Installation Of Absorptive Materials

f The way materials are installed affects their ability to absorb sound (Fig. 52-1). Installing absorptive materials directly on a wall or ceiling gives the least effective sound absorption. A layer of air between the absorptive material and a rigid surface works almost as well in mid-range frequencies as if the same cumulative thickness of absorptive material were used, which is useful to know because air is cheaper than other materials. To get the best low-frequency absorption, you need a deep air space on the ceiling, and you should treat the walls as well. A hung ceiling 41 cm (16 in.) below the structural slab is too shallow to even absorb midrange frequencies well. The absorption coefficient ratings for materials are always given with mountings corresponding to American Society for Testing and Materials (ASTM) requirements.

Hanging the absorptive material below the ceiling and supporting it away from the walls works better than attaching material firmly to walls or ceilings. The best way to install acoustically absorbent material is to hang cubes or tetrahedrons from the ceiling. When you use very thick blocks installed at a distance from each other, the edge absorption is very large, especially in the high frequencies. However, these objects become major architectural elements in the space. As this may not be appropriate for all uses, louvers or baffles offer a somewhat less effective but simpler option.

For best results, treat the ceiling, floor, and wall op- 3 posite the sound source approximately equally. Treating the ceiling alone may miss highly directive high-frequency waves, which may not reach the ceiling until the third reflection off a surface.

Materials absorb high frequencies better than lower frequencies. The amount of absorption is not always proportional to the thickness of the material, but depends on the material and its method of installation. Beyond a certain point, added thickness does little to increase absorption, except at very low frequencies. The lowest musical frequencies can't be absorbed efficiently by ordinary thicknesses of porous material. Let's look at how some specific materials absorb sound.

Rig idly fastened to concrete

Nailed to furring

Suspended from ceiling and walls

Figure 52-1 Installing absorptive materials.

Suspended from ceiling and walls

Figure 52-1 Installing absorptive materials.

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