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Two Types of Resiliently Suspended Ceilings
The most common is a lightweight mechanical ceiling that contains the lighting fixtures, the outlets for the air conditioning system, etc. These lightweight ceilings consist of light steel framing drop-in absorptive tiles that are generally 24" x 24" or 24" x 36". The primary purpose is to absorb sound within the room and to lower the reverberation rate. Because the material is so light, there is virtually no
reduction in transmitted noise either in or out of the room.
Acoustical barrier ceilings are entirely different. In years past they might have been plaster on wire lathe, but modern construction is two layers of 5/8" gypsum board screwed together with staggered joints. Every effort is made to seal the perimeter as well as any penetrations. While these ceilings are still lightweight as compared to concrete floating floors, they do have sufficient mass to act as sound barriers and the fact that they are carefully caulked and sealed puts them in a totally different category than the mechanical ceilings described above.
Barrier ceilings are primarily used to reduce noise transmission from the floor above. In most cases an architect will choose either a floating floor in the equipment room or a barrier ceiling in the space below. However, the two methods are sometimes used in conjunction with one another.
In other applications the ceilings help contain noise. Thus, an equipment room may have a suspended ceiling to complete the isolated wall and floor design. This is a common procedure for adjacent music practice rooms, particularly when the double partition walls do not reach all the way to the structural ceiling.
Barrier ceilings are light as compared to floating concrete floors, so the effectiveness of the ceiling is far more dependent on the air gap than mass or rigidity. Since the air must allow for the inclusion of the hangers and support steel, a minimum air gap is about 12".
Lightweight fiberglass bats are placed over the barrier ceiling to further improve the performance. The building service ducts, electrical conduits, etc., pass beneath it and above a removable tile mechanical acoustical ceiling. The acoustical hangers are located in the supporting rods or wires common to both ceilings. When wires, rods or straps penetrate the barrier ceiling, these members must be isolated by means of resilient sleeves and they should be caulked as well.
While the double ceiling method is probably the most effective approach to the problem, vibration hangers are commonly used to support single ceiling systems as well. If the single ceiling is of the sound barrier type, the vibration isolator helps to prevent the passage of structural noise just as in the case of the double ceiling. Hangers used to support simple mechanical ceilings prevent rattling of the ceiling members.
A mounting that "looks into" or rests on a rigid structure has a simpler task than one working against something that is flimsy. In the case of floating floors, the neoprene isolators or springs rest on the main structure, which is comparatively rigid. In the case of ceiling hangers, we often start with the noise and vibration at the concrete building structure and move down a rod or wire to the vibration control hanger and then on to the suspended ceiling. Under the best of circumstances, when this is a plastered ceiling, it is still a very flexible diaphragm without concentrated mass as compared to the concrete floor that a floor mounting rests on. Therefore, a hanger must be very carefully designed or it will not have the comparative flexibility to do the job.
Very little test work has been done to show the effectiveness of acoustical ceilings using isolation hangers. In 1969 we tested lightweight components. We started with a 3" gypsum concrete floor with an STC of 41 and suspended a single 5/8" gypsum board ceiling using W30N hangers with 1" static deflection. The air gap was 12". The STC went up to STC 50 for an improvement of nine as tabulated in Test Four. Most ceilings are made up of two layers of 5/8" gypsum board with lightweight fiberglass bats laid over the top. Therefore, it is safe to assume that the average barrier ceiling provides an improvement of STC 14.
| Transmission Loss Test (KAL 714-9-69) |
||
|---|---|---|
| Frequency (Hz) | Lightweight Gypsum 3" Floor Only | 3' Lightweight Gypsum Floor & Suspended 5/8" Ceiling |
| STC | 41 | 50 |
| 125 | 27 | 35 |
| 160 | 26 | 32 |
| 200 | 31 | 36 |
| 250 | 32 | 39 |
| 315 | 30 | 39 |
| 400 | 33 | 43 |
| 500 | 38 | 47 |
| 630 | 38 | 50 |
| 800 | 41 | 53 |
| 1000 | 43 | 57 |
| 1250 | 44 | 59 |
| 1600 | 45 | 64 |
| 2000 | 48 | 67 |
| 2500 | 51 | 69 |
| 3150 | 51 | 71 |
| 4000 | 54 | 76 |
We manufacture a very wide range of ceiling hangers in order to be competitive when other vendors are specified. In this bulletin, however, we are discussing only two major categories consisting of the ISOH2A and ISOH3A. Our suggestions are as follows:
ISOH2B - Steel coil spring vibration hangers are far superior to the neoprene designs because the higher deflection spring element will serve to isolate building vibration. The design includes a neoprene cup in series with the spring that acts as a partial high frequency noise barrier.
ISOH3A - Combination hangers make use of the WHD neoprene element in series with the W30 spring. Thus the design combines the best features of the all neoprene and the spring hangers and we recommend them for all critical applications.
15° Misalignment Tolerance - Both our spring and combination spring neoprene hangers are designed so that the hanger rod or wire can be off vertical by as much as 15° without rubbing on the steel hanger box and transmitting noise. We continue to manufacture lower priced competitive hangers that do not have this angular tolerance, but we invented the 30° sweep design, because most field problems stem from a contractor's difficulty in lining up what may be hundreds of hangers perfectly. If they do not, the wires and rods rub.
Precompression - We strongly recommend that all spring hanger installations have the spring elements partially precompressed at the factory before they are installed. If the springs are not precompressed, the ceiling will descend as much as a 1" when the spring deflects as weight is added. The contractor will have great difficulty in preventing cracks in plaster ceilings or finishing with a flat ceiling at the proper elevation. When the spring hangers are precompressed 70% of the total travel, the ceiling will not descend at all until the installation is about completed and the travel will only be 0.30" to completion.
