Page 215 - The Master Handbook Of Acoustics

190 CHAPTER NINE

Glass Fiber: Boards
This type of glass fiber usually used in the acoustical treatment of
audio rooms is in the form of semi-rigid boards of greater density than
building insulation. Typical of such materials are Owens-Corning
Type 703 Fiberglas and Johns-Manville 1000 Series Spin-Glass, both of
3 lb/cu ft density. Other densities are available, for example, Type 701
has a density of 1.58 lb/cu ft and Type 705 a density of 6 lb/cu ft. The
Type 703 density, however, is widely applied in studios.
These semi-rigid boards of glass fiber do not excel cosmetically,
hence they are usually covered with fabric. They do excel in sound
absorption.

Acoustical Tile
During the 1960s and 1970s many top-line manufacturers of acoustical
materials offered their competitive lines of 12″- -12″-acoustical tiles.
Surface treatments of the tiles included even-spaced holes, random
holes, slots, or fissured or other special textures. They continue to be
available from local building material suppliers. Such tiles are rep-
utable products for noise and reverberation control as long as they are
used with full knowledge of their limitations. One of the problems of
using acoustical tile in critical situations is that absorption coefficients
are rarely available for the specific tile obtainable. Going back into the
earlier literature, the average of the coefficients for eight cellulose and
3
mineral fiber tiles of 4″ thickness is shown in Fig. 9-7. The range of
the coefficients is indicated by the vertical lines. The average points
3
could be used for 4″ tile for which no coefficients are available. Coef-
1
ficients 20% lower would be a fair estimate for 2″ tiles.

Effect of Thickness of Absorbent

It is logical to expect greater sound absorption from thicker materials,
but this logic holds primarily for the lower frequencies. Figure 9-8
shows the effect of varying absorbent thickness where the absorbent is
mounted directly on a solid surface (mounting A). In Fig. 9-8, there is
little difference above 500 Hz in increasing the absorbent thickness
from 2 inches to 4 inches, but there is considerable improvement
below 500 Hz as thickness is increased. There is also a proportionally
greater gain in overall absorption in a 1-inch increase of thickness in