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Post by ultrasonic on May 20, 2019 8:29:38 GMT
Hi all, I'm just about to start experimenting with using acoustic panels to try to improve the sound in my domestic listening room. My starting point will be twelve 4" thick 2' x 4' panels of the approximate UK equivalent to OC 703 (Rockwool RW3), which I'll use for a mix of first reflection point treatment and corner bass traps. I'm aware of OC panels with FRK layers being advised for use as bass trap panels placed across corners (FRK facing the room) to improve low frequency absorption and to stop the room sounding too dead by reflecting some higher frequency sound. The panels I have are without any such layers but I was thinking it would be a good idea to glue one to the front of the bass trap panels, and so my question is what material would be best for this? Two options I've found so far are one which is at least superficially similar to OC FRK, formed from one layer of kraft paper, one layer of low density polyethylene and one layer of aluminium foil: www.ebay.co.uk/itm/Anderton-Three-in-One-Damp-Proof-Foil-Backed-Lining-Paper-10m-x-54cm/190169910835?epid=1605480194&hash=item2c47028e33:g:PWMAAOSw4shX6lE2A cheaper option would just be some thick paper, like this: www.amazon.co.uk/gp/product/B00WID9TGS?pf_rd_p=330fbd82-d4fe-42e5-9c16-d4b886747c64&pf_rd_r=G0RRNE02Y6R0DYRTPQFH For initial testing I will leave the panel fronts 'naked' but if they stay long term then I would be covering them in suitable fabric. While I'm posting I do have one other related question. I've seen mixed views elsewhere on whether adding a thin membrane layer onto the back of 4" thick first reflection point panels might be beneficial or not. What's the current thinking? (I will be spacing these panels 2-4" from the walls/ceiling.) Thanks for any help .
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Post by ultrasonic on May 20, 2019 16:20:30 GMT
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Post by rock on May 20, 2019 22:29:46 GMT
The answer to question regarding the exact material and thickness is somewhat ambiguous but Ethan has stated, as I recall, paper like a paper grocery bag so it looks like the brown wrapping paper is fine. Also acceptable is thin plastic I believe about .7 mil. (thousandths of an inch). Spray glue to the front but go easy on the glue.
No membrane on the back of either type of panel. If your panel is flush to the wall, it won't matter but if you gap it will, so just don't do it.
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Post by ultrasonic on May 21, 2019 8:08:09 GMT
The answer to question regarding the exact material and thickness is somewhat ambiguous but Ethan has stated, as I recall, paper like a paper grocery bag so it looks like the brown wrapping paper is fine. Also acceptable is thin plastic I believe about .7 mil. (thousandths of an inch). Spray glue to the front but go easy on the glue. No membrane on the back of either type of panel. If your panel is flush to the wall, it won't matter but if you gap it will, so just don't do it. Thanks for taking the time to reply . I'm in the UK and so not actually familiar with how thick US paper grocery bags are. I did spend a couple of weeks in Seattle in around 2003 but my memories are of more interesting things than bags! I actually ordered some of the foil faced paper before seeing your post, thinking the cost wasn't that different to the white paper I'd gone on to look at, and I'd hope should be the closest match to the FRK for which there is the best evidence of a benefit. Before I stick this to my panels though, can I check there isn't an obvious problem with what I've bought? (First link in my first post.) Regarding the membrane on the back of first reflection point panels, has there been some recent data proving this is a bad idea? Over on Gearslutz there was quite a lot of discussion about this a few years a go, with at least one member arguing strongly for this based on his own testing (with 4" panels), and I've seen one post from Ethan in favour of it: www.gearslutz.com/board/showpost.php?p=8506966&postcount=7I've seen a few posts suggesting that the HF RealTraps have a membrane on the back too, but I'm not sue if this is actually true? This is all from a few years ago though, which is why I was wondering if there was better information now? Oh, and I appreciate it would make no difference if the panels are mounted flush to the wall. I was asking specifically in relation to first reflection point panels offset from the wall, with a membrane on the back (and obviously not on the front). The idea behind using a back membrane would be for the 4" panel to absorb what you wanted for good first reflection performance, and that the membrane could then help add a bit of extra low frequency absorption too.
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Post by Hexspa on May 21, 2019 9:18:38 GMT
Hi ultra and welcome. Glad you had a good time in the Pacific Northwest. Maybe my opinion is worth something here. As far as I've seen, there is no comparative data below 125Hz on the effect of FRK vs not. For reasons which are unclear to me, it's not something that's part of standardized tests. The only hard data I've seen is Ethan's density test but that's just one test in one room - far from establishing a broad sample size. You just have to take it on limited evidence and authority that this works since, in addition, neither are there concrete reports as to why this even works in the first place. The basic informal hypothesis is that the FRK works as a pressure element which is in turn dampened by the absorber akin to how a membrane resonator works but without an internal air chamber. For what it's worth, I don't see where any GS member is arguing for placing FRK behind his RFZ panels so, unless they have proof, I'd take it with a grain of salt. Nothing is stopping you from experimenting on your own. Put your FRK then flip your panels between tests and post the results - we'd love to see. Again, since so much of this is grey-area science, there is no standard for FRK thickness nor material but, above all, keep it simple. Some of the above could be wrong but it's true as far as my experience is concerned.
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Post by ultrasonic on May 21, 2019 9:41:21 GMT
Hi ultra and welcome. Glad you had a good time in the Pacific Northwest. Maybe my opinion is worth something here. As far as I've seen, there is no comparative data below 125Hz on the effect of FRK vs not. For reasons which are unclear to me, it's not something that's part of standardized tests. The only hard data I've seen is Ethan's density test but that's just one test in one room - far from establishing a broad sample size. You just have to take it on limited evidence and authority that this works since, in addition, neither are there concrete reports as to why this even works in the first place. The basic informal hypothesis is that the FRK works as a pressure element which is in turn dampened by the absorber akin to how a membrane resonator works but without an internal air chamber. For what it's worth, I don't see where any GS member is arguing for placing FRK behind his RFZ panels so, unless they have proof, I'd take it with a grain of salt. Nothing is stopping you from experimenting on your own. Put your FRK then flip your panels between tests and post the results - we'd love to see. Again, since so much of this is grey-area science, there is no standard for FRK thickness or material. Above all, keep it simple. Some of the above could be wrong but it's true as far as my experience is concerned. Thanks for your reply. I'd seen Ethan's density test report, which is what convinced me there may be a useful LF benefit from FRK in the first place. Here's one GS thread where the topic of FRK on the back of RFZ panels is discussed, with some measured data in post #12. www.gearslutz.com/board/bass-traps-acoustic-panels-foam-etc/511421-oc-703-frk-foil-not-foil.htmlI'll be honest I'm not at all convinced about the quality of the measurement though, as it's not clear the microphone wasn't moved between the tests. From my basic understanding the idea of having FRK on the back of a RFZ panel appears to make logical sense though, plus there was that post of Ethan's I posted above seemingly in favour of it. My current inclination is to not put a membrane on the back of RFZ panels since it doesn't appear to be common practice, but on the other hand if it's a simple way to boost performance I may as well. You make a fair point about testing for myself, but I had planned to make the RFZ panels first, and it would be a bit of a faff to retrospectively add a backing later on. For ease I was probably not going to make a frame for all of the bass traps too, meaning particularly for the ceiling I couldn't easily swap them. I will definitely be making measurements though, and if I have anything I think may be of interest I'll share it.
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Post by Hexspa on May 21, 2019 11:05:57 GMT
I misread Ethan's post but it was from 2012 and I still haven't seen any conclusive evidence one way or the other. While I can't doubt the logical validity to the assertion, it remains a hypothesis to be tested nonetheless. All you can do is try it out and see what works best in your space. Cheers.
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Post by ultrasonic on May 21, 2019 12:11:05 GMT
I misread Ethan's post but it was from 2012 and I still haven't seen any conclusive evidence one way or the other. While I can't doubt the logical validity to the assertion, it remains a hypothesis to be tested nonetheless. I fully agree, which is why I thought I'd ask here to see if there was any more up-to-date info. on the subject . If Ethan Winer happened to have time to comment I'd obviously be very interested in his current thoughts too .
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Post by Ethan Winer on May 21, 2019 15:24:30 GMT
I haven't tested this stuff in ages, but I do remember that FRK on the rear of a reflection panel is better (absorbs bass more) than plain rigid fiberglass without FRK. I believe this was the result of our tests at IBM's acoustic lab years ago. So pay the extra $1 and get the FRK type.
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Post by ultrasonic on May 21, 2019 15:40:41 GMT
I haven't tested this stuff in ages, but I do remember that FRK on the rear of a reflection panel is better (absorbs bass more) than plain rigid fiberglass without FRK. I believe this was the result of our tests at IBM's acoustic lab years ago. So pay the extra $1 and get the FRK type. Thanks . Is there any acoustic downside that should be balanced against the improved bass performance? (FRK backed panels aren't actually available where I live in the UK but I will be manually adding this where appropriate.)
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Post by Ethan Winer on May 21, 2019 16:17:59 GMT
I don't know of any downside to having FRK on the rear. Of course, you can't just put it up against the wall! You'll need at least a small air space to allow the FRK to vibrate. Also, if you make your own FRK using heavy "shopping bag" type paper, use spray glue to securely bond it to the insulation material.
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Post by ultrasonic on May 21, 2019 16:56:17 GMT
I don't know of any downside to having FRK on the rear. Of course, you can't just put it up against the wall! You'll need at least a small air space to allow the FRK to vibrate. Also, if you make your own FRK using heavy "shopping bag" type paper, use spray glue to securely bond it to the insulation material. Great, thanks! I'm surprised this isn't spoken about more. I guess it may be now... I'd understood there would need to be an air gap behind the panel for the membrane to be effective, and I intend to add it to 4" thick panels that will be fixed 2-4" from a wall or ceiling. Is the type of glue important? The cheapest spray glues seem to be of a higher viscosity type intended for really strong bonding, but I suspect more expensive craft types may be a better bet so there isn't a thick/hard glue layer? I was asking about what to stick to panels to mimic the FRK layer above. I've now ordered some damp proofing paper that says it has one layer of kraft paper, one layer low density polyethylene one layer aluminium foil that I hope will be roughly equivalent. It won't be enough for all of my panels if I'm doing the RFZ ones too though so I may try some thick paper for these instead. Edit: this is what I bought: www.ebay.co.uk/itm/Anderton-Three-in-One-Damp-Proof-Foil-Backed-Lining-Paper-10m-x-54cm/190169910835?epid=1605480194&hash=item2c47028e33:g:PWMAAOSw4shX6lE2
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Post by ultrasonic on May 21, 2019 19:56:42 GMT
A local office supply store happened to be closing down so I picked up some of this cheaply which I hope will do the job.
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Post by Ethan Winer on May 22, 2019 15:40:34 GMT
I'm sure that 3M spray glue will be fine.
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Post by Hexspa on May 24, 2019 2:56:03 GMT
If OP has made up their mind then that's fine. However, for anyone else, I'm hypothesizing that this is not going to be a best practice. Let me explain why.
1. FRK in front of panels is known to work - at least at 125Hz. Absorption is incident-dependent which means it works best when parallel to a sound source. The sound isn't coming from behind the panel but reflected against any surface behind it or diminished through the absorbent. Therefore, any FRK behind a panel will be less effective at any frequency.
2. The RFZ is for mid-to-high targeting. By targeting bass, you're mixing uses. There's almost always a downside to any compromise - even if we don't know what it is yet.
3. The gap is being partially negated. The gapping of panels relies on that gap being unobstructed. Again, since RFZ panels are targeting a certain range, we can expect FRK of any kind to interfere with that process across the gap.
4. RFZ has little to do with bass. Even in headphones, stereo information below 100Hz becomes less meaningful. You will gain no stereo clarity by using FRK but in fact may harm it.
5. Beginner status. It's a classic pitfall to over-think your first two years of any endeavor. I, and others experienced in a variety of areas, usually recommend novices to follow proven methods before attempting innovation. Rear-facing FRK is experimental, generally unused, mostly unproven, and totally unnecessary for treating a room in probably nearly all cases.
For these reasons I predict that this is not going to be ideal and perhaps even counterproductive. But since none of us can produce a broad sample of reliable comparative data, it's all going to be speculation until then anyway. If this counter-stance is wrong then let the evidence prove it. Until then, anyone attempting this method of using FRK should consider the above before moving forward.
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