Page 238 - The Master Handbook Of Acoustics

213
ABSORPTION OF SOUND

B 2
A C
1
3
4

FIGURE 9-27

The method of stretching the plywood or hardboard skin over the poly bulkheads shown
in Fig. 9-26.

completes poly B. Poly C is mounted in a similar fashion and so on to
the end of the series of polys. The end result is shown in Fig. 9-28.
Notice that the axes of symmetry of the polys on the side wall are per-
pendicular to those on the rear wall. If polys were used on the ceiling,
their axes should be perpendicular to both the others.
It is quite practical and acceptable to construct each poly as an
entirely independent structure rather than building them on the wall.
Such independent polys can be spaced at will.

Membrane Absorbers

Building insulation commonly comes
with a kraft paper backing. Between walls
this paper has no significant effect, but if
building insulation is to be used as a
sound absorber on walls, perhaps behind a
fabric facing, the paper becomes signifi-
cant. Figure 9-29 compares the sound
absorption efficiency of R-19 (6 inch) and
R-11 (3.5 inch) with the kraft paper back-
ing exposed and with the glass fiber
exposed to the incident sound. When the FIGURE 9-28
paper is exposed it shields the glass fiber Finished poly array of Fig. 9-26 mounted on the wall
from sound above 500 Hz but has little above a low-frequency absorber structure. Note that
the axes of the polys on the rear wall are perpendic-
effect below 500 Hz. The net effect is an
ular to the axes of the polys on the other wall. (Moody
absorption peak at 250 Hz (R-19) and 500 institute of Science photo, reprinted with permission of
21
Hz (R-11), which may be important in Journal of the Audio Engineering Society. )