Using plenum barriers to comply with sound isolation requirements

The existence of these noise flanking paths is 
well-known in the architectural acoustics industry. Previous studies by the National Research Council 
of Canada (NRC) concluded even if ceiling panels with high transmission loss are used, the attenuation between rooms is limited by such leaks. (R.E. Halliwell and J.D. Quirt’s “Controlling Interoffice Sound Transmission through a Suspended Ceiling,” which appeared in the September 1991 issue of the Journal of Acoustical Society of America, has more information.)

The findings of this earlier phase of the research program showed typical noise flanking paths in a suspended acoustic ceiling (caused by penetrations for lights and air-distribution devices alone) decreased ceiling attenuation class (CAC) by 10 points. (A piece co-written by this article’s author and A. Heuer, “Effects of Noise Flanking Paths on Ceiling Attenuation Class [CAC] Ratings of Ceiling Systems and Inter-Room Speech Privacy,” was part of the Proceedings of Inter-Noise 2015, edited by Courtney Burroughs and George Maling.) CAC is the acoustic metric quantifying the ceiling panel’s sound-blocking performance when the demising wall stops at the height of the ceiling. A 16-mm (5/8-in.) thick mineral fiber ceiling panel tested at CAC 37, but when four lights, one supply diffuser, and one return-air grille were added to the ceiling system, the value decreased to CAC 27. More importantly, the decrease in performance was not consistent across all frequencies.

Figure 1: These images depict laboratory ceiling attenuation class (CAC) tests with the noise flanking paths through the return grilles, supply diffusers, flexible ducts, and light fixtures remediated. At left, noise-control measures include an internally lined elbow duct over the return-air grille. Insulation and mass-loaded vinyl (MLV) over the lights and diffusers and around flexible ducts can be seen in the image on the right.

High-frequency isolation (i.e. 1000-Hz octave band and higher), which is more relevant to whether or not speech is intelligible, decreased by 15 to 22 dB. Therefore, ‘CACpanel’ must be differentiated from ‘CACsystem’—the former being what is tested and reported by manufacturers for their ceiling panels, and the latter being how the whole ceiling system with common penetrations actually performs.

While noise flanking paths through ceiling systems can be remediated at times, the noise-control measures required to do so are often labor-intensive (and therefore costly) and can prevent practical access to the ceiling plenum. (See “Optimizing Ceiling Systems and Lightweight Plenum Barriers to Achieve Ceiling Attenuation Class [CAC] Ratings of 40, 45, and 50,” written by the same authors in note 4. It was part of Proceedings of Noise-Con 2016−Revolution in Noise, edited by Burroughs and Gordon Ebbitt.) Figure 1 illustrates the noise-control measures used during this early phase of testing to increase CACsystem to equal CACpanel.

Reducing sound transmission
The Building Science Branch of the Alberta Public Works, Supply, and Services has conducted extensive research on sound isolation between offices with suspended ceilings. (For more, read K. Kruger’s piece, “The Effect of Various Parameters on the Sound Isolation between Offices with Suspended Ceilings,” from Canadian Acoustics [16 (2)] 
in 1988.) It states attempting to match 
the isolation performance of a ceiling to that of the demising wall can lead to disappointing results. It is important to know the combined effect of the wall, ceiling system, and any flanking that might be introduced through the ceiling.

Figure 2: A full-height demising wall, shown on the left, is required by most U.S. acoustic standards. The center image illustrates use of lightweight plenum barriers above partial-height demising walls, as required by some Canadian acoustics standards. Open plenums, like the one on the right, are not permitted by U.S. or Canadian acoustic standards—they do not meet occupant expectations of sound privacy.

Another conclusion is the most effective method of reducing sound transmission through the ceiling is to introduce a barrier into the plenum (Figure 2). The plenum barrier can be limited in length. It only has to be positioned above the demising wall between the two adjacent rooms, and does not need to extend around the entire perimeters. This would allow return air to still flow freely through the plenum.

When plenum barriers do need to surround the room’s entire perimeter, a hole of the appropriate size based on air volume and desired velocity should be cut in the plenum barrier over the door into the room. Unless the door is heavy, with gaskets, and acoustically rated at STC 40 or above, it will likely transmit more noise than the return-air opening in the plenum barrier above the ceiling.

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