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Natural Science Forum / Physics / Acoustics / July 2007Acoustic Steel Deck Installation
I've been trying to determine how to properly install a steel deck with perforated webs so that the specified acoustic properties are achieved. I've read that there should be a rigid fiberglass board above the deck, but does anyone know how thick or what density? > I've been trying to determine how to properly install a steel deck > with perforated webs so that the specified acoustic properties are > achieved. I've read that there should be a rigid fiberglass board > above the deck, but does anyone know how thick or what density? The perf steel deck (may I presume that it's roof deck?) manufacturer will either supply, or specify, the fiberglass material to be set behind the perforations. Sound absorption data from 125 Hz upwards should be readily supplied by said mfr. Below 125 Hz, the absorption is reasonable since it arises from the flexible nature of thin sheet metal and roofing membranes. Likely it is about the same as the 125 Hz value. Angelo Campanella I believe you may have already obtained this information from contacting my office, but for the benefit of everyone else I will clarify. The most typical ribbed steel acoustical deck with perforations in the web are often tested in a way different from the way they are actually installed. The manufacturers will test them with both fiberglass in the rib and two or three inches of fiberglass over the top. In the real world the material over the top is usually a foam insulation that has no acoustical absorption. Fortunately, the Steel Deck Institute has sponsored some tests with this more common installation to demonstrate the difference. The difference is that you get much less absorption at the low frequencies when you use foam board insulation instead of fiberglass. With fiberglass results look good at lower frequencies. With foam board on top and fiberglass just in the ribs, the absorption is concentrated primarily at 500 and 1000 Hz. > I've been trying to determine how to properly install a steel deck > with perforated webs so that the specified acoustic properties are > achieved. I've read that there should be a rigid fiberglass board > above the deck, but does anyone know how thick or what density? Thanks for the responses. After looking at the test reports from the Steel Deck Institute it is amazing how poor the low frequency absorption is when foam board is used. Hi "mgleason", Could you give me web links which point to the acoustic test reports by Steel Deck Institute ?? I have searched Steel Deck Institute webpage ("http://www.sdi.org/") but still could not find the reports. Many Thanks. CSL <mgleason@novusengineering.com> ???????:1183639267.184916.160290@n2g2000hse.googlegroups.com... > Thanks for the responses. After looking at the test reports from the > Steel Deck Institute it is amazing how poor the low frequency > absorption is when foam board is used. As far as I know they are not posted. Someone told us about them several years ago and we contacted SDI by phone and found the right person and got copies faxed to us. Noral Stewart > Hi "mgleason", > [quoted text clipped - 11 lines] >> Steel Deck Institute it is amazing how poor the low frequency >> absorption is when foam board is used. > As far as I know they are not posted. Someone told us about them several > years ago and we contacted SDI by phone and found the right person and got > copies faxed to us. It would help us if you could scan the reports and distribute them, with SDI permission. We need to know this information. Ang. C. For those interested I am posting the test results below. These are based on Riverbank lab tests. The deck in each case was laid over 3 inches of expanded polystyrene foam on the floor with fiberglass in the perforated ribs. Ribs in one case were 1. 5 inches high and in the other 3 inches, both 2 inches wide. The report did not indicate the distance between ribs. The tested area was 81 sq ft for the 1.5 inch deck and 72 sq ft for the 3 inch deck. Freq 1.5 3 100 .11 .17 125 .11 .14 160 .11 .17 200 .14 .27 250 .20 .36 315 .31 .51 400 .38 .66 500 .63 .89 630 .82 .97 800 .94 .97 1000 1.04 .95 1250 1.04 .79 1600 .86 .61 2000 .66 .53 2500 .55 .38 3150 .46 .38 4000 .36 .34 5000 .31 .33 NRC .65 .70 Note that these tests were done with the deck laid over foam on a hard floor. Results in actual installation may be different especially at low frequency. I have long thought that products such as this should be tested mounted in an opening in the chamber wall to simulate the sky beyond. >> As far as I know they are not posted. Someone told us about them several >> years ago and we contacted SDI by phone and found the right person and [quoted text clipped - 4 lines] > > Ang. C. I find it pretty distressing that all the steel deck manufacturers publish this misleading data. One major manufacturer of steel decks has a footnote that sort of discribes to actual test conditions (not at all clearly) and then gives the lower NRC ratings (but not octave band data) for the decks tested under more realisitic conditions. My General Contractor is checking with his roofing contractor to see if he will install the roof over 2" of 6 pcf fiberglass. The problem is that the fiberglass is much more compressible than the Polysio insulation board. It will compress up to 10% at under heavy snow loads. This could cause the screws holding down the insulation to push up and could threaten the roof membrane. Also you loose a bit of R value going from polyiso at 6.5 per inch to FG at 4.5 per inch. So why, pray tell, do they test the decks this way???????? Mark Bagdon, P.E. |
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