deanm
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Post by deanm on Oct 12, 2018 20:57:18 GMT
Oh... Umm... okay, then. That's a good point.
I'll happily go back to the original, easier, plan of making the framework non-resonant.
Other than the part about it being below 50Hz, did the theory of what I proposed make any sense?
-d
(one fact that I've proven so far: as little as I know about acoustics, I know even less about carpentry.)
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Post by rock on Oct 13, 2018 13:00:57 GMT
I never built resonant panel absorber but I read Ethan's article a few years ago. He built them on 2x4 frames attached and sealed to his walls and or ceiling. Sealing is important, it needs to be closed. The fiberglass extends the bandwidth (lowers the Q). The fiberglass should not touch the resonant panel; it should vibrate freely.
Answers to questions: 1-Yes 2-No 3-Panel should not touch fiberglass
Look for Ethan's article IIRC the title is "building a better bass trap"
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deanm
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Post by deanm on Oct 14, 2018 0:18:09 GMT
These would be much simpler (and less effective) traps than the membrane pressure traps you're talking about, rock. These are simply lengths of 2x4 that span width-wise under the ceiling; they're there to hold the foot of fluffy insulation that will cover the entire ceiling. Some dumb (dumb, obsessive, over-thinking, etc) part of my brain keeps saying I should also try to tune them to resonate at 47 Hz (since that's the mode that hits them perpendicularly, in the direction that they naturally resonate), and then glue the fluffy insulation to them to increase damping. Envision 7 2x4s that have been used to beat muppets, constrained at each end, left free to vibrate in the middle. Sealing these in any sense is not possible, because their primary function is to hold up a big 10' x 10' x 12" velocity trap. Hence, glue the wood to the fiberglass, so they don't just float next to each other. If I can also get a little more absorption by letting them resonate sympathetically at 46hz, with only a little more effort on my part, then it seems like a thing to do. I'm not expecting a lot of absorption out of this, but hey, all absorption is cumulative, right? And I hear Hexpa's advice to draw the line somewhere when it comes to modal ringing, and I get that Ethan's line is at 50Hz, which is even more strict than EBU guidelines. But my 46 Hz mode is on the cusp, and even though it's most resonant at 46, it still has impact a major third or so in either direction. So I shouldn't go very far out of my way for it, but maybe it's still worth a shot. ...so says my stupid brain. "FOR JUST A FEW DOLLARS MORE!!!"
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Post by Hexspa on Oct 14, 2018 3:30:38 GMT
If you're set on treating that mode, 6' thick absorption will do it. You could also do 3' with a 3' gap. Conceivably, you can have a good effect with 24" fluffy with a 2.5x gap. We need to remember that our hearing is most sensitive between 1.5 and 5kHz. When we get graphy, I think we see sub frequencies better than we hear them. I'm no audiophile and, to me, sub is just level. It has little-to-no character, unlike midrange bands; on, off, volume, duration. I'd like to participate in a test where a resonant absorber is introduced vs. absent to hear whether I notice a 100ms and/or 6dB improvement in ringing at 45Hz during program material and test tones.
In my room, I tried plucking around E1 and F1 to trigger my 42Hz monster mode, vs an octave up. The difference was very 'meh' and I was surprised with just how low that cycle sounds. 84Hz was a little tighter but even if I never get technically impressive decay in that region, I'll never lose sleep or fail to complete a project over it.
Treating your room with regular absorption will improve your 46Hz mode, don't forget. It's not like modezilla is going to breathe wretched sub fire all over your super chunks. I think in this room I'm like 10dB out of whack to get the illusive Perfect Decay.
FWIW, if you have a dimension which rings at 138Hz, 3x46Hz, you can leave that un- or less treated to help negate your sub region due to destructive interference. In fact, my X and Z dimensions do just that which I discovered in hindsight after having tried the conventional advice to make your rear wall 100% absorbent. For this room, treating the side walls as much as possible while leaving the Z axis relatively untouched produced the best result. The corollary of that is that if you treat this unicorn mode at the expense of balance, you might make other, more audible, bands worse.
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deanm
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Post by deanm on Oct 14, 2018 7:18:39 GMT
I don't see it as a unicorn, no... I'm not planning on going far out of my way for that particular mode. Certainly not gonna extend my room back 6' for it. ("Honey, I gotta take the wall out. And the hallway. And the bathroom, too, but only the sink. I never use that sink anyway.")
It's just more like, as long as I'm at this point in construction of the ceiling trap, it's an idea that might improve that mode a tad.
"modezilla"... heh!
>>Treating your room with regular absorption will improve your 46Hz mode, don't forget.
Yuppers. This fact also prevents any useful testing of the "resonant 2x4 trap", because I'm not committed to the academia of it enough to do a 'control' measurement. I have neither the resources, nor the inclination, to build it one way, test it, tear it down, rebuild it the other way, test that, etc.
Interesting thought about the destructive interference between modes working to your advantage. I don't think I could apply it here, though.
-d
(If my angry wife kicks some holes through the sheetrock into the adjoining room, that's a Helmholtz resonator!)
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Post by Hexspa on Oct 14, 2018 20:12:08 GMT
I don't see it as a unicorn, no... I'm not planning on going far out of my way for that particular mode. Certainly not gonna extend my room back 6' for it. ("Honey, I gotta take the wall out. And the hallway. And the bathroom, too, but only the sink. I never use that sink anyway.") I don't mean it disparagingly, it just seems like a pet project. We all have those. Also, I forgot your room dimensions so please pardon me for giving non-applicable advice. Cheers.
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Post by rock on Oct 14, 2018 21:24:44 GMT
deanm wrote "I need some advice having to do with pressure absorption. I realize the "best bang for your buck" approach is focusing on broad-band velocity absorption, which is what I'm doing. I think I may also be able to get some pressure absorption for free (or just a few dollars more) out of the construction of the ceiling velocity trap, if I'm clever."
At this point I'm confused. You asked for advice on pressure traps and I sent you to Ethan's article. Now you say you don't want to follow Ethan's design concept and you have some other idea (at least that's the best I can figure). I really don't understand exactly (or even vaguely) how your pressure trap will work so I'm sorry I can't even begin to tell you anything more except:
In general, pressure or resonant panel traps are not recommended (at least on this forum) in favor of velocity absorbers. Cheers.
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deanm
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Post by deanm on Oct 15, 2018 1:23:17 GMT
Hexspa: >>I don't mean it disparagingly, it just seems like a pet project. We all have those.
Yes, this. Definitely. lol... This has become my latest obsession.
rock: So, I may have neglected it earlier, but, thank you for your advice, including the reference to Ethan's design. I think everything I've gotten from you has been good, and appreciated. But, I think I haven't been able to ask my questions clearly. So your answers, while they are truths, and while I agree with them, are not answering the questions I have in my mind.
To specify:
>> pressure or resonant panel traps are not recommended (at least on this forum) in favor of velocity absorbers.
I agree with this advice, and I'm following it. All of the treatments I have solidly planned out for this room are velocity absorbers - a fully absorbent ceiling, chunks in the front corners, absorbent side RFZ panels, absorbent SBIR front panels, and I'm also gonna make the back wall as absorbent as I can. (I may decide to put some diffusers up on the rear side walls to treat Y axis flutter, because they look so much sexier than FG, but that's another issue.)
>>Now you say you don't want to follow Ethan's design concept and you have some other idea (at least that's the best I can figure).
Yes, this is true, although without any sort of belligerent undertones to it. If I want to build a pressure trap, from scratch, which will have the primary function of being pressure trap, I will follow Ethan's sealed membrane trap design.
But that isn't what's going on in my room. Here's what is:
I'm seeing a possibility of turning some components of the fully absorptive ceiling, (namely, the long 2x4s), into resonant traps. Cuz the 2x4s are already there, as part of the construction of the velocity trap. So maybe, for just a little more effort, I can make those 2x4s also absorb some of the modal ringing from front to back.
>> I really don't understand exactly (or even vaguely) how your pressure trap will work
I possibly don't either, TBH. My idea is just a guess at something that may work, based only on my novice understanding of pressure / resonance trapping.
Because of my incomplete understanding, I'm not good at explaining the idea, or asking questions about it. Here it is again in different words:
I have a 2x4 board which spans from left side to right side of my room, suspended under the ceiling. This board is constrained at both ends, but nowhere else. It vibrates easily back and forth between the front and rear of my room, and resonates with a fundamental of 10 Hz. Due to the orientation, it doesn't vibrate easily up and down. (Actually, I have 7 of these 2x4s mounted up there, all in parallel. But let's just focus on one. If the one doesn't work, then 7 will also not work equally well.) These boards are the framework for the velocity trap that I'm currently building, which will cover the entire ceiling in 12" of fluffy FG.
My primary font-to-rear mode is at 46.5 Hz, with a pretty massive SPL peak and ring time.
So that's what I have. Here are the extra steps I'm contemplating as a part of building the ceiling trap:
- Tune the fundamental resonance point of the 2x4 to 11.625, so that it can resonate sympathetically with the front-to-rear mode of 46.5 Hz. I'd try this by adding a crosspiece to brace near one end, shortening the board's length. It probably won't be this simple, because the overtones probably aren't harmonic, but for the sake of discussion, let's say that I find a way to make the 2x4 resonant at 46.5. - Glue the edges of the fluffy FG (which will already be there as part of the ceiling velocity trap) to the board, so that it adds damping to the 2x4's resonance (i.e., make it look like a muppet-beating stick).
And that's all I'd do differently to the ceiling trap. It'll still function primarily as a broad-band velocity trap for sound waves hitting the ceiling.
Here's how I hope it works: When the air mass of the room resonates modally, front-to-rear, at 46.5 Hz, some of that energy transfers, sympathetically, to the 2x4 causing it to resonate transversely (the 2x4s span from side to side of the room). The energy removed from the sound wave is thus stored as resonant vibration in the 2x4. The vibration is then transferred from the 2x4 to the attached fluffy FG insulation, where it breaks down and becomes heat. The FG damps the resonance of the 2x4, and by extension, damps the resonance of the air mass. This reduces the modal ringing time.
I'm not expecting a lot of added value from this. I understand that a better pressure trap design would work better, and that additional velocity traps would probably work better than that. I'm aware that the fluffy FG in the ceiling trap will also absorb a sizeable portion of the front-to-rear mode, even without the 2x4s resonating. The resonant 2x4 idea is most definitely NOT my main plan for treating the front-to-rear mode. But even if the resonant 2x4s gives me just a little more absorption of that 46.5 hz mode, then it adds value, which is good.
If it works at all.
So, therein is the question, to anybody who understands pressure / resonance traps: Will this work at all?
I'm more interested in having you shoot holes in the "how" of the plan, not in the "why".
Thanks, -Dean
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Post by Hexspa on Oct 15, 2018 21:59:27 GMT
So, 46Hz has a 24' wavelength. That'll be reinforced by a 12' dimension. Y axis is height. Your ceiling, at 8', actually helps to negate the 46Hz resonance. If you want to kill this mode, you need treatment on your length, (Z axis), and less treatment on your height.
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deanm
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Post by deanm on Oct 17, 2018 1:39:35 GMT
Ohhhh... Umm, I've always thought that in a 3-dimensional architectural or engineering context, x = width, y = depth, and z = height. This is apparently not as standard as I thought it was (my bad), and I can see how that ambiguity makes my explanation more confusing. Sorry 'bout that. I've gone back and edited the last explanation to clear that up. Hopefully it's better now.
This may all become moot in the next day or two anyway. I'm reaching the decision point with the construction. Unless I'm certain that this whole "resonant 2x4" idea will help, or at least, that it won't make this room worse, then I'll can it and just make the framework rigid.
I'm giving up on trying to "tune" the 2x4s. The lowest audible resonant overtone when I thump them varies over a major third, at least. This is probably due to variance in how rigidly they're constrained at the ends, and also probably to variance in their internal rigidity.
It now seems to me like it's all a question of damping. If I don't damp the 2x4's at all, they will add unwanted LF resonance to the room. If I make them rigid, they won't make it worse, but they also won't make it any better. If I can reach a sweet spot (at or near critical damping?) between the two extremes, the 2x4s should "bleed off" some of the LF energy from the room through sympathetic vibration, and convert it to heat via the fiberglass damping. Damping should also lower the Q of each board, widening the bandwidth(s) they resonate at.
(EDIT: critical damping is defined as the amount of damping needed to reach equilibrium as quickly as possible, without overshooting and oscillating. Looking at the plans for Ethan's membrane traps, it's clear that the desired damping level is much lighter.)
I'm continuing to learn about how pressure traps work, and realizing that my idea here is probably too primitive to call a "trap". I'm not sure what to call it. Maybe just a "damped resonator"? Maybe just "rubbish"? *shrug*
-d
"trap" is a euphemism anyway. I'm not trying to trap the bass. I'm trying to kill it.
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Post by rock on Oct 17, 2018 13:50:55 GMT
I think one of the reasons Ethan un-recommends his panel absorbers is that tuning is not adjustable except for rebuilding them with different material. Looks like your 2x4's are similar in that regard but IDK. Thanks for explaining your plan. My gut feeling is that they may influence the acoustics the way you hope but even if they do, the surface area is so small, they will have little effect. If you really want to know, I think you need to make a series of acoustic measurements with them in and out so you can see the difference.
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Post by Hexspa on Oct 17, 2018 19:43:49 GMT
Ohhhh... Umm, I've always thought that in a 3-dimensional architectural or engineering context, x = width, y = depth, and z = height. This is apparently not as standard as I thought it was (my bad), and I can see how that ambiguity makes my explanation more confusing. Sorry 'bout that. I've gone back and edited the last explanation to clear that up. Hopefully it's better now. Oh, that may very well be. I have no construction experience besides crafts and I hadn't considered there's a different architectural standard. When I refer to x=width, y=height, and z=depth, I mean it as you'd see on a 3D plot such as a waterfall graph. Kind of weird that those would be different.
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deanm
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Post by deanm on Oct 18, 2018 9:53:04 GMT
I think one of the reasons Ethan un-recommends his panel absorbers is that tuning is not adjustable except for rebuilding them with different material. Looks like your 2x4's are similar in that regard but IDK. Thanks for explaining your plan. My gut feeling is that they may influence the acoustics the way you hope but even if they do, the surface area is so small, they will have little effect. If you really want to know, I think you need to make a series of acoustic measurements with them in and out so you can see the difference. Yeah, these are also good points - thanks again. I'm ditching the idea. I'll file it under "Inventions that I have learned won't work". TBH, though, I'm actually a little surprised at how much surface area there really is. It's kind of like my surprise at seeing how much surface area you gain by opening up the sides of a velocity trap. 7 boards x 3.5 inches wide x 116 inches long = 2,842 square inches = 19.75 square feet. If I imagine that both sides of the board would be pushed on, I double that, to 39.5 square feet. That's a tasty sized panel! BUT, here are a few new reasons it won't work. More gut feelings and theories, but I'm pretty confident about these ones: - 3.5" is not enough width to block, or appreciably attenuate, a LF sound wave. The pressure behind the board will nearly equalize to the pressure in front very quickly. So both sides of the board would be pushed on at the same time, with about the same pressure, in opposite directions. So the "two-sided" thing works against me, not for me. Now I think I get *why* membrane traps are sealed. - My surface area to mass ratio is way high. The 2x4s are 1.5" thick. (gotta love how they measure and label dimensional lumber.) Each square inch of surface area is tasked with pushing 1.5 cubic inches of wood. Compare this with Ethan's plan, which uses 1/4" plywood, IIRC. My ratio is 6 times that. You probably wouldn't make a membrane trap with a 1.5 inch thick face for audio purposes. - The placement of the 2x4s, in relation to the predicted high-pressure zones for the fundamental mode, sucks. The "best placed" board is 15" away from the front or rear wall. The next-best is 30". Etc. The only high-pressure zones I would possibly hit well are the intermediary zones for the harmonics of the mode, and it'd be haphazard at best. Moot point, but I never did figure out any way to objectively measure the performance of the 2x4s. Keep in mind, the 2x4's are also a component of the ceiling velocity trap. The fluffy fiberglass is integral to damping the 2x4s for the pressure trap idea. The 2x4s are integral to supporting the FG for the velocity trap. I can't measure one at a time. BTW, I ordered Ethan's book. I'm looking forward to learning why many of the other inventions in my head won't work. -d
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deanm
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Post by deanm on Oct 21, 2018 10:48:04 GMT
Hey esteemed gurus with much more experience than I,
I've seen lots of threads about good ways to use FRK (or an equivalent facing) with rigid fiberglass. Do the rules for using FRK stay the same for thick (9.5 inches + 2 inch air gap) fluffy fiberglass, too? Can I expect the same increase in reduction under 100hz, and a decrease in reduction above 2Khz?
I watched Ethan's presentation on Doug Ferarra's "Completely absorbent ceiling" for his basement studio and control room, and it doesn't mention any use of FRK. (Ethan, if you happen to be lurking, I'd love clarification on this.)
I'm using 9.5" unfaced fluffy FG for my ceiling trap, which leaves a 2" air gap behind it. I'm trying to decide whether or not to glue a paper facing onto it around the front and side perimeters, and and on the rear half of the trap - i.e., use FRK anywhere that won't be dealing with early reflections.
Thanks, -d
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deanm
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Post by deanm on Oct 23, 2018 20:38:28 GMT
just a followup on that last question, in case anyone else wants to know: i found some relevant info here: www.bobgolds.com/AbsorptionCoefficients.htmit looks like FRK on fluffy FG only helps absorption at 250Hz. Lowers absorption everywhere else. So it only appears useful for reflecting highs if the room gets too dead. makes this job easier :-) movin' forward... -d
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