Page 247 - The Master Handbook Of Acoustics

222 CHAPTER NINE

Figure 9-36 is identical to Fig. 9-35 except that the perforated ply-
wood is spaced 8″ and glass fiber of 4″ thickness is mounted in the cav-
ity. The general effect of these changes is a substantial broadening of
the absorption curve.
It would be unusual to employ such perforated panel absorbers
without acoustic resistance in the cavity in the form of glass fiber batts
or boards. Without such resistance the graph is very sharp. One possi-
ble use of such sharply tuned absorbers would be to control specific
troublesome room modes or isolated groups of modes with otherwise
minimum effect on the signal and overall room acoustics.
Table 9-3 includes the calculated frequency of resonance of 48 dif-
ferent combinations of airspace depth, hole diameter, panel thickness,

1.0

C
0.8
D 4"
8"

Absorption coefficient 0.4 A
0.6

0.2 B

0
100 1,000 4,000
Frequency - Hz
FIGURE 9-36
Actual absorption measurements on the same perforated panel absorbers of Fig. 9-35
except that the air space is increased to 8″, half of which is taken up with mineral fiber.
5
Panel thickness is 32″. (A) Perforation 0.18%. (B) Perforation 0.79%. (C) Perforation
9
1.4%. (D) Perforation 8.7%. (Data from Mankovsky. )