86 Chapter 4
the plates with 6 inch separation is included in Fig.
4-37. The superior performance of laminated glass
comes with a higher cost.
Managing Cavity Resonance. The TL measurements
in Fig. 4-37 were made with no absorbing material
around the periphery of the space between the two glass
plates. By lining this periphery with absorbent material,
the natural cavity resonance of the space is reduced. An
average 5 dB increase in TL can be achieved by
installing a minimum of 1 inch absorbent on these
reveals. The use of 4 inches of absorbing material,
covered with, perhaps, perforated metal, further
improves low-frequency transmission loss.The practice of using glass plates of different thick-
ness is substantiated by shallower coincidence dips in
Fig. 4-37 as compared to Fig. 4-35. Resonance associ-
ated with the plates or the cavity tend toward the
creation of acoustical holes, or the reduction of TL at
the resonance frequencies. Hence, distributing these
resonance frequencies by the staggering of plate thick-
ness and use of laminated glass is important.Homemade Acoustical Windows. The essential
constructional features of two types of observation
windows are shown in Fig. 4-38. Fig. 4-38A is typical
of the high TL type commensurate with walls designed
for high loss. The high TL of the window is achieved by
using heavy laminated glass, maximum practical
spacing of the glass plates, absorbent reveals between
the glass plates, and other important details such as a
generous application of acoustical sealant. It is very
important to note that the windowsill and other elements
of the frame do not bridge the gap between the two
walls and thereby compromise the double wall
construction. Bridging the double wall construction at
the window is a very common error that must be
avoided if the STC of the partition is to be maintained.Fig. 4-38B shows a window for a single stud wall, a
more modest TL. The same general demands are placed
on this window as on the one in Fig. 4-38A, except that
scaled down glass thickness and spacing are appropriate.Inclining one of the plates, as shown in Fig. 4-38,
has advantages and disadvantages. Slanting one pane
reduces the average spacing, which slightly reduces the
TL. However, slanting one window as shown espe-
cially in a studio (as distinct from a control room) will
have the beneficial effect of preventing a discrete reflec-
tion right back at a performer standing in front of the
window. The principal benefit of such plate inclination
is really the control of light reflections that interfere
with visual contact between the rooms.Proprietary Acoustical Windows. Many of the same
companies that build proprietary acoustical doors also
build acoustical windows. IAC builds a line of windows
ranging from STC-35 to STC-58. The STC-53 window
from IAC is shown in Fig. 4-39 and Fig. 4-40. It should
be noted that the same warning about bridging a double
wall construction applies to proprietary windows as
well as to home-made ones.Figure 4-36. Sound TL characteristics of single panels of
laminated glass. Courtesy Libbey- Owens-Ford Co. (After
Reference16.)
Figure 4-37. Spacing two dissimilar glass plates improves
transmission loss. Glass of 2 and 3 inch thickness used in
all cases.
605040302010(^0) 100 200 500 1k 2k 5k 10k
Transmission loss—dB
3/4" laminated
(3/8" × 2)
1/2" laminated
(1/8" + 3/8")
1/4" laminated
(1/8" × 2)
Frequency—Hz
50
40
30
20
10
0
60
100 200 500 1k 2k 5k 10k
Frequency—Hz
Transmission loss—dB
laminated glass
airspace 6:
Airspace 2"
Airspace 4"
Airspace 6"