Massive Materials

Many of the structural materials used in building construction attenuate airborne sound very well. Heavy, dense materials (Fig. 51-1) prevent outdoor sound from carrying to the inside of the building.

Brick has substantial mass, and is good at attenuating sound. When a wall is made of two layers of brick used side by side, but without connection, the level of sound attenuation is very high. Brick walls absorb very little sound, and reflect sound at all frequencies.

Normal weight concrete is one of the best materials for attenuating sound. Lightweight concrete is less effective. Concrete absorbs virtually no sound. Concrete will carry and transmit impact sounds, however. Aerated concretes are porous, and absorb sound fairly well.

Concrete masonry units (CMUs) with hollow cores can attenuate sound quite well, especially when the CMU is of normal weight concrete, and when the hollow cores are filled with concrete, sand, or grout. Walls of two unconnected CMU layers have exceptionally high attenuation.

Massive materials keep sound from traveling from one side to the other.

Figure 51-1 Massive materials.

Reflective materials bounce sound back into the space of its origin.

CMUs, especially cinder blocks, are slightly porous unless painted or sealed. If sealed, CMUs can reflect all frequencies well. Other forms of masonry vary, but are similar to brick, concrete, and CMUs.

Stone, including reconstituted materials such as terrazzo, can be used for massive, load-bearing walls, stone veneer facing, or paving. Thick, well-sealed stone walls attenuate sound very well. Marble is among the most acoustically reflective materials. Some stone is naturally porous, and therefore less reflective.

Plywood has a modest amount of mass, and is relatively ineffective for attenuating sound. Thin plywood furred out from a solid wall is a good absorber of low frequencies. Plywood is quite reflective at high frequencies.

Figure 51-2 Reflective materials.

Glass is massive but thin, so its ability to attenuate sound is marginal. Well-separated double-glazing offers superior sound attenuation, as do some types of laminated glass. Laminated glass consists of two or more sheets of glass with interlayers that provide damping as the glass sandwich is flexed. Some types of laminated glass have substantially better attenuation than equal thicknesses of glass alone. Glass reflects higher frequencies almost completely. Because glass resonates, it will absorb good amounts of low frequencies.

When designing interior glazing, glass lights with i( laminated glass set in resilient framing have more mass and offer better damping than plain glass in rigid frames.

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