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Post by Hexspa on Sept 28, 2022 9:44:54 GMT
Ok, two hours later and I think I have an answer. 6" (152.4mm): Fire N Sound, 20000 Pa*s/m²: much better 200Hz absorption than rockboard 60, slightly worse 20-70Hz Safe N Sound, 10000 Pa*s/m²: an exaggeration of the above Rockboard 60, ~38000 Pa*s/m² you can gap it up to improve <70Hz with upper bass compromise www.acousticmodelling.com/8layers/porous.phpEveryone seems to refer back to that calculator. MKS rayls/m = Pa*s/m² = airflow resistivity = GFR. There are some cases where LF performance around 30Hz worsens as you thicken the absorber. But after a thickness point, there's no difference in the calculation. For R19 fluffy @ 2900 MKS rayls/m, you don't really get crumpling but you do get a loss of improvement around 48" For SnS, you can go to 12" without the 'crumpling' effect (more sub, less upper bass) For Fire N Sound, crumpling happens around 8" For Rockboard 60, the crumpling happens around 4-6" More GFR means the panel starts to perform differently sooner in terms of thickness. My take has been that the improvement in LF performance is worth a little 'crumple', whether from GFR side-effects or gapping since this can be offset by a greater ft2 coverage; what you lose, you can easily recover with more panels. Since I like simple, 6-8" is fine for rigid, 6-12" is best for safe n sound and 12-48" is best with fluffy. It goes without saying that 48" of fluffy is more effective than a 6" rigid panel and resistance (thickness x resistivity) is why. TL;DR: Fire N Sound should perform better than at 6" thickness from 125Hz and below (assuming, random incidence no air gap). However, if you double the thickness of both to 12" then Safe N Sound outperforms it with the exception of 20Hz because resistivity, thickness (resistance), and air gap all play a role. Thank you for your question, this has been validating for me. I just bought four bags of sns to make 12" panels. At the time of purchase, I was under a vague impression that 8" is good for rigid, 24" and up is good for fluffy and sns is in the middle. At 9" sns and 12" fluffy, the latter takes the lead but with 12" each, sns has better LF response (though crumpled in comparison). By 24", fluffy dominates above 22Hz. <8": rigid 6-18": sns 12-48" fluffy crossover ranges give edge to higher GFR material in LF range (6-8" rigid vs sns, 12-18" sns vs fluffy) >18" sns, 24" fluffy is diminishing returns Or even simpler, the ideal thickness for a given absorber: 6" rigid 12" safe n sound 24" fluffy
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Post by Ethan Winer on Sept 28, 2022 16:12:06 GMT
Andre is a genuine expert. You don't need to ask anyone else.
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Post by mtl777 on Sept 28, 2022 21:19:37 GMT
Andre is a genuine expert. You don't need to ask anyone else.
Yep! And Andre said he is okay with Thermafiber Fire and Sound Guard as well as Thermafiber SAFB for my needs (both have 1.37 absorption coefficient at 125 Hz for the 6" thickness).
Unfortunately, the Thermafiber Fire and Sound Guard has been discontinued and has been replaced with the Thermafiber Fire and Sound Guard Plus, which, after looking at its data sheet, I see is not good. So my final choice will be the Thermafiber SAFB.
Here's my updated comparative worksheet with Thermafiber SAFB added. It's interesting to note that Thermafiber SAFB and Thermafiber Fire and Sound Guard have the exact same absorption coefficients across all the different frequencies and thicknesses in their specs!
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Post by Hexspa on Sept 30, 2022 2:45:09 GMT
Safe n' Sound is 10-11,000 rayls and Thermafiber SAFB is 10,142 (awfully specific). These numbers come from gearspace. If you compare to a material with higher resistance (OC703 @ 20,000 rayls) then you get increased sub bass performance but better upper bass performance with the lower GFR material. The result is similar with or without random incidence applied or not but these numbers also include a rigid backing which might not apply in your case. I imagine that the relative differences will remain. 6" absorber, 1" gap; semi vs rigid: A 1' wavelength is about 1100Hz - well within the range of any absorber. I understand that half that frequency will also be reinforced so 550Hz and this is where the lower GFR material shines. However, if you're able to fill the entire 1' gap with fluffy, you'll get better performance overall. The numbers I have give a range of fluffy resistivities from 2450 mks rayls/m R11 to 5000 R15. 6" rigid 1" gap vs 12" fluffy (R15 @ 5000 rayls) no gap: Fluffy vs semi-rigid at 1' each is more of a toss up and is a similar difference to rigid vs semi rigid in the first example. This makes sense because we're going up a class of rigidity and doubling the thickness: I see that you only have 7" to work with so I'd go with Andre's advice. Be sure to use that last 1" for an air gap between the panel and the wall. You get the same absorption response but just a little lower (no compromise). Here's the thing, though: 1.1kHz and 550Hz are pretty directional. Since that passive radiator cuts off down at 40Hz, I think a rigid absorber would handle that better. I understand that it needs a foot to work but that doesn't mean much in the way of high frequencies are coming out of it. 125Hz and below, speakers are progressively omnidirectional so, behind the speaker, there's more energy that a rigid absorber is better at.
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Post by Hexspa on Sept 30, 2022 3:09:57 GMT
Andre is a genuine expert. You don't need to ask anyone else. Are you saying I need to study more before giving advice? In any case, here's the amroc simulation of your room: amcoustics.com/tools/amroc?l=15&w=7.5&h=8&ft=true&r60=0.6I think I was getting confused about room gain vs boundary interference. The speaker is omnidirectional at LF but the room gain creates peaks at the room's modal frequencies. The higher frequencies are the ones that give SBIR issues, above Schroeder I believe. With your 7.5' dimension reinforcing 150 and 75Hz, pulling it out 1' places it between a peak and a null so that's good. The rigid panel is better at 75Hz and just about as good at 150Hz.
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Post by mtl777 on Sept 30, 2022 5:56:36 GMT
I'm going to put a plastic tarp material (the one that painters use) on the front of the absorber to act as a membrane and further increase its LF absorption. Plastic tarp is available in different thicknesses such as 0.5 to 8.0 mil. What thickness should I use?
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Post by Hexspa on Sept 30, 2022 6:07:45 GMT
I'm going to put a plastic tarp material (the one that painters use) on the front of the absorber to act as a membrane and further increase its LF absorption. Plastic tarp is available in different thicknesses such as 0.5 to 8.0 mil. What thickness should I use? Ethan said yesterday that he never measured the various thicknesses and that anything from a large trash bag to proper FRK to a trader joes grocery bag is fine. the-audio-expert.freeforums.net/post/7518/threadPersonally I wouldn't go too thick. Thicker than a cheap grocery bag but thinner than a outdoor tarp ideally. 2-3mm is what I'd go for. "For reference, a thin plastic shopping bag is about . 5 Mil, a bread bag is about 1.5 Mils and a thick, retail plastic shopping bag, like you'd get at clothing or shoe store, is about 2 Mils."www.nrcm.org/wp-content/uploads/2016/05/whatisareusablebag.pdfAs a point of reference, the standard kitchen bag is around . 9 mil while heavy-duty contractor-grade trash bags will usually start out at 3.0 mil. Use higher mil count for heavier, larger, and more jagged trash.www.aaapolymer.com/mil-vs-micron/#:~:text=7%20mils%20to%204.0%20mils,larger%2C%20and%20more%20jagged%20trash.
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Post by Ethan Winer on Sept 30, 2022 15:30:50 GMT
Are you saying I need to study more before giving advice? OMG No! At this point you probably know more than I do.
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Post by Hexspa on Sept 30, 2022 16:07:58 GMT
Are you saying I need to study more before giving advice? OMG No! At this point you probably know more than I do. Thanks, Sir. I'm trying to learn.
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Post by Ethan Winer on Sept 30, 2022 16:26:13 GMT
Hexie, you're doing fine! I was just pointing out that whatever Andre says should be taken seriously.
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Post by mtl777 on Sept 30, 2022 18:55:33 GMT
I'm going to put a plastic tarp material (the one that painters use) on the front of the absorber to act as a membrane and further increase its LF absorption. Plastic tarp is available in different thicknesses such as 0.5 to 8.0 mil. What thickness should I use? Personally I wouldn't go too thick. Thicker than a cheap grocery bag but thinner than a outdoor tarp ideally. 2-3mm is what I'd go for.
You mean 2-3 mils, right? (2-3 mm is 79-118 mils)
Thanks!
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Post by mtl777 on Sept 30, 2022 20:52:52 GMT
I see that you only have 7" to work with so I'd go with Andre's advice.
Andre at first suggested Roxul Safe N Sound. Then I showed him the absorption coefficients of OC Thermafiber Fire and Sound Guard and OC Thermafiber SAFB, which both have the same acoustic specs comparing each other, and have better coefficient of 1.37 at 125 Hz than Roxul Safe N Sound's 1.11. Andre said he's also okay with OC Thermafiber Fire and Sound Guard and OC Thermafiber SAFB. So, which of Andre's advice of the three should I go with?
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Post by Ethan Winer on Oct 1, 2022 16:06:00 GMT
Which of the three? Ask Andre! Or if he says they're all the same, go with what's cheaper or easier to get.
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Post by mtl777 on Oct 1, 2022 22:17:39 GMT
Ok, two hours later and I think I have an answer. 6" (152.4mm): Fire N Sound, 20000 Pa*s/m²: much better 200Hz absorption than rockboard 60, slightly worse 20-70Hz Safe N Sound, 10000 Pa*s/m²: an exaggeration of the above Rockboard 60, ~38000 Pa*s/m² you can gap it up to improve <70Hz with upper bass compromise www.acousticmodelling.com/8layers/porous.phpEveryone seems to refer back to that calculator. MKS rayls/m = Pa*s/m² = airflow resistivity = GFR. There are some cases where LF performance around 30Hz worsens as you thicken the absorber. But after a thickness point, there's no difference in the calculation. For R19 fluffy @ 2900 MKS rayls/m, you don't really get crumpling but you do get a loss of improvement around 48" For SnS, you can go to 12" without the 'crumpling' effect (more sub, less upper bass) For Fire N Sound, crumpling happens around 8" For Rockboard 60, the crumpling happens around 4-6" More GFR means the panel starts to perform differently sooner in terms of thickness. My take has been that the improvement in LF performance is worth a little 'crumple', whether from GFR side-effects or gapping since this can be offset by a greater ft2 coverage; what you lose, you can easily recover with more panels. Since I like simple, 6-8" is fine for rigid, 6-12" is best for safe n sound and 12-48" is best with fluffy. It goes without saying that 48" of fluffy is more effective than a 6" rigid panel and resistance (thickness x resistivity) is why. TL;DR: Fire N Sound should perform better than at 6" thickness from 125Hz and below (assuming, random incidence no air gap). However, if you double the thickness of both to 12" then Safe N Sound outperforms it with the exception of 20Hz because resistivity, thickness (resistance), and air gap all play a role. Thank you for your question, this has been validating for me. I just bought four bags of sns to make 12" panels. At the time of purchase, I was under a vague impression that 8" is good for rigid, 24" and up is good for fluffy and sns is in the middle. At 9" sns and 12" fluffy, the latter takes the lead but with 12" each, sns has better LF response (though crumpled in comparison). By 24", fluffy dominates above 22Hz. <8": rigid 6-18": sns 12-48" fluffy crossover ranges give edge to higher GFR material in LF range (6-8" rigid vs sns, 12-18" sns vs fluffy) >18" sns, 24" fluffy is diminishing returns Or even simpler, the ideal thickness for a given absorber: 6" rigid 12" safe n sound 24" fluffy Thank you for the Porous Absorber Calculator!
What do you mean by "rigid"? Are you referring to the Fire and Sound as the one you call "rigid"?
Also, I'm wondering why does the calculator not show the absorption coefficient at 125 Hz as published (1.37 for Fire and Sound and 1.11 for SNS)? It's showing only about half the published specs. I understand there could be a difference but half of published specs seems to be too much of a difference.
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Post by Hexspa on Oct 3, 2022 11:30:36 GMT
By "rigid" I mean something like OC705 or IIG1280 where the material will hold its shape without sagging or compressing. This seems to be a sliding scale from fluffy to the materials I mentioned with something like Safe n Sound in the middle. I have four packs of Safe n Sound behind me in my room but I'm waiting for my frame material to arrive before unpacking them. Rigidity, Gas Flow Resistivity and R-value seem related when it comes to frictional absorbers. What I mean to say is that fluffy, with a GFR from about 2500-5000 mks rayls/m is not rigid. OC 701-703 range from about 14000-27000 (depending who you ask) are rigid. Safe n Sound is about 10-11000 and, while I haven't removed them from the packaging, I anticipate it to fall somewhere in between in terms of rigidity. The less rigid a material, the more care you need to take when making absorbers. Fluffy will compress under its own weight whereas the rockwool I have has been suspended from my ceiling (although randomly rotated) for 12 years without any deformation. Since their resistivity is related to their rigidity, I refer to that as shorthand; as I've seen others do. Technically, it's not the rigidity or R-value that matter but they're mostly related. Check out this list of resistivity and density. I don't know where he got all those numbers but it's a list anyway. There's not always a 1:1 correlation between density and resistivity but it's more frequent than not, from a brief glance. I presently do not understand the math among air flow resistance, air flow resistivity and specific airflow resistance. I'm pretty sure resistivity Pa.s/m² is independent of thickness. Somehow, resistance is related to volume (Pa.s/m³) and specific airflow resistance (Pa.s/m) is also related to thickness. Passed that, you can refer to this thread. If I've screwed up the nomenclature somewhere, somebody please help out. That calculator works with resistivity aka (mks rayls/m) aka (Pa*s/m²) and not absorption coefficient. These are also related metrics but not identical. I don't know how they measure GFR but I'm pretty sure they measure absorption coefficients with sound. Here's an article I haven't read that explains how absorption coefficients are created. Regarding mils vs mm, I'm sorry I didn't know there was a difference. Yes, 2-3mm is 79-118 mils as you noted. You make a good point about using the facing on your panel. I hadn't thought of that. Unfortunately, that calculator doesn't let you add facing. I have Fire and Sound as 20000 rayls so that should be rigid, although I've never seen it. Some of these resistivity numbers don't line up depending who you ask. In the end, that's why I like to accept a margin of error. For a given thickness, the one with lower GFR will absorb more in higher frequencies but less in low compared to a higher resistivity material. 6" of rigid beats 6" of semi-rigid in sub bass but not at 200Hz, and so on up until a certain point of thickness where the lower-gfr material just becomes better overall. I guess this is the reflection factor that people talk about: higher-resistance materials aren't as good passed a certain thickness. It's all fun but eventually we have to make decisions. As others have noted, availability is no small consideration. That plus how easy the material is to work with is pretty much all I need to know. The last factor is performance-per-frequency but that needs to be evaluated based on your measurements or placement. Cost is also an issue. By default, having already reviewed these issues, I still think that the 6/12/24" guideline is sound bearing in mind that the higher-gfr material is better for sub bass at the transition thicknesses. If you need maximum absorption, go thick and light (24"). If you are space-constrained, rigid is your friend. Somewhere in the middle and you use a Safe n Sound equivalent. You can always add an air gap for better sub with a compromise in mid bass (although for 24" of fluffy, this is silly), or a facing which should also improve bass absorption but at the expense of mid-to-high frequency reflections.
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