Page 257 - Master Handbook of Acoustics

FIGURE 12-11 The low-frequency absorption of 1-in glass-fiber board is improved materially by
spacing it out from the solid wall.

Effect of Density of Absorbent

Glass fiber and other materials come in various densities ranging from soft thermal insulation batts to
semirigid and rigid boards. All of these have their proper place in the acoustical treatment of spaces.

Generally sound is able to penetrate the interstices of a high-density, harder surface material as well
as a soft type. Figure 12-12 shows that there is 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.