Page 250 - Master Handbook of Acoustics

suspended ceilings with lay-in panels. (See Table 12-1.)

Mid/High-Frequency Absorption by Porosity

Materials that have a porous composition, that is, with interstices between their matters, can operate
as porous sound absorbers. The key word in this discussion of porous absorbers is interstices, the
small cracks or spaces in porous materials. If a sound wave strikes a wad of cotton batting, the sound
energy causes the cotton fibers to vibrate. The fiber amplitude will never be as great as the air
particle amplitude of the sound wave because of frictional resistance. Restricted as this motion is,
some sound energy is changed to frictional heat as fibers are set in motion. The sound penetrates into

the interstices of the cotton, losing energy as fibers are vibrated. Cotton and many open-cell foams
(such as polyurethane and polyester) are excellent sound absorbers because of their open-cell
porosity that allows sound waves to penetrate the material. On the other hand, closed-cell materials
(such as polystyrene), such as some used for thermal insulation, do not allow sound to penetrate the
material, and are relatively poor absorbers. The better the air flow through a porous material, the

better its ability to absorb sound.
Porous absorptive materials most commonly used as sound absorbers are usually fuzzy, fibrous
materials in forms such as boards, foams, fabrics, carpets, and cushions. If the fibers are too loosely
packed, there will be little energy lost as heat. On the other hand, if fibers 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 that depend 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

may fill the surface pores and greatly reduce sound penetration, and thus absorption. However, if
spray-painted with nonbridging paint, the absorption may be reduced only modestly. Acoustical tile
painted at the factory minimizes the problem of reduced absorption. Under certain conditions, a
painted surface can reduce porosity but act as a diaphragm that might actually become a fair absorber
on a different principle, that of a damped vibrating diaphragm.
In some rooms, the acoustical treatment may overly prescribe carpeted floors and drapes, which

emphasizes a shortcoming of most porous absorbers—that of poor low-frequency absorption. Tiles of
cellulose fiber with perforated faces are also deficient in low-frequency absorption. Porous
absorbers are thus proficient at reducing high-frequency sound energy, but do not address a major
problem of room acoustics—low-frequency standing waves.

To show the general similarity of the absorption characteristics of sound absorbers depending on
porosity for their effectiveness, a comparison is made in Fig. 12-7. The acoustical tile, drapes, and
carpet show highest absorption above 500 Hz and relatively low absorption in the low-frequency
region dominated by room modes. Coarse concrete blocks show a typical high-frequency absorption
peak, but also a more unusual absorption peak around 200 Hz.