Page 234 - The Master Handbook Of Acoustics

209
ABSORPTION OF SOUND

applications in which reflected mid/high-frequency sound from the
panel absorber might create problems, a facing of glass fiber board
would not interfere with the low-frequency absorbing action if it was
spaced to avoid damping of the vibration of the plywood panel. Chap-
ter 15 emphasizes that all room modes terminate in the corners of a
room. A corner panel absorber could be used to control such modes.

Polycylindrical Absorbers

Flat paneling in a room might brighten an
interior decorator’s eye and do some good
acoustically, but wrapping a plywood or
hardboard skin around some semicylindri-
cal bulkheads can provide some very
attractive features. These polycylindrical 1 /4"– /2" Airspace
1
elements (polys) are coming back into
fashion. A few are used in top-flight
recording studios today. Visually, they are
rather overpowering, which can be good or
bad depending on the effect one wants to
achieve. With polys it is acoustically pos-
sible to achieve a good diffuse field along
with liveness and brilliance, factors tend-
1"–1 /2" Glass or
1
ing to oppose each other in rooms with flat
mineral fiber
surfaces.
One of the problems of using polys has
been the scarcity of published absorption
coefficients. The Russian acoustician, V. S. 1 /4"– /16" Plywood
1
Mankovsky, has taken care of that in his
9
book. As expected, the larger the chord
dimension, the better the bass absorption.
Above 500 Hz there is little significant dif-
ference between the polys of different
sizes.
The overall length of polys is rather
immaterial, ranging in actual installations
from the length of a sheet of plywood to FIGURE 9-23
the entire length, width, or height of a Typical resonant panel absorber for wall mounting.