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Post by nlt6mor on Aug 25, 2018 15:05:48 GMT
I finally scored a box locally of OC 2" unfaced 703 to build some traps for my front corners for 58 bucks. They also has two pcs of faced panels that I also picked up. Not sure of my plan yet, but intend to build four stackable triangular boxes. Use the 703 across the front and fill behind somehow with Safe and Sound or/or pick fluffy. Should I give any thought to using the faced 703 on the bottom or the top? My speakers will sit in front of these traps and I have 12" passive radiators on the back side of the speakers. So question is does it make a difference if the backs of these speakers fire into the foil facing (bottom triangle boxes) or put the foil facing on top where maybe higher frequency reflections are wanted? Second question is there any importance to filling the entire cavity behind the OC 703 tightly, or should there be some air space? Thanks for any advise.
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ths61
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Post by ths61 on Aug 25, 2018 20:07:14 GMT
Here are some calculators to experiment with. You can use the multi-layer calculator to experiment with filled backs and air space backs as well as model layers of different density porous absorbent material. Filled backs are generally preferred if budget permits, but air space backs are the next best if filled backs are not practical or economical. I have seen both commercial and DIY traps made using OC on the front and other materials to fill the back of corner traps. The FRK is recommended on the front only (not middle or back). If you don't have enough FRK for both the top and bottom, you can use kraft paper or thin plastic as a substitute for the unfaced panel if desired. The membrane is primarily used to help reflect mid/high frequencies and absorb the low frequencies. Your passive radiator is most likely emitting low frequencies so they should pass through the FRK if it is on the bottom.
REW and a calibrated mic can help you see what the effects your traps have before, during and after room treatments.
HTH
NOTE: The model depicts that there is a transition point in thickness where the pink fluffy starts out performing the rigid fiberglass.
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deanm
New Member
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Post by deanm on Aug 25, 2018 22:43:14 GMT
here are some resistivity numbers that I've seen for OC rigid fiberglass, to help you with the multi-layer calculator:
OC 701 1.5 pcf = 24 kg/m3 = 8.000 MKS Rayls/m = 8.000 K Pa.s/m2
OC 703 3.0 pcf = 48 kg/m3 = 16.000 MKS Rayls/m = 16.000 K Pa.s/m2
OC 705 6.0 pcf = 96 kg/m3 = 30.000 MKS Rayls/m = 30.000 K Pa.s/m2
OC Fluffy pink = about 5000 KPa.s/m2
there may be more "official" numbers for airflow resistivity out there - if you find some, please post them back here for me. That applies for everyone - if you have more accurate numbers, or, if you think the ones posted are correct, please correct / reassure me.
thanks,
-d
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Post by Hexspa on Aug 26, 2018 0:38:59 GMT
Second question is there any importance to filling the entire cavity behind the OC 703 tightly, or should there be some air space? Thanks for any advise. You don't want to compress fluffy. If you want to fill the gap, you'll get an estimated 20% improvement. The effect of spacing a panel is to extend the the effectiveness down an octave with the expense of moderately notched low-mid absorption. |
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Post by nlt6mor on Aug 26, 2018 6:11:44 GMT
Thanks all for your answers. To recheck: 1, no problem with the passive radiator pointing straight into the RFK corner trap. 2, fill all the space behind the front OC 703 with what ever fits, "loosely". I will be using mainly Safe and Sound and filling small areas with pink fluffy. 3, RFK has its advantage so cover the the other two triangular trap fronts with paper bag or other membrane. 4, buy microphone for REW so I can see what might be happening! One other question occurred to me. as these triangular trap will be enclosed on all three sides with only some type of fabric, it is not a pressure trap or tuned trap that I have read about. Is there any acoustical need for the edges to be flush and tight to the walls? Maybe only visually nice to do. I don't really want to fit around the base board to allow the upper portion to be tight to the wall. There is no acoustical reason to do that correct? Thanks again.
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deanm
New Member
Posts: 38
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Post by deanm on Aug 26, 2018 9:21:00 GMT
my 2 cents:
I think you're correct about the spacing to the wall, nit6mor. As you point out, this is a velocity trap. The wall forces a node to exist where velocity = 0, so immediately next to the wall, there's not much to slow down.
-d
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ths61
New Member
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Post by ths61 on Aug 26, 2018 17:05:24 GMT
... Is there any acoustical need for the edges to be flush and tight to the walls? Maybe only visually nice to do. ... FWIW, a couple of sources on the web (both commercial and DIY) state that having the edges flush against the wall of a straddling corner trap will cause a bump in absorption @ 100Hz. Pulling it away from the wall eliminates the bump in absorption @ 100Hz. Have not heard this statement being made specifically about filled SuperChunk types of traps, only straddling corner traps.
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deanm
New Member
Posts: 38
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Post by deanm on Aug 26, 2018 22:10:50 GMT
Interesting - I wonder why this is. Maybe the gap allows the lower, less-directional waves to get "around" the trap, by following a path of lesser resistance.
I guess when I said:
...I wasn't considering a wave traveling parallel to the wall. I stand corrected.
That's a 2" gap. I wonder if a smaller gap makes a proportionally lesser difference?
Thanks for posting this info, ths61
-d
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Post by Hexspa on Aug 27, 2018 0:34:59 GMT
I'm not seeing a reference for that graph inside of the link. All I can tell you is that, for my room, angling panels across corners in the conventional way was less effective than angling them vertically i.e. leaning them against the wall and mirroring that from the ceiling.
I'm seeing a lot of super-technical chat here. Part of me wants the young guns to discover something great but my inner grandpa wants to laugh and say you guys are making this way too intellectual. I realize this is a fascinating field but it's really much easier than this to get good results if you want to.
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ths61
New Member
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Post by ths61 on Aug 27, 2018 1:07:30 GMT
I'm not seeing a reference for that graph inside of the link. All I can tell you is that, for my room, angling panels across corners in the conventional way was less effective than angling them vertically i.e. leaning them against the wall and mirroring that from the ceiling. I'm seeing a lot of super-technical chat here. Part of me wants the young guns to discover something great but my inner grandpa wants to laugh and say you guys are making this way too intellectual. I realize this is a fascinating field but it's really much easier than this to get good results if you want to.
From their testing link pages, they indicate their tests were done at Riverbank Acoustical Laboratories in Geneva, Illinois, but I do not specifically see this graph as originating from the data gathered there. If you are curious, maybe a clarification question on that thread maybe in order.
As for leaning them up against the wall, if you are doing it in the upper and lower corners, you are probably leveraging the tri-hedral wall-2-wall-2-[ceiling|floor] junctions which supposedly yield higher results. They may also be reflecting some of the mid/highs in vertical directions versus just horizontal.
As for young guns, if your picture is recent and you are referring to chronological age, I am probably old enough to be your dad. 
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Post by nlt6mor on Aug 27, 2018 1:39:30 GMT
Not sure what it all means, but with my baseboard being only 3/4 inch out from the wall, I would suspect a lesser difference than what is shown in the 2" gap. If I am concerned, I should just have another opportunity to stuff some more material in the 3/4 crack and cover with a wood trim pc. I am planned to put my Safe and Sound right up tight to the back side of the Oc 703 . Is there a reason for the gap in shown in the link? I only have 8 pcs of OC 703 2". Four for the corner traps filled behind with S&S, two pcs planning to mount horizontally in opposing first reflection spot with a 2" air gap, and two pcs straddling the wall and floor behind my TV and AMP equipment. Not only is it a corner, but will help conceal the wires and other parafernalia going on behind the screen! lol. Thanks
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ths61
New Member
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Post by ths61 on Aug 27, 2018 7:15:55 GMT
Not sure what it all means, but with my baseboard being only 3/4 inch out from the wall, I would suspect a lesser difference than what is shown in the 2" gap. If I am concerned, I should just have another opportunity to stuff some more material in the 3/4 crack and cover with a wood trim pc. I am planned to put my Safe and Sound right up tight to the back side of the Oc 703 . Is there a reason for the gap in shown in the link? I only have 8 pcs of OC 703 2". Four for the corner traps filled behind with S&S, two pcs planning to mount horizontally in opposing first reflection spot with a 2" air gap, and two pcs straddling the wall and floor behind my TV and AMP equipment. Not only is it a corner, but will help conceal the wires and other parafernalia going on behind the screen! lol. Thanks Baseboards, crown molding and electrical outlets can prove to be obstacles to SuperChunks. You would think builders would take that into consideration.  |
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Post by Hexspa on Aug 28, 2018 17:16:18 GMT
I'm not seeing a reference for that graph inside of the link. All I can tell you is that, for my room, angling panels across corners in the conventional way was less effective than angling them vertically i.e. leaning them against the wall and mirroring that from the ceiling. I'm seeing a lot of super-technical chat here. Part of me wants the young guns to discover something great but my inner grandpa wants to laugh and say you guys are making this way too intellectual. I realize this is a fascinating field but it's really much easier than this to get good results if you want to.
From their testing link pages, they indicate their tests were done at Riverbank Acoustical Laboratories in Geneva, Illinois, but I do not specifically see this graph as originating from the data gathered there. If you are curious, maybe a clarification question on that thread maybe in order.
As for leaning them up against the wall, if you are doing it in the upper and lower corners, you are probably leveraging the tri-hedral wall-2-wall-2-[ceiling|floor] junctions which supposedly yield higher results. They may also be reflecting some of the mid/highs in vertical directions versus just horizontal.
As for young guns, if your picture is recent and you are referring to chronological age, I am probably old enough to be your dad.
Touche. However, I believe that my panels are doing two things by being angled: acting over a broader range of frequencies which, in turn, effect my z plane modal response less which results in the flattest response I can get. They do cover the corners but not just the corners. I tried that and it didn't work well at all. This is why it pays to try things out and be flexible to surprises.
I guess some people just find this topic interesting in and of itself. That's fine, I'm just saying it's better to take measurements because you're likely going to be surprised by real-world performance in your particular space. Plan and measure but not so accurately that your pieces are no longer at least semi-modular. You guys are going to learn as you go and will probably want to make changes. Don't lock yourselves into narrow models, no matter your age |
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