Ceiling Plenum Acoustical Dilemma
Acoustical privacy/separation is one of the most important considerations in interior design.  Some years ago, we did some research to get at the real facts regarding the issue so we could provide effective guidance for our clients.  We found out that many popular assumptions about acoustical separation were erroneous.
A. PROBLEM

Popular Plenum - Modern offices usually have acoustical tile ceilings with lay-in panels that are suspended from the roof or floor structure above the space. This forms a space or "plenum" between the ceiling and structure. The plenum space is used for piping, sprinkler lines, ductwork, recessed lights, electrical wiring, communications wiring, and as a space to allow air to circulate from individual rooms back to the heating and air conditioning units. This plenum, with the accessibility provided by the lay-in ceiling tile system, provides a very economical, flexible, and functional way to accommodate these functions.

Sound Flanking - The plenum space works best to house the systems listed above when it is completely open. That means partitions should stop at the ceiling rather than going through the plenum to the structure. Extending partitions through the plenum is quite costly and makes it more difficult to make future modifications to systems in the plenum. But with the partitions stopping at the ceiling, sound can readily travel around the partition-- through the ceiling with thin panels and cracks between tiles, into the open plenum space, and back down through the ceiling of the adjacent room. This "flanking path" creates serious sound transmission problems.

Air Plenum Return - The problem is particularly difficult with an "air plenum ceiling", where the plenum is used as a giant air duct for returning air from each room to the main heating and air conditioning unit. Each room has grilles or slots in the ceiling to allow air to enter the plenum. Sound can easily travel from the grilles or slots in the ceiling of one room, through the plenum space, and down through the grilles or slots of the ceiling in the adjacent room.

B. BASIC CONCEPTS

STC - Sound Transmission between rooms is measured by STC, Sound Transmission Class. It is frequently considered to be the decibels (dB) of sound that will be lost when the sound goes through a wall or ceiling. But it is actually more complicated. Testing is done at different frequencies and composite results are analyzed to determine the STC.

Limitations - STC data, while it provides the best approach to designing partitions and ceilings, is not very precise. Several points of difference in STC are not really detectable. It can be misleading (i.e., a partition with a relatively good STC might do a relatively poor job with sounds of a particular frequency). And actual conditions and people's attitudes can make a big difference.

Path of Least Resistance - Sound will take the path of least resistance. Therefore, if the ceiling system has an STC of 25 and the partition has an STC of 40, increasing the STC of the partition wouldn't do any good. The sound will go around the partition through the ceiling. Even within a partition, flanking paths such as cracks at the edges are usually the key problem, so plugging the cracks will usually help more than adding thickness to the partition.

Absorption Helps - Absorbing sound helps to reduce sound transmission. For example, loose fiberglass insulation in the ceiling plenum would reduce the passage of sound through the ceiling. It "muffles" the sound, reducing it's intensity.

C. SUGGESTIONS

Starting Point - A basic partition, with 5/8" gypsum board on both sides of 3-5/8" metal studs provides an STC of approx. 40-44. A basic ceiling, with 5/8" mineral acoustical tile in a lay-in suspension system provides an STC of 35-39, but with if there are air plenum openings in the ceiling (i.e. return air registers), this is reduced to approx. STC of 25-29.

Suggestion #1, Return Air - The slots or grilles in the ceiling for return air are a major problem. There are some steps that can reduce the sound transmission through return air grilles:

  1. The grilles can be located as far from adjacent rooms as possible so that the sound has to travel farther to get to the adjacent room.
  2. An enclosure or "boot" can be fabricated from fiberglass duct material and installed above each return air grille. The boot forces the sound to turn some corners and travel some distance before it enters the plenum, and the fiberglass surface of the boot absorbs some sound along the way.

We will make the conservative assumption that these steps would increase the STC by 5 (recovering of the damage done by the openings), therefore making the ceiling STC approx. 30-34. Lighting troffers which have slots around the edge of the fluorescent light to allow return air to enter the plenum should not be used. They do not allow the use of boots and their locations can not be based solely on sound considerations.

Suggestion #2, Acoustical Tile - Acoustical ceiling tiles are not all created equal. Some manufacturers (including Armstrong) will spray a coating on the back of the tile panels to increase the STC by approx. 5, and there is very little additional cost for the coating. Also, the higher the sound absorption (NRC), the better, so we recommend using tiles with an NRC of at least .65-.75. This suggestion should raise the ceiling STC to approx. 35-39.

Suggestion #3, Ceiling Insulation - The next logical step to increase the ceiling STC is to install a layer of fiberglass batt insulation on top of the ceiling tiles. This increases the thickness and effectiveness of the ceiling assembly, helps to seal some cracks, and also helps to absorb sound in the plenum. The question is whether to install the insulation only in the area within 4 ft. of partitions or whether to install it over the entire ceiling. We suggest only installing it in the area within 4 ft. of partitions, for the following reasons:

  1. It is difficult to lift ceiling tiles where there is insulation above.
  2. Installing the insulation against or above light fixtures requires the use of special light fixtures with thermal cut-offs.
  3. There can be panel sag and condensation problems where large areas of insulation are used.

This can add approx. 7-10 to the STC, and we will use the lower number due to concern with the flanking route of the return air grilles. This makes the ceiling STC approx. 42-46. The literature seems to indicate that there is not a great deal of difference between covering the entire ceiling and placing the batts only within 4 ft. of partitions.

Suggestion #4, Stud Insulation - Adding fiberglass batt insulation between studs in the partitions adds approx. 5 to the STC, making it 45-49. It probably doesn't help the overall STC much, as the limiting factor is the ceiling, rather than the wall. But it does have an important benefit-- it helps to reduce sound transmission loss where there is equipment recessed in the partition (i.e. electrical outlets).

Suggestion #5, Seal Perimeter - The design STC of partitions assumes that the perimeter of the partition is carefully sealed. It is very important. At the base of partitions, fiberglass sill sealer should be used under the floor track, and the joint should be caulked with the bottom edge of the gypsum board held 1/4" above the floor to permit proper installation of the caulking. Similar provisions should be made where a gypsum board partition terminates against another material or against another gypsum board partition without joints being taped.

Suggestion #6, Seal Penetrations - Wherever electrical boxes or other recessed fixtures are installed in gypsum board partitions, the space between the gypsum board opening and the box should be caulked.

D. OTHER OBSERVATIONS

1. Doors & Windows - Doors and windows are a particular problem for sound transmission. With doors, the problem is the cracks around the edges, particularly the large gap at the bottom. With windows, the problem is the glass. Our suggestions are:

  1. Where acoustical privacy or noise are a particular problem, install automatic door bottoms to seal the gap at the bottom of the door. They can easily be installed at any time in any wood door.
  2. Where acoustical privacy or noise are a particular problem and windows are needed, double glazing should be used and operable windows gasketed.
  3. Install hollow metal frames that include gasketing for all doors. This permits sealing of any door in the future simply by installing the automatic door bottom.
  4. Wherever possible, stagger doors/windows across corridors and maintain maximum separation between doors/windows to make sound travel farther between spaces.

2. Sound Masking System - A sound masking system, with speakers above the ceiling that emit unintelligible rumbling/humming sounds, can be an effective way to solve some sound transmission problems. By increasing the "ambient noise level", the system makes any particular sound less noticeable or intelligible. It is particularly effective where the problem is people being able to hear the people in the adjacent room. It does have a significant cost, but would probably be a worthwhile alternative to creating barriers in the ceiling plenum, and may only be necessary in certain areas.

E. CONCLUSIONS

1. Typical Separation - We suggest that the following steps be taken to provide a min. STC of approx. 42-46 between all spaces:

  1. Partitions should be constructed with 1 layer of 5/8" gypsum board on each side of 3-5/8" metal studs spaced 24" o.c. and stud space filled with fiberglass batt insulation. Partitions should be terminated just above ceiling.
  2. Acoustical ceiling tile should have min. .65 NRC and min. 40-44 STC. Grilles should be used for return air (no slotted lights), with boots installed above all grilles. Batt fiberglass insulation should be installed over ceilings in area within 4 ft. of all partitions (light fix. kept out of these areas).
  3. Seal around all partitions and around all penetrations in partitions.

2.Special Sound Separation - Where additional acoustical separation is required, including rooms with noisy equipment, lavatories, and conference rooms, use the following construction to provide a min. STC of approx. 55-59:

  1. Construct partitions with 1 layer of 5/8" gypsum board on each side of 3-5/8" metal studs spaced 24" o.c., with resilient channels on one side and stud space filled with fiberglass batt insulation. Extend partitions through ceiling up to underside of structural deck and seal against deck and all pipes, ducts, etc.
  2. Provide automatic door bottoms on doors and provide double glazing and gasketing for windows.

F. WHY NOT?

1. Why not add more layers to partitions? It wouldn't do any good. The ceiling is the limiting factor, not the partitions. Any further steps taken in the ceiling construction would result in significant costs and disruption of the plenum.

2. Why not install thin plenum barriers, such as rigid insulation, above partitions, between partitions and structural decks? The barriers would disrupt the plenum, particularly the use of the plenum for return air, and our research indicates that they would not provide any greater benefit than the insulation above the ceiling beside the partitions.


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Ambrose Design Group, Inc., 99 Pratt Street, Hartford, CT 06103
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Last Updated 3/17/99