panel trap experiment outline

Speak now or forever live with not voicing your opinion: Attached is a sketch to accompany this description of the proposed test. Test 1: reference wall 1 5/8" drywall 2x4 studs 24" OC, R13 fiberglass 5/8" drywall Test 2: reference wall 1 with panel traps added per ethans basic designs, including the "high bass trap" 5/8" drywall 2x4 studs 24" OC, R13 fiberglass 5/8" drywall Ethan panel trap recommendations Test 3: repeat of 2, sans high bass trap Test 4: reference wall 1 with panel traps tuned to lower frequencies added 5/8" drywall 2x4 studs 24" OC, R13 fiberglass 5/8" drywall Low frequency panel traps Test 5: repeat of test 4 with a second layer of drywall added to the non-trap side, and both layers of drywall on the non-trap side not attached to the structure so 5/8" drywall 5/8" drywall no attachment to studs 2x4 studs 24" OC with R13 fiberglass 5/8" drywall low-frequency panel traps Test 6: repeat of test 5 with low frequency traps unsealed Test 7: repeat of 5 and 6 with Ethans traps Test 8: repeat of 5 and 6 with no traps, as a second reference, so: 5/8" 5/8" no attachment to studs 2x4 studs 24" OC with R13 fiberglass 5/8" drywall This protocol is subject to change at my discretion. That is an ambitious and potentially time consuming set of tests. The tests are competing with some important-to-GG Company work, and some possibility that the entire test protocol cannot be done exists. It seems very clear to me that part of how destructive panel traps will be to TL relates to the rleative locations of the resonances in the wall and the reosnances in the traps, and as such it is necessary to add the lower frequency traps as well. I presume that <100hz panel traps are not uncommon anyway. And further, the potential for contesting whatever results arise from the test almost mandate a broader set of tests lest whatever 1 or 2 tests are run wind up of use to nobody and simply the subject of additional debate. Low frequency trap shall consist of 3.5" deep cavity with R13 fiberglass insulation and double 5/8" drywall.

Brian, Is it possible to use a distance keeper to keep the wool away from the front panel of the trap? In your case you build the horizontally, which makes this probably a bit annoying. That's how it explicit adviced in this trap design of Ethan. Keeping the wool spaced from the front panel. Brian, this indeed should be nice work, and even when it doesn't work out as itended or planned due to lack of time, energy or whatever, many thanks for this more than generous offer.

well, i will get as much of it done as is at all possible. alot of those changes will only take 10 minutes or so, and would allow a fairly rapid re-test. i willt ry to space the insulation away, is a simple thumbtack or two an acceptable method of doing this.

Brian, > That is an ambitious and potentially time consuming set of tests. < No kidding - I still can't believe you're willing to do this! Outstanding!!!!

> Low frequency trap shall consist of 3.5" deep cavity with R13 fiberglass insulation and double 5/8" drywall. < The design calls for one inch thick 703 rigid fiberglass. For the purposes of these tests I imagine fluffy fiberglass would be okay, but in that case you should make it two or even three inches thick instead of only one inch. Even three inches will fit in a 3.5 inch cavity, and will still leave enough clearance to not touch the vibrating front panel. --Ethan

Maybe it's just too late or I'm just blind but what's the diff between test 1 and 3? Also, if TL is a large concern, I'd suspect that most folk would use the double 5/8" as the backing - potentially even with a 3/4" MDF backer on the 'trap'. I know you can't test everything - just trying to simulate the reality that most folks would encounter. Besides, if I don't bitch now, I have no future right to :wink: :D Bryan

bpape wrote:

Maybe it's just too late or I'm just blind but what's the diff between test 1 and 3? Also, if TL is a large concern, I'd suspect that most folk would use the double 5/8" as the backing - potentially even with a 3/4" MDF backer on the 'trap'. I know you can't test everything - just trying to simulate the reality that most folks would encounter. Besides, if I don't bitch now, I have no future right to :wink: :D Bryan

Bryan, test 1 has no traps, test 3 has panel traps but not the all-fibrous "high bass" trap. On studiotips, Rod Gervais hypothesized that the high bass trap and its effect on TL should be determined so its included once. My guess is that it won't do much of anything for TL, so if that proves to be true it won't be included on other tests, leaving some "regular" wall space. Ethan, when i said "deep bass" trap, i meant one even deeper, tuned to 50hz or so. It is important in my estimation to model a trap with resonance NOT IN LINE WITH that of the walls. This should be in line with the decoupled wall, but not the other wall, giving a broader perspective. All, we're building those suckers tonight, and i'll post pictures if i can.

Brian, > when i said "deep bass" trap, i meant one even deeper, tuned to 50hz or so. It is important in my estimation to model a trap with resonance NOT IN LINE WITH that of the walls. This should be in line with the decoupled wall, but not the other wall, giving a broader perspective. < Excellent. More information is always better than less. --Ethan

Brian Ravnaas wrote:

Rod Gervais hypothesized that the high bass trap and its effect on TL should be determined so its included once. My guess is that it won't do much of anything for TL, so if that proves to be true it won't be included on other tests, leaving some "regular" wall space.

Brian, My best guess is the same as yours - I don't believe it will have any effect - or such a small effect as to be almost unmeasurable. But - I am one who always hates unanswered questions - so I figure if we build them like this - we should test them like this. with fluffy fiberglass insulation the thumbtacks won't do much good - the glass will just form around them - and if you have enough tacks so that doesn't happen - then the glass will effectively damp the panel. If you want to use the fluffy stuff - perhaps you would be better off using some nylon strapping to hold the glass at least an inch from the panel - you could just staple it (the strapping that is) to the sides of the frame. I hope that helped. Sincerely, Rod

hey Rod, 10-4 on your logic and i think its sound. I just would rather run the tests w/o the absorption in one of the rooms as the standard advises against high absorption rooms. But its a good idea. I am having trouble finding 703 here in Fargo, and could not find any on Friday. Any suggestions?

Brian, > I am having trouble finding 703 here in Fargo, and could not find any on Friday. < Mineral wool? High-quality non-sculpted acoustic foam? --Ethan

Brian Ravnaas wrote:

hey Rod, 10-4 on your logic and i think its sound. I just would rather run the tests w/o the absorption in one of the rooms as the standard advises against high absorption rooms. But its a good idea.

Brian, If we have it in one test then it establishes a baseline to measure against. Past that point I can't see why we would need it.

I am having trouble finding 703 here in Fargo, and could not find any on Friday. Any suggestions?

Try using Johns Manville Linacoustic - it's 3pcf material as well. You should be able to get it here: Macarthur Company 1449 43rd St W, Fargo, ND, 58102 Phone:[removed] Sincerely, Rod

Thanks Brian - figured I was just having a senior moment. Looking forward to the results. Bryan

that's awesome, rod! its on the schedzule for tomorrow, i'll call those guys in the AM and post pics if possible.

Brian Ravnaas wrote:

that's awesome, rod! its on the schedzule for tomorrow, i'll call those guys in the AM and post pics if possible.

No probs Brian, it was my pleasure. I look forward to the results, Sincerely, Rod

got the goods at the place rod mentioned. and even better, they had 703 on hand. so we're all set. pictures soon!

sorry for the bad pic quality, camera phone. traps are built to Ethan's spec including 2x low bass 1x mid/high bass 1x mid/high absorber (not panel trap) the high bass trap has 1/8" hardboard instead of 1/8" plywood. This material is a bit heavier than the plywood, but we couldn't find 1/8" plywood. The insulation is 703, and is spaced away from the wood surfaces as prescribed. should be fun.

the first set of tests is completed. a reference wall of 5/8" drywall 2x4 studs 24" OC with 3.5" R13 fiberglass insulation 5/8" drywall had added to it the full set of Ethans traps, these being: 2x deep bass trap 1x high bass trap 1x mid/high absorber Then, per discussions above or on Studiotips, the mid/high absorber was removed. The results are presented below. I can't discuss in any length as we're still here at the lab working, but i thought i'd post them on-the-fly. The overall weight added by all the traps was about in line with adding another layer of drywall (just a bit less), and the overall gains at low freq's were smaller than adding another layer of drywall. However, and a bit surprisingly, at higher freq's the traps helped and (the surprising part) the mid/high absorber seemed to help at high freqs as well.

Brian:

However, and a bit surprisingly, at higher freq's the traps helped and (the surprising part) the mid/high absorber seemed to help at high freqs as well.

I was expecting it to be more mass, so it would improve TL above 250hz. From IRC 02-108

Inserting a panel in the middle of a double wood stud partition, making it a triple leaf partition, substantially increases the transmission loss above 250 Hz (frequencies for human speech) so long as the panel does not create a mechanical link between the two rows of studs. The use of a fiberboard panel seems to be superior to that of a gypsum board for this application. However, inserting a third panel, wether it be made of gypsum or fiberboard, reduces the transmission loss at low frequencies (stereo systems, home theater) as well as the STC rating of the partition.

What's the 1DOF frequency estimate of the panels (depth, surface mass)? Or more to the point, is it 65hz ?

1DOF http://www.ethanwiner.com/BTPlans.gif Assuming a 1/4" sheet of plywood usually weighs about 25 pounds, that's 0.78125 psf. d = 28900 / (f ^2 * m) where d = depth in inches f = resonance frequency in hz m = surface density in pounds per square foot becomes f = sqrt( 28900 / (m * d) ) Deep Bass Trap: f = sqrt( 28900 / ( 0.78125 * 3.5 )) = 102 hz Mid High Absorber: f = sqrt( 28900 / ( 0.78125 * 2.5 )) = 121 hz High Bass Trap: f = sqrt( 28900 / ( 0.78125 * 1.5 )) = 157 hz Clearly those are all wrong, at least on TL.

Bob, > Deep Bass Trap: f = sqrt( 28900 / ( 0.78125 * 3.5 )) = 102 hz > Mid High Absorber: f = sqrt( 28900 / ( 0.78125 * 2.5 )) = 121 hz > High Bass Trap: f = sqrt( 28900 / ( 0.78125 * 1.5 )) = 157 hz When I measured the high- and low-bass traps at IBM (third octave resolution unfortunately), the low-bass type peaked at 100 Hz and the high-bass at 180 Hz. So I'd say your calcs are pretty close. Note that the "mid high" absorber is one inch of 703 only with a 2-inch air gap. So it's not a resonant trap, as Brian correctly noted above. > Clearly those are all wrong, at least on TL. < Or put another way, this type of trap does not appear to reduce isolation at its resonant frequency, as has been suggested. --Ethan

Brian, Outstanding. I owe you much more than a beer my friend. --Ethan

Ethan:

Or put another way, this type of trap does not appear to reduce isolation at its resonant frequency, as has been suggested.

That's not the way I read what's happened. The 1DOF calculations should correspond with a lab measurement if the walls of the lab were made out of 1' thick concrete or something (similar to infinite, relatively speaking). In this case the test wall is fairly thin. The trap functions in combination with the wall, to create at least a 2DOF with a new lower MSM system, in this case around 63hz. That's where it resonanates, and probably also where the system absorbs the most from the room. This lower resonance is clearly shown in Brian's TL chart. I was expecting it to also create a higher resonance, but that did not appear on the TL chart. At 63hz, with the two resonant traps in place, the TL fell from 16.5dB down to 13.9 dB, which is a 2.6dB drop, or 16% (100*2.6/16.5) loss in TL. (I don't know what the margin of error is in this test. Presumably comparing one wall to another right after it, the relative error would be low). Hmm, I don't think % is a correct way to manipulate this (0dB isn't zero, etc), but 2.6dB is significant there.

Note that the "mid high" absorber is one inch of 703 only with a 2-inch air gap. So it's not a resonant trap,

mea culpa, I didn't notice a) that the mid/high absorber had fabric instead of plywood b) that the high bass trap is using 1/8" plywood instead of 1/4" (see recalculation below) 1/8" italian light plywood is .21 psf 1/8" mahogany plywood is .33 psf 1/8" birch 3 ply plywood is .44 psf 1/8" Marine Meranti (Phillipine Mahogany) plywood is .59 psf <- I'm cheating ! 1/8" marine douglass fir plywood is .71 psf ( http://www.westwindhardwood.com/wood_marine.html ) Deep Bass Trap: f = sqrt( 28900 / ( 0.78125 * 3.5 )) = 102 hz Mid High Absorber: see absorption coefficients High Bass Trap: f = sqrt( 28900 / ( 0.59 * 1.5 )) = 180 hz

Well, the calculated performances of these walls are: reference: STC=41, OITC=29, tennekes=24.2, flat noise = 38.1, with all traps: STC=47, OITC=30, tennekes=24.9, flat = 41.3 without mid/high absorber: STC=45, OITC=30, tennekes=24.5 , flat=40.4 Some simple analysis of these results: a total of 135lbs was added with all these traps, equivalent to about 1 more layer of drywall on one side, which should give a 2-3 point/dB improvementin all of the measurements. So for the mass addition, the triple leaf performs worse than keeping the double leaf. The improvements at higher frequencies relate to another air cavity with very flexible panels and so forth, and aren't entirely unexpected. Most surprising to me is the high freq effect of the mid/high absorber. The low frequency effect of adding the traps does not seem to fall in line with the predicted resonance points of the traps. An effect is discernible, but it is not extreme. The bulk of the weight of the traps is in the frames (which had 2x4's for the deep bass traps instead of 1x4s for installation practicality) The reference wall was installed and tested. With no changed at all to that wall the traps were added on top of it and sealed. Weather stripping + additional caulk was used to seal them. For the test without the mid/high absorber, it was simply removed and the test was re-run. attached picture shows the middle of the installation process w/the lab guys doing the work so i can drink coffee.

ehtan and Z, you're very welcome & i'll try to get back to answer questions as best i can when i can. For the oher folks, i hope if you have time to peek at the data you can comment. In short, from an isolation standpoint this is (like all triple leaves) essentially a waste of materials. But this data supports the basic hypothesis that adding to create a triple leaf isn't necessarily bad. It is feasible that it could be bad in some particular situation, but in this case the results were not worse. The same traps will be installed on hopefully at least one more wall before weeks end. I wouldn't expect to see any dramatic problems. hope its interesting/helpful

Brian:

Most surprising to me is the high freq effect of the mid/high absorber.

That is neat and unexpected. (Dennis Erskine's design of of covering the 65% of the walls in absorption is to increase HF TL? :) , or did it just make your source room quieter )