Achieving Acceptable Noise Levels: Design Considerations in Acoustical Wall Partitions

There are three strategies to reduce noise in buildings: eliminating the source of the noise; using sound-absorbent rather than sound-reflective materials; installing sound barriers to prevent sound transmission into adjoining areas. Most of the solutions in current use fall into the third category. Several techniques can be used alone or in conjunction with others to achieve a higher STC rating. With the goal of decreasing the amount of sound transmission through the partition, the following five variables can have an impact on the ability of the partition to provide this desired loss.

Ways to Affect Sound Transmission
Increasing Mass. Increasing the amount of material sound waves must penetrate to reach the adjoining room is an obvious tactic. More mass means sound waves must spend more energy to flow through the wall. Using multiple layers of gypsum board is one solution that can reduce sound transmission from room to room. Increasing mass in a cost- and space-effective way presents its own challenges, however. The sheer amount of mass needed to reduce decibel levels significantly may require a wall thickness that is impractical for a building.

Eliminating Stiffness. Flexibility in the wall assembly will promote sound absorption; conversely, too much stiffness will reduce transmission loss. For that reason, replacing wood studs with metal studs, which inherently have more give to them, is a preferred practice and results in better acoustical performance. In general, less framing will achieve superior acoustical results, with 24-inch o.c. framing spacing outperforming 16-inch o.c. framing.

Increasing Damping. Damping is a technique for dissipating vibrational energy in a structure, thereby decreasing the amount of energy allowed to pass through to the next room. Damping consists of one or a combination of materials applied to the wall interior to boost its ability to reduce mechanical energy. All materials demonstrate some damping, though many materials are actually effective radiators and transmitters of sound.

Increasing Cavity Depth. Increasing the depth of the wall cavity between wall panels in two adjoining units can increase the thickness of the overall partition, and thus increase the amount of sound transmission loss, particularly when the cavity is filled with acoustical insulation.

Providing Cavity Absorption. Another way to control airborne sound from room to room is through the use of materials that absorb sound by converting sound waves into heat. Adding sound-absorbing material such as fiberglass or mineral-fiber insulation to the cavity of a partition will increase the amount of sound transmission loss. The sound absorbing material should completely fill the cavity but not be compacted or compressed in any way. Any gaps in the wall cavity should be filled.

Sound Reduction Solutions

There are a range of products that will improve the acoustical performance of a wall partition through one or a combination of strategies outlined above.

Acoustically Enhanced Gypsum Board. Mass and damping are important features in a wall's ability to achieve a higher STC. These features are incorporated in acoustically enhanced gypsum board, a new product on the market to reduce sound transmission. This product incorporates increased mass by using gypsum board with a high-density core in an arrangement known as constrained layer damping.

While the introduction of structural damping will increase the amount of sound transmission loss, constrained layer damping, in particular, is more effective. Constrained layer damping can be a three-layer "sandwich" system constructed from attaching a base layer to a damping layer and adding a third constraining layer. In acoustically enhanced gypsum board, constrained-layer damping, a new form of noise reduction, is achieved through the use of a middle layer containing viscoelastic polymer sandwiched between two pieces of enhanced high-density gypsum board. A form of plastic, the viscoelastic polymer absorbs and dissipates noise by converting the vibrational energy of sound waves into negligible heat. Acoustically enhanced gypsum board is appropriate for use as single-layer application or as a component of multi-layered wall assemblies where sound transmission between rooms or dwelling units is a concern. The gypsum board can be mold-resistant, if need be.

A key advantage of this type of product is that the entire wallboard panel can be produced in a typical 5/8-inch gypsum board for use in the construction of high-STC-rated wall assemblies. Because the solution relies on constrained-layer damping rather than structural isolation, no additional depth is required in the wall cavity, as is the case with many typical sound-reduction solutions. As a result, architects and building owners can benefit from the ability to maximize floor space in each unit.

Because acoustically enhanced gypsum board can be installed like traditional gypsum board, it is able to offer a more reliable and less complicated solution than alternative methods that require clips and channels, and where the possibility of improper installation is a key concern. Another advantage of acoustically enhanced gypsum board is that it is easily finished and can be decorated in the same manner as standard gypsum board.

Fire Resistance Ratings

While most acoustically enhanced gypsum board cannot be used as a substitute for 5/8-inch board in a fire-rated assembly, it can be used as an additional layer in all UL fire-rated assemblies without compromising the fire rating. Upgrading such an assembly with an acoustically enhanced product can increase the STC rating by as much as 10 points while maintaining its fire rating. As an option, the acoustically enhanced product may be used as an additional layer on one or both sides of fire-rated wall assemblies. When acoustically enhanced gypsum board is installed between the framing and the UL-classified gypsum board, the UL-classified gypsum board layer(s) required for the design should be installed as indicated in the design as to fastener type and spacing, except that the required fastener length should be increased by a minimum of 5/8 inch. For UL-rated walls of the U300, U400 and V400 series designs, the outer layer should be treated with joint compound and paper tape.

Acoustically enhanced products are available in 4-foot-wide panels in lengths of 8 feet, 9 feet, 10 feet and 12 feet, and in a variety of profiles, including nonrated wood- and metal-stud designs and 1-hour-rated metal-stud wall designs with STCs ranging from 57 to 61.

Architects should be aware that the use of acoustically enhanced products in actual installations may not produce the same results as were achieved in controlled laboratory conditions.