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Post by Brian F. on Aug 27, 2018 15:19:05 GMT
Hello, I made some panels using Roxul ComfortBoard 80 2" semi-rigid rockwool. The foil is facing out because I wanted to weaken modes at low frequencies. The reverb times are low and pretty consistent vs. freq, so that seemed to work OK.
But for the early reflections, would it make sense to put something over the foil that absorbs better at higher frequencies? For example, a thin piece of open cell foam or even something like a towel. It could be done from the back without having to remove the cloth covering. Is it worth the trouble?
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Post by rock on Aug 27, 2018 18:10:58 GMT
Hi Brian, Welcome to the forum!
Foil faced rigid is used for corner bass traps were reflections are not an issue. Normally it's suggested to use non-foil type rigid for reflections. It's also suggested to space the panels off the wall a distance equal to the thickness to extend the low freq performance, but that only works with non-foil type. If you want to use the foil type instead, if you place the foil backing directly on the wall, I imagine it will work the same as a non-foil panel mounted directly on the wall. I'm not sure of any tests but to my thinking, the foil in contact with the wall will reflect the sound back into the panel pretty much the same as the wall without the foil.
If you have extra foil panels built, use them in other corners, a rectangular room has 12.
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Post by Brian F. on Aug 28, 2018 11:54:30 GMT
Hi Brian, Welcome to the forum! Foil faced rigid is used for corner bass traps were reflections are not an issue. Normally it's suggested to use non-foil type rigid for reflections. It's also suggested to space the panels off the wall a distance equal to the thickness to extend the low freq performance, but that only works with non-foil type. If you want to use the foil type instead, if you place the foil backing directly on the wall, I imagine it will work the same as a non-foil panel mounted directly on the wall. I'm not sure of any tests but to my thinking, the foil in contact with the wall will reflect the sound back into the panel pretty much the same as the wall without the foil. If you have extra foil panels built, use them in other corners, a rectangular room has 12. Thanks, rock! I'm glad I found this forum. I would like to have been able to put proper triangular or round style bass traps in some of the vertical edges. But my listening room has doors, a hallway, a window with AC, and an opening in or near these 4 corners. In 2 of these corners, I put flat panels floor to ceiling as close as I could get. If I do it again, I'll try to put bass traps some of the 4 horizontal edges along the ceiling. But for now what I have seems to be working OK judging from the reverb time measurements. Right now the unfaced side of the panel is spaced about 1" from the wall. Your comments about the foil and the wall have gotten me thinking. I agree that putting the foil right on the wall is probably not helpful. But I'll have to think more about what effect it might have if the foil side is facing the wall but spaced off. Brian F. |
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Post by rock on Aug 28, 2018 13:34:42 GMT
So the way an un-faced panel works is that the sounds goes though it from the front, what's left reflects off the wall and goes through it again on the way back out. When you space it off the wall, the same thing happens but the frequencies effectively absorbed are extended lower.
With faced panels, foil on the front reflects the mids and highs right off the bat and the foil somehow enhances the LF performance. The spacing off the wall works the same as above. As far as exactly HOW the foil enhances the LF performance, from what I know, the science is not understood. (If I'm wrong about this, and I hope I am, I'd love to understand what's going on here)
Now with the foil-faced panel with the foil toward the wall, my guess is if you're spacing off the wall, you're just not getting full advantage of the space behind the panel. If you want to experiment with measurements, please share your results. To look at this another way, if you want to space off the wall, use un-faced panels. BUT, even with your foil-faced panels spaced off the wall, I don't think you will gain anything by removing the space and mounting them flush, it's just that I suspect the spacing has gained you little to nothing.
With foil in contact with the wall, my best guess in that there is no difference from an un-faced panel mounted the same way.
As always, hopefully Ethan and or other members can add, correct or confirm my comments.
Cheers,
Rock
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Post by Hexspa on Aug 28, 2018 17:32:07 GMT
As far as exactly HOW the foil enhances the LF performance, from what I know, the science is not understood. (If I'm wrong about this, and I hope I am, I'd love to understand what's going on here) I just flipped through that section in Acoustic Absorbers and Diffusers and, while they do mention this phenomenon, from what I remember, they don't give any explanation as to why it occurs. There's a lot we don't know! Moreover, we don't know what the various combinations of flipping things around, layering several types of absorbers, facing, and spacing all do in general. The best way to find out any given combination of room, absorber, and accoutrements is to experiment and measure; science, in short. |
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Post by rock on Aug 28, 2018 20:07:49 GMT
So Hex, other words are you suggesting we actually figure it out ourselves!!?? I guess you're right, you can't learn everything thing out of a book. BTW, one man's accoutrements is another man's stuff.  Cheers, Rock |
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Post by Brian F. on Aug 29, 2018 13:06:07 GMT
I can think of reasons why it might be better to have the foil on the room side, and others why it would be better on the wall side with a gap. I wish I had the test equipment to do these types of measurements.  |
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Post by Hexspa on Aug 29, 2018 16:36:11 GMT
So Hex, other words are you suggesting we actually figure it out ourselves!!?? I guess you're right, you can't learn everything thing out of a book. BTW, one man's accoutrements is another man's stuff. Cheers, Rock On the contrary - I want everyone to remain enslaved by the loud-mouths, politicians and shame propagators.
I've got a dirty joke for that second comment but I don't want Ethan to get too excited
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Post by rock on Sept 1, 2018 17:04:51 GMT
I wish I had the test equipment to do these types of measurements. You might. All you need is a computer, REW free SW, a mic and an interface/soundcard (I have used an old Technics test set hacked to plug directly into my Mac audio input) You should use an omni measurement mic but you can start with any old mic as you learn how the use REW. Here's what Ethan says: realtraps.com/art_measuring.htm |
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Post by Brian F. on Sept 2, 2018 3:47:23 GMT
You might. All you need is a computer, REW free SW, a mic and an interface/soundcard (I have used an old Technics test set hacked to plug directly into my Mac audio input) You should use an omni measurement mic but you can start with any old mic as you learn how the use REW. Here's what Ethan says: realtraps.com/art_measuring.htmThanks, Rock. This has been occupying a corner of my brain in the last few days. I've used REW with a UMM-6 calibrated mic before. It should be possible (maybe even easy) to measure how much gets through a panel, but I really want to measure what gets through, hits the wall, and then goes back through again. It's probably not this simple like light going through tinted filters. I want to emit and measure on the same side. So I was thinking it would have to be time-resolved, which I don't know how to do or maybe can't do with REW, a speaker, and a mic. Maybe it would be fun and useful to do the measurements that can be done and worry about the other stuff later. Brian F. |
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Post by rock on Sept 2, 2018 5:07:21 GMT
Check out Ethan's tests with rigid in his empty room and use it as a template for your tests. Make up panels of different designs and put and measure them in the same room. Compare the results.
Keep the mic in EXACTY the same place for all tests. The 38% position is a good place but where ever you measure from, keep it consistent with all samples and trials.
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Post by Ethan Winer on Sept 23, 2018 15:18:40 GMT
If you have foil-faced material toward the wall for absorbing reflections, it's still a good idea to space it off the wall. The foil is behind so that's not a problem, and the air space still extends the absorption to lower frequencies which is always useful.
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