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Acoustical Plenum Barriers: What helps block sound transfer best?

(This is the second article in a three-part series on using acoustical plenum barriers for sound privacy.) Why can't I use the ceiling to block sound through the plenum? The short answer is because suspended, acoustic ceilings are for sound absorption. They are not good at blocking sound transfer. They are very porous and lightweight. Compare a sheet of gypsum board (drywall) used in partitions to a ceiling panel. The gypsum board is much heavier. We paint walls, sealing up any pores. Partitions are constructed of at least two, and sometimes up to four, layers of gypsum board. The ceiling panel is just no match.

Most ceiling panels have sound blocking capacities (Ceiling Attenuation Class, where the sound passes through the ceilings in both rooms) of only 20 to 35 points, which is 10 to 25 points less than what is required by standards for sound privacy between offices.

Please remember that the ceiling panel's sound blocking capacity is based on the panel alone. It does not account for the fact that we then penetrate the ceiling with lighting, air devices, speakers, etc. causing significant noise leaks.

Research shows that adding these penetrations can decrease the overall ceiling system's sound-blocking capacity by up to 10 points. This means that a ceiling system's performance can be 25 even though the ceiling panel is rated at 35. That 10-point decrease roughly means that the noise coming through the ceiling is twice as loud as without the penetrations and four times as loud as the sound coming through the wall. Ceilings are for sound absorption.

Can I just use a full-height wall?

Yes, absolutely. In fact, most acoustic standards in the U.S. either require or prefer the use of full-height partitions sealed airtight. That will definitely work.

But ask yourself if you are over-designing. The goal is to match the sound-blocking capacity of the partition below ceiling level with the sound-blocking capacity of the architecture above the ceiling level. The overall isolation is only as good as the weakest link.

While the ceiling is not sufficient as a sound blocker by itself, it is not completely useless. The ceiling can provide about 20-25 points of sound blocking. If you extend your STC 45 partition full height above the ceiling, the noise has to transmit through the ceiling, through the upper wall and back through the ceiling in the adjacent room. That path is much more robust than the path directly through the lower portion of the wall. While full-height walls are the right way to go from the performance and compliance perspectives, you may be adding unnecessary costs in some cases.

The guiding rule is that if the sound-blocking requirement between rooms is 45 points or less, plenum barriers should be seriously considered. Plenum barriers, when combined with a stone wool ceiling, can provide up to 50 points of sound blocking. This is also where full-height walls should be considered equally with plenum barriers.

Once the sound-blocking requirement exceeds 50 points, full-height walls should be used instead of plenum barriers. For example, Canada's Workplace 2.0 Fit-up Standards requires plenum barriers for standard offices, meeting rooms, training rooms and quiet rooms to achieve enhanced speech privacy. It's not until rooms, such as executive offices requiring secure speech privacy, that full-height walls are specified.

Author: Gary Madaras, Ph.D., Assoc. AIA, Rockfon Acoustics Specialist for North America

This is the second article in a three-part series on using acoustical plenum barriers for sound privacy. The first article described what plenum barriers are and why you need them on projects going forward. Here, we tackle how the ceiling and full-height walls contribute to blocking sound transfer.



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