Page 254 - Master Handbook of Acoustics

expanded metal, metal lath, hardware cloth, or perforated vinyl wall covering can be used as a cover.
Absorption coefficients greater than 1.0 can be obtained by glass-fiber panels.

Glass-Fiber High-Density Boards

Semirigid boards of glass fiber can be used in the acoustical treatment of audio rooms. This type of
glass fiber is usually of greater density than glass-fiber building insulation. Typical of such materials
are Johns-Manville 1000 Series Spin-Glass and Owens-Corning Type 703 Fiberglas, both of 3 lb/ft 3
density. These are available in different thicknesses (e.g., 1 to 4 in) yielding different R values (e.g.,

3
4.3 to 17.4). Other densities are available; for example, Type 701 has a density of 1.5 lb/ft and Type
3
705 has a density of 6 lb/ft . These semirigid boards of glass fiber do not excel cosmetically; hence
they are usually covered with fabric. However, they do excel in sound absorption and are widely

used for room treatment.

Glass-Fiber Acoustical Tile

Manufacturers of acoustical materials offer competitive lines of 12 × 12 in acoustical tiles. Surface

treatments of the tiles include even-spaced holes, random holes, slots, fissured, or other special
textures. These tiles are typically available from local building material suppliers. Such tiles are
reputable products for noise and reverberation control provided 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 not always available for a specific tile. The average of the coefficients for eight
cellulose and mineral fiber tiles of 3/4 in thickness is shown in Fig. 12-9. The range of the

coefficients is indicated by the vertical lines. The average points could be used for 3/4-in tile for
which no coefficients are available. Coefficients 20% lower would be a fair estimate for 1/2-in tiles.
When acoustical tiles are used in a drop ceiling, they function primarily as porous absorbers.
However, the plenum airspace above the tiles causes them to also perform somewhat as panel
absorbers.