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Post by purtatoe on Aug 14, 2021 16:25:21 GMT
Quick question:
I understand the benefits of FRK/Foil/Paper on the front of bass traps. But what I don't understand is, if FRK does not prevent low frequency absorption into the bass traps then what's the point of needing "breathable" material to cover them in the first place? Why worry about if it's breathable or not? We're always told the point is so air can pass through, but that doesn't make sense if air can pass through FRK too. What am I missing?
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Post by Hexspa on Aug 15, 2021 2:19:48 GMT
That's a good point but I think it's better not to add any extra resistance. After all, why would you use a different fabric for your FRK panels and your non-FRK panels? Specifically, the panels in your reflection free zone will not have FRK.
If for some reason, all your panels were solely to manage bass, then I guess you could just glue a thicker fabric to the absorption.
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Post by rock on Aug 15, 2021 13:32:11 GMT
Your question is quite reasonable but I can't answer because I really don't know exactly how the "FRK, paper or plastic" actually works, but tests show that they do enhance LF absorption and reflect some mid/high to some degree. But that won't keep me from speculating  The difference between a fabric covering and FRK etc. is that the FRK is glued or attached in some way to the absorber where the fabric is usually or often stretched across a frame (rather than glued to the absorber) for cosmetic purposes. For this reason, you would want it to be acoustically transparent as to not interfere with the absorber, one way or the other, but you already know this. Aside from cosmetics, I suppose you could lightly glue a non-transparent fabric to the front and get some LF performance enhancement...But... the FRK, paper or plastic is possibly less massive or different in other properties a than your selected non-transparent fabric and your results may be unexpected or unknown in some way. If you really want to know what the difference is, you'll probably need to run some acoustic tests like in Ethan's materials found in the stickies above. If you do this, please let us know how it goes. Practically, I think it's much easier and more efficient to go with one kind of transparent fabric for all your absorbers and just use FRK etc. for bass traps in non-reflection zones. |
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Post by purtatoe on Aug 15, 2021 18:30:55 GMT
Your question is quite reasonable but I can't answer because I really don't know exactly how the "FRK, paper or plastic" actually works, but tests show that they do enhance LF absorption and reflect some mid/high to some degree. But that won't keep me from speculating The difference between a fabric covering and FRK etc. is that the FRK is glued or attached in some way to the absorber where the fabric is usually or often stretched across a frame (rather than glued to the absorber) for cosmetic purposes. For this reason, you would want it to be acoustically transparent as to not interfere with the absorber, one way or the other, but you already know this. Aside from cosmetics, I suppose you could lightly glue a non-transparent fabric to the front and get some LF performance enhancement...But... the FRK, paper or plastic is possibly less massive or different in other properties a than your selected non-transparent fabric and your results may be unexpected or unknown in some way. If you really want to know what the difference is, you'll probably need to run some acoustic tests like in Ethan's materials found in the stickies above. If you do this, please let us know how it goes. Practically, I think it's much easier and more efficient to go with one kind of transparent fabric for all your absorbers and just use FRK etc. for bass traps in non-reflection zones. You might have a point there about the FRK being glued directly to the absorder. I do recall that being essential. I wish Ethan wood chime in on this though. Still a bit confusing. Thanks. |
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Post by Hexspa on Aug 15, 2021 20:04:26 GMT
Like I said, FRK is to enhance low frequency performance of absorbers by adding a membrane. These membranes act on pressure as opposed to velocity - the opposite of how insulation works. The insulation acts as a damper for the membrane.
Think of a tennis net. You can't run through it but air can. That's how FRK works where air is the low frequencies and you are high frequencies.
If the membrane isn't attached to an absorber, it'll just vibrate in sympathy, pass low frequencies, and reflect high ones. It's not the glue that's important, it's the even, light, contact between the two materials.
We've already gone over this in other threads. Perhaps the detail you're missing is that not all absorbers get FRK - only ones which you want to optimize for bass. Any absorber whose job is to manage mid-to-high frequencies needs no FRK and needs acoustically-transparent fabric.
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Post by purtatoe on Aug 16, 2021 0:14:06 GMT
That's a good point but I think it's better not to add any extra resistance. After all, why would you use a different fabric for your FRK panels and your non-FRK panels? Specifically, the panels in your reflection free zone will not have FRK. If for some reason, all your panels were solely to manage bass, then I guess you could just glue a thicker fabric to the absorption. I wouldn't want to add different fabric. I was just thinking out loud more or less. My point was that DIY bass trap builders always seem obsessed with making sure they get the right "breathable" fabric to cover their traps, but if it didn't matter then the choices of fabric would be that much more plentiful and people wouldn't have to give it a second thought. I know I myself obsessed over the "breath-ability" and because I built over 30 bass traps for my space money was a HUGE factor and I wanted materiel that was as cheap as possible. I ended up going with Fabric called "OLY-fun" which I highly recommend for those on a budget. |
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Post by purtatoe on Aug 16, 2021 0:24:38 GMT
Like I said, FRK is to enhance low frequency performance of absorbers by adding a membrane. These membranes act on pressure as opposed to velocity - the opposite of how insulation works. The insulation acts as a damper for the membrane. Think of a tennis net. You can't run through it but air can. That's how FRK works where air is the low frequencies and you are high frequencies. If the membrane isn't attached to an absorber, it'll just vibrate in sympathy, pass low frequencies, and reflect high ones. It's not the glue that's important, it's the even, light, contact between the two materials. We've already gone over this in other threads. Perhaps the detail you're missing is that not all absorbers get FRK - only ones which you want to optimize for bass. Any absorber whose job is to manage mid-to-high frequencies needs no FRK and needs acoustically-transparent fabric. Maybe I'm not making myself clear. I'm not missing that details. I understand FRK vs no FRK. All I'm saying is if a bass trap can absorb the low-end just as well WITH FRK or paper attached to the insulation,.... why then, regardless or whether or not the bass trap has FRK, would the finish fabric need to be breathable? |
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Post by Hexspa on Aug 17, 2021 6:46:57 GMT
Because mid-to-high frequencies are reflected more as the fabric becomes less permeable. Let's remember that we're not making 'bass traps' but 'broadband absorbers'. The thicker the panel, to an extent, the lower the absorbed frequency. In that sense they're bass traps but, regardless of thickness, 20kHz will be fully absorbed so long as the fabric is acoustically transparent.
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Post by rock on Aug 17, 2021 22:23:36 GMT
Hex, thanks for the FRK/membrane explanation!
So I'm understanding that what the OP (Mr. purtatoe) wants to do is to use Breathable fabric for his broadband absorbers but would like to use Non-breathable fabric for his dedicated "Bass traps" instead of using FRK. So if I'm correct, if non-breathable fabric will behave just like FRK does, his bass traps will work the way we all expect. This of course means the OP will have to source and buy two different covering materials. Do I have that right? Make sense so far?
OK, but I think the problem may be that it's very possible the non-breathable fabric may not work exactly the same as FRK...but I don't know so that's why I suggested testing. One reason is that we usually stretch fabric tightly over a wood frame. FRK is not stretched across the frame, that's a difference. Or maybe you need to lightly wrap the non-breathable fabric. If someone knows the non-breathable fabric will work like FRK, it would be great to hear the details and then the OP should be good to go.
I do believe the OP has a valid question but at some point I'm thinking "Just use FRK and breathable fabric." "If it works, don't fix it." If Ethan or someone has some useful insight, that would be great but in the end, is it that hard to find breathable fabric? You're gonna need it for your broadband units anyway? If you can't find FRK, you can use paper or plastic.
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Post by Hexspa on Aug 18, 2021 16:07:55 GMT
That's a good point but I think it's better not to add any extra resistance. After all, why would you use a different fabric for your FRK panels and your non-FRK panels? Specifically, the panels in your reflection free zone will not have FRK. If for some reason, all your panels were solely to manage bass, then I guess you could just glue a thicker fabric to the absorption. I wouldn't want to add different fabric. Rock, I understand him to say he wants to use just one fabric. You make a good point about wrapping the frame. I neglected to assume that because my build order is insulation/fabric/frame. The way you're saying reverses the last two. Actually I made my super chunks in the latter order. If I remember, I think I told OP he can glue the fabric to the material directly if its more dense. Why would a thin dense fabric behave much differently than plastic or paper? I just think we're splitting hairs. Who has measured paper FRK vs plastic vs dense fabric? That sounds like a good idea for a video. Not sure if or when I'll do it haha. |
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