-37
shows the effect of three different spacings. In all cases
the same ½ inch and ¼ inch glass plates are used, but
the air space is varied from 2 inches to 6 inches. The
effect of spacing the glass plates is greatest below
1500 Hz. There is practically no increase in transmis-
sion loss by spacing the two glass plates above
1500 Hz. In general, the 2 inch increase from 2 inches
to 4 inches is less effective than the same 2 inch spacing
increase from 4 inches to 6 inches. Many observation
windows in recording studios utilize spacings of
12 inches or more to maximize the spacing effect.
When two glass plates are separated only a small
amount, such as glass widely used for heat insulation,

the sound TL is essentially the same as the glass alone

from which it is fabricated. There is practically no

acoustical advantage using this type of glass in observa-

tion windows. This is one of the few cases where

thermal insulation does not correspond to acoustic isola-

tion. The single case of using laminated glass for one of

Figure 4-34. IAC STC 43 Door. Courtesy IAC.

Head

Magnetic acoustic

seal (typ.)

Wall

thickness^3 / 8 "

2" (10)

(45)

3"

(76)

13 / 4 "

(45)

JAMB (right)

23 / 4 "

(45) (^3) /
8 "
(10)
Wallthickness
13 / 4 "
(45)
2"
(45)
Astragal (double leaf only)
13 / 4 "
(45)
Finished floor
Compression seal
Section A SILL
Head
"H"Clean opening
Sill
Figure 4-35. Sound TL characteristics of single glass (plate
or float) panels. Courtesy Libbey-Owens-Ford Co. (After
Reference 16.
50
40
30
20
10
0
Transmission loss—dB
3/4"
1/2"
1/4"
100 200 500 1k 2k 5k 10k
Frequency—Hz