Wood Fiber Sound Board.This semi-rigid building material composed of wood fibers and binders, compressed under heat and pressure into large panels of various sizes, presents another solution for reducing sound transmission. Sound board can be used as a damping layer between studs and gypsum board to improve STC ratings. The extra labor and material cost combined with a relatively minor performance improvements are the main disadvantages of wood-fiber sound boards as an effective sound solution.

Installation Concerns

To realize the full potential of any sound-reduction product, correct installation is critical. In many cases, what tests well in the laboratory may not perform up to its advertised STC rating in real-life situations. One reason for this is the complicated installation required in several sound-control solutions discussed above.

To recap, resilient furring channels are particularly prone to installation mistakes; any "hard" connections between gypsum board and studs will reduce the assembly's flexibility and thus virtually eliminate the promised benefit of sound transmission loss. Other existing solutions rely on adding multiple layers of gypsum board to increase mass. All of these solutions involve added material and labor costs and result in deeper wall cavities, which consumes valuable floor space. A typical wall assembly using resilient channels can be up to 6 1/2 inches thick. A double-stud assembly can be even thicker-up to 9 inches or deeper in some cases.

When specifying an acoustically enhanced gypsum board, the issues for the architect include criteria for sound control on one hand, and fire ratings within the prescribed budget on the other. Yet another critical point is whether the wall assembly is contractor friendly-so that actual installed performance meets the designer's expectations for sound control. Acoustically enhanced gypsum board is installed like regular gypsum board as described in the Gypsum Association publication GA 216-2004. With this relatively easily installation process, acoustically enhanced boards have a high potential for delivering consistent STC ratings in the field. Because of their reliance on constrained layer damping as opposed to structural damping, acoustically enhanced products are not prone to "short-circuit" events as they are not dependent on maintaining separation between the gypsum board and the framing studs.

Cutting acoustically enhanced gypsum board requires scoring deeply from both sides of the board before snapping, or the use of a hand or power saw. Installation recommendations are relatively straightforward and involve staggering board joints from one side of the wall to the other; limiting wall penetrations to one stud per cavity; and using ASTM-approved acoustical sealants, caulk, and putty pads. Installing acoustically enhanced gypsum board over an insulating blanket, applied continuously across the face of the framing members, is not recommended. Blankets should be recessed and flanges attached to the sides of the studs or joists. When storing acoustically enhanced gypsum board off the ground-and ideally under cover-sufficient risers must be used to ensure support for the entire length of the gypsum board to prevent sagging.

Flanking Sound. Unfortunately, wall assemblies are often specified to meet the minimum building-code criteria for STC ratings, but the details are ignored-a scenario that can have adverse impact on overall sound isolation. That oversight can be exacerbated by the fact that few local building-inspection departments require field tests to validate compliance with sound-isolation measures. A key consideration here is specifying and designing measures to prevent "flanking" sound-transmission pathways. Flanking pathways are construction conditions that let noise snake around the main wall or floor.

Sound will find its way through any weaknesses in the building's design; these areas are known as flanking paths. In laboratory testing, flanking paths are eliminated and only the direct path is measured. In the real world, the transmission loss values of a structure can be lower due to sound passing through roof voids, small holes, and cable glands in the structural system. Doors and windows, for example, are often acoustically weak and can significantly reduce the acoustic insulation of a façade or wall. Often the transmission along these flanking paths seriously reduces the sound insulation between units. Proper use of acoustical sealant or caulk can reduce its effect. Lack of caulking, blocking, or other measures that will prevent flanking sound transmission can reduce the overall STC significantly. For best results, all perimeter gaps and non-uniform gypsum board edges should be completely sealed off with acoustical sealant or caulk. An unsealed 1/4-inch gap around the perimeter of a 100-square-foot wall partition with an STC in the 50s will result in a loss of over 20 STC points. This kind of design and installation prevention can potentially save a project from a post-construction lawsuit.

Best-practice methods dictate that acoustically enhanced gypsum board joints should be staggered from one side of the wall to the other. A 1/4-inch gap should be allowed along all perimeter edges and completely sealed with acoustical sealant or caulk. All wall penetrations should be sealed with acoustical sealant as described in ASTM C 919 and putty pads consistent with ASTM E 90.

Acoustically enhanced products can be finished and decorated like standard gypsum board. In order to determine the level of finish required to ensure a properly prepared surface that accepts the desired decoration, architects should refer to Gypsum Association GA-214, "Recommended Specification for Levels of Gypsum Board Finish."

In conclusion, reducing the transmission of airborne sound between spaces within buildings is a goal of a wide spectrum of residential, commercial, and institutional construction projects. Moreover, in many places it is also mandated by law. Industry has expanded its product range to assure architects of reliable, cost-effective, and easy-to-install wall partitions that minimize the intrusion of noise in homes, hospitals, hotels, schools, and other facilities.