Glass fiber and other materials come in various densities from the flimsy thermal insulation batts to the semi-rigid and rigid boards used widely in industry. All of these have their proper place in acoustical treatment of spaces, but the question right now is, "What effect does density-the packing of the fibers-have on sound absorption coefficient?" In other words, is the sound able to penetrate the interstices of the high-density, harder surface material as well as one of the flimsy kind? The answer appears in Fig. 9-10, which shows relatively little difference in absorption coefficient as the density is varied over a range of almost 4 to 1. For very low densities the fibers are so widely spaced that absorption suffers. For extremely dense boards, the surface reflection is high and sound penetration low.

Figure 9-10: The density of glass-fiber absorbing material has relatively little effect on absorption in the range of 1.6 to 6 lb/cu ft. The material is mounted directly on solid wall.