Originally published in Interiors & Sources

04/15/2009

Acoustical Wall Design

By Travis Lawrence and Mei Wu

 
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    Figure 1 : Transmission Loss Comparison of a Wall with a Door (larger image)

When designing a new education or healthcare facility, there are acoustical guidelines you need to follow from the American Institute of Architects (AIA) and the American National Standards Institute (ANSI). These organizations provide sound transmission class (STC) requirements for noise-sensitive rooms in education and healthcare facilities. These requirements from ANSI and the AIA focus on walls that perform at an STC of 50, which satisfies building-code requirements. Here are your options for walls in these types of spaces …

Common Wall Build-Up
A common wall build-up (floor to floor) is a wall for spaces that have no speech privacy or noise-isolation requirements. With these three elements below, you gain an STC rating of 37:

  • 18-gauge (3 5/8-inch) steel studs.
  • One layer of 5/8-inch GWB on each side.
  • 3 inches of batt insulation in stud cavities.

Increasing the STC of this wall type to achieve an STC 50 rating requires additional layers of drywall to each side. You’d need to use:

  • 18-gauge (3 5/8-inch) steel studs.
  • Two layers of 5/8-inch GWB on each side.
  • 3 inches of batt insulation in stud cavities.

The use of resilient channels (RCs) within wall build-ups is a low-cost approach for increasing the STC rating. Although laboratory STC ratings of such wall build-ups confirm these STC benefits, it’s not uncommon for RC field installation to be rendered nonfunctional by using the wrong products and/or an incorrect installation. This can eliminate anticipated STC gains and accrue wasted labor and material costs. You can reach an STC rating of 50 by using:

  • 18-gauge (3 5/8-inch) steel studs with resilient channels.
  • One layer of 5/8-inch GWB on each side.
  • 3 inches of batt insulation in stud cavities.

The issues concerning construction costs, time, and building space needed for such a wall type introduce a need for alternative wall designs. Achieving the STC 50 rating desired by AIA and ANSI guidelines can be accomplished with several alternatives, which all have their strengths and weaknesses.

Alternative STC 50+ Wall Build-Ups
High STC ratings can be achieved without the use of special acoustical products if the gauge of the steel studs is soft. A soft metal stud will reduce the efficiency of transmitted vibrations (noise); however, the soft gauge studs reduce the stiffness of a wall and limit the amount of weight the wall can hold, and can also limit the use of a room. To reach an STC rating of 50, use:

  • 25-gauge (3 5/8-inch) steel studs.
  • One layer of 5/8-inch GWB on each side.
  • 3 inches of batt insulation in stud cavities.

Acoustical Building Products
One of the transmission paths of airborne noise between spaces occurs when a shared partition vibrates from a noise source. The sound pressure level (SPL) of the noise source and the sound transmission loss of the shared partition are key factors in the SPL of the receiving room. When the transmission loss of the shared partition is increased, the magnitude of transmitted noise is reduced. The use of specific “acoustical building products” in wall build-ups is effective at increasing STC ratings with minimal loss of space, but carries a premium price.

Acoustical drywall (soundproof drywall) has more mass than regular drywall, and incorporates increased dampening by use of constrained layers of sheet metal and damping compounds. You can achieve an STC rating of 52 by using:

  • 18-gauge (3 5/8-inch) steel studs, 24-inch OC.
  • One layer of 5/8-inch acoustical drywall on one side of the wall.
  • One layer of 5/8-inch GWB on the other side.
  • 3 inches of batt insulation in stud cavities.

Dampening compounds are a type of non-hardening glue that increases dampening between two layers of drywall. The compound is sandwiched between two layers of regular drywall and mitigates the vibrations responsible for transmitted noise. These types of products are effective in post-construction projects as well. An STC rating of 52 can be achieved by using:

  • 18-gauge (3 5/8-inch) steel studs, 24-inch OC.
  • Dampening compound sandwiched between two layers of 5/8-inch GWB on one side.
  • One layer of 5/8-inch GWB on the other side.
  • 3 inches of batt insulation in stud cavities.

Resilient sound isolation clips (RSICs) are refined resilient channels that eliminate incorrect installation issues that occur with traditional resilient channels. The RSIC isolates the drywall from the wall studs and uses a rubber element to reduce the vibration path between rooms, thereby increasing the STC rating. An STC rating of 53 can be achieved by using:

  • An RSIC-V clip acoustic assembly.
  • 7/8-inch x 25-gauge furring Channel @ 24-inch OC, RSIC-V clips @ 48-inch OC.
  • 18-gauge (3 5/8-inch) steel studs.
  • One layer of 5/8-inch GWB on each side.
  • 5.5-inch batt insulation in stud cavities.

Variation of Stud Rows
Wall types that have a single row of studs are coupled systems that transmit vibrations (noise) more efficiently than decoupled systems. By separating certain components within these coupled systems, these efficient vibration paths are reduced and result in higher STC ratings (less transmitted noise). More floor space must be used when using a decoupled system, but the building materials and construction methods won’t incur the increased cost that occurs with special acoustical products. An STC rating of 50 can be achieved by using:

  • 18-gauge (3 5/8-inch) staggered steel studs.
  • One layer of 5/8-inch GWB on one side.
  • Two layers on the other side.
  • 3 inches of batt insulation in stud cavities.

Or, an STC rating of 57 can be achieved by using:

  • 18-gauge (3 5/8-inch) double row steel studs.
  • One layer of 5/8-inch GWB on each side of wall.
  • 1-inch gap between the two rows.
  • 3 inches of batt insulation in both cavities.

Common Noise Leaks
The laboratory STC rating generally stated in a wall build-up design isn’t always achieved in the field. Different factors cause this, but it’s most commonly due to building elements that don’t properly seal one room from another. When these improper seals occur, they’re referred to as “noise leaks,” and such leaks can have adverse effects on the field STC value of a partition. Building elements where noise leaks commonly occur include doors, wall penetrations, window mullions (fake and real), HVAC ducts, and gaps at the top and bottom of a wall.

A comparison of the transmission loss of a wall with and without a door has a difference of 9 dB in the mid- and high-frequency bands, and is demonstrated in Fig. 1.

Travis Lawrence is senior acoustic consultant and Mei Wu is principal consultant at Redwood City, CA-based Mei Wu Acoustics.

 

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When choosing a metal-clad building for your next construction project, consider Morton Buildings, Inc., and their designBUILD team, we’ll make your dream a reality.

Visit our website today to learn about the design flexibility of a Morton building and the endless possibilities of partnering with our designBUILD team.

Wood construction is both cost and energy efficient. Check out Morton Buildings and our designBUILD team online today to discover all the benefits of post-frame construction.

We Can Help You Reduce Energy by 30%

Our mission is to help our customers manage their buildings' energy costs, improve reliability, and enhance performance while having a positive impact on the environment.
CLICK HERE to find out how.


Mitsubishi Electric’s H2i R2-Series heat pumps provide 100% heating capacity down to 0° F and simultaneous heating and cooling down to -4° F delivering year-round comfort, regardless of climate zone.

 
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