Page 213 - The Master Handbook Of Acoustics

188 CHAPTER NINE

Air space
Mounting
Mounting A
depth
16"

Mounting E-405
FIGURE 9-6
Commonly used standard mountings associated with listings of absorption coefficients. With Mounting A the
1
material is flat against the backing. Mounting B (not shown) is similar to Mounting A but with a 16″ air-
space resulting when acoustical materials are cemented to a surface. Mountings B and A are essentially the
same. Mounting E-405 applies to suspended ceilings with lay-in panels. (See Table 9-1.)

more fibers are vibrated. Cotton is an excellent sound absorber that has
been specified in studio treatment in Africa where it was plentiful and
cheap, and because imported materials were out of the question.
Porous absorptive materials most commonly used as sound
absorbers are usually fuzzy, fibrous materials in the form of boards,
foams, fabrics, carpets, cushions, etc. If the fibers are too loosely
packed, there will be little energy lost as heat. On the other hand, if
they are packed too densely, penetration suffers and the air motion
cannot generate enough friction to be effective. Between these
extremes are many materials that are very good absorbers of sound.
These are commonly composed of cellulose or mineral fiber. Their
effectiveness depends on the thickness of the material, the airspace,
and the density of the material.
The absorption efficiency of materials depending on the trapping
and dissipating of sound energy in tiny pores can be seriously
impaired if the surface pores are filled so that penetration is limited.
Coarse concrete block, for example, has many such pores and is a fair
absorber of sound. Painting that block fills the surface pores and
greatly reduces sound penetration, and thus absorption. However, if
spray painted with a nonbridging paint, the absorption may be
reduced very modestly. Acoustical tile painted at the factory mini-