Figure 4 shows the improvement in sound isolation performance when double-layer stone wool plenum barriers are combined with a standard stone wool ceiling relative to the performance of the ceiling alone. In one case (bold dashed line), the ceiling systems included recessed light fixtures, open return air grilles, and supply air diffusers. Additionally, there was a supply air duct, three polyvinyl chloride (PVC) pipes, and three metal conduits in the plenum, penetrating the plenum barriers. (In this study, the penetrations were caulked, but over the course of three years of testing, the researchers have come to realize the caulking is unnecessary unless the installation was sloppy and resulted in gaps.) This most accurately represents actual applications in real buildings. The tested performance was STC/CAC 48—three points higher than the requirement of STC/CAC 45 in Workplace 2.0 for enhanced sound privacy.
When the plenum barrier was tested without the penetrations in the ceiling and plenum barrier (bold continuous line), the performance increased to STC/CAC 52. This is high enough to meet the most stringent level of privacy in Workplace 2.0, secured speech privacy, in Assistant Deputy Minister (ADM) offices. Numerous penetrations through the plenum barrier and ceiling should be avoided or acoustically remediated. For example, one might use suspended light fixtures instead of recessed light fixtures and use a return air grille silencer instead of leaving the grille open to the plenum.
Optimizing the results
When comparing the double-layer plenum barrier results of this phase to those from prior phases of the research program, the air space between the plenum barrier layers was found to substantially increase performance. Single-layer, monolithic slabs of stone wool (some with foil facings on both sides), with total thickness greater than the two thinner layers combined, performed significantly worse than a double-layer plenum barrier of thinner material and an interstitial air space.
The foil facing on the plenum barrier material improves acoustic performance significantly. Without it, the performance decreases 10 points. Only plenum barrier material with a foil facing should be used to meet Workplace 2.0 enhanced sound privacy requirements. Substitutions of unfaced materials will likely not result in the required performance.
Covering the vertical seams between plenum barrier panels with metal tape may not be necessary from an acoustic performance perspective. Adding the tape over the seams did not increase wideband STC/CAC or one-third octave band transmission loss. However, taping the vertical seams between the plenum barrier panels tied all of the pieces together into a system that appeared to be more durable and professionally installed. Covering the seams with tape may also help acoustic performance or durability on actual project sites if gaps occur between panels, or if the plenum barriers are penetrated by structural or mechanical elements inside the plenum. Caulking the plenum barriers is generally not required, but might be considered by installing contractors in limited locations if a large gap is observed.
The findings of this research show specifiers, architects, interior designers, and contractors the materials and installation methods for complying with the plenum barrier requirements and various sound privacy levels laid out in Workplace 2.0.
If a space is normally unoccupied, such as with a storage room, one can specify an STC 35 wall and stop it at the underside of a suspended ceiling. No plenum barrier is required. For private offices, meeting rooms, training rooms, and similar spaces, one could call for an STC 45 wall that stops at the underside of the ceiling (CACpanel 20 or higher) and place a double-layer, foil-faced stone wool plenum barrier as described in this article above the wall. Even with penetrations through the ceiling and plenum barrier for building services, the required performance will be met. For the limited spaces requiring secured privacy, using an STC 52 wall and limiting the penetrations in the ceiling and plenum barrier or extending the wall full-height from floor slab to floor slab or roof will achieve the goal.
Gary Madaras, PhD, Assoc. AIA, is an acoustics specialist at Rockfon and the director of the Optimized Acoustics Research Program. He helps designers and specifiers select the appropriate acoustical ceiling products and apply them effectively. Madaras is a member of the Acoustical Society of America (ASA), Canadian Acoustical Association (CAA), and Institute of Noise Control Engineering (INCE). He authors technical articles and speaks publically on the topic of optimizing acoustic experiences. Madaras can be reached at email@example.com.