Tuned panel absorbers... - AVS Forum
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post #1 of 7 Old 11-28-2012, 05:07 AM - Thread Starter
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Hey guys.

I'm lost on a few things that I couldn't really find in my searching.

I understand how panel absorbers work and how to build them, I'm just not sure how large to build them, nor how thick the plywood or how deep the unit has to be for a certain frequency.

I'm guessing the tuning frequency depends on the thickness of the front material and depth of the panel while the larger it's made, the more efficient it is at absorbing said frequency?

Also, my problem areas are length modes at 30hz and 60hz. I might be able to get rid of the 60hz just by x-over and distance settings but the 30hz is a killer. The room is 18' x 10.5' x 7', would a large enough panel placed on the back wall be able to take care of my problem frequency, in theory anyway? Or am I missing something?

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post #2 of 7 Old 11-28-2012, 06:35 AM
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Hey Chris - this is exciting. I'll be interested to see what results you have.

Keeping in mind that my experience with this is non-existent and my "knowledge" only academic - it sounds like you haven't seen this article published in sound on sound. Part 2 includes practical tips for panel absorbers (as well as helmholtz). The author (Paul White) includes an equation, which I assume is accurate, for the design frequency of a panel trap. F=170/sqrt(M*D) where F is the design frequency, M is the mass in pounds per square foot, and D is the depth of the air space in inches. This basic formula (constant divided by the square root of the product of mass and depth) seems to be incomplete to me, since it doesn't account for the rigidity of the panel membrane directly, instead assuming that it is "stiff" I think, as opposed to something flexible like a fabric (I assume that MLV and an equivalent mass of plywood should behave differently). That said, and as the author explains, with absorption inside the air space the Q of the system gets lowered significantly, making the exactness of the design frequency less significant. My point here is that while the overall size of the panel you build (wall sized to picture frame sized) will influence the total energy it can dissipate, the depth influences the design frequency.

So we've got several parameters to optimize:
  • design frequency - influenced by panel material choice and panel depth;
  • total energy absorption capability - influenced by the wall real estate you devote to the device;
  • and system Q - influenced by the internal damping of the cavity - more damping broadens the effective range, while decreasing the potency at the design frequency
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post #3 of 7 Old 11-28-2012, 06:46 AM
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Quote:
Originally Posted by HopefulFred View Post

........This basic formula (constant divided by the square root of the product of mass and depth) seems to be incomplete to me, since it doesn't account for the rigidity of the panel membrane directly..............

+1

That was my first thought as well. I think there are a lot of people that would be interested in how well this would work in practice!

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post #4 of 7 Old 11-28-2012, 06:51 AM
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post #5 of 7 Old 12-01-2012, 08:16 PM - Thread Starter
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I was just thinking of something. As of a little while ago, I was under the assumption along with your confirmation Fred, that the depth of the absorber and thickness of the front panel affects the frequency while the overall size of the device adjusts the effectiveness of said frequency. This just came to mind...

I'm thinking along the lines of, say, a 3 inch by 3 inch piece of 1/8" aluminum/steel, and then take a 3 foot by 3 foot piece of the exact same material. By hand, I could much easier bend the larger piece of material via laverage, does this matter to the frequency that I'm targeting or is it that the frequency will flex the piece no matter what, but the easier the piece is to flex, the more effective the absorber will be?

The more I think about it, the more I realize that it was probably pointless to ask in the first place :P
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post #6 of 7 Old 12-04-2012, 09:48 PM
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Chris, it hurts my brain to be unable to figure this out. I've been reading at gearslutz, since those guys - being in the business of making money based on the performance of their rooms - take this seriously. I've seen threads discussing this type of thing, but no one seems to do the math in public. They all must rely on spreadsheets and software, I suppose. The other challenge here is the access to appropriate materials. I think we can conclude that I allowed myself to believe in an oversimplified version and should have known better.

This thread: http://www.gearslutz.com/board/studio-building-acoustics/667929-my-experiment-metal-panel-absorber.html illustrates my points pretty nicely. However, I bet it's the place to find answers if you are a careful and patient reader. I'm having trouble devoting the time and concentration that would be required - especially when sifting through the jargon and abbreviations - not to mention that a lot of the posters there are European and have access to different materials than we can find readily here.

I suspect that something like RPG's Modex products are what you need, like the modex plate http://www.rpginc.com/ProductDocs/MDXP_Modex%20Plate/Modex%20Plate_2%20Page%20Brochure.pdf which is advertised to be effective to 50Hz. I have no idea what it costs - and they only make one size. I also hate their website - couldn't they make it easier to get an idea what these products cost or how a person can get them? (No, that would open them up to a mass market, and undermine their exclusive image and relationships with pro designers)

To be honest, these are the reasons I decided to build my room in a way that would allow me to try to use Helmoltz resonators. While not as sexy and compact as these more complicated engineered solutions, I have more confidence that I can engineer a Helmoltz device based on a single cavity resonance. The key (not explained at this link) is the port are compensation - which means you have to add something like 60-80% of the port diameter to the length of the port to get the effective port length. You can basically build a ported subwoofer box with no driver, and use the port dimensions and box dimensions to tune the thing to absorb at its fundamental (and odd multiples, I think). I'm hoping to build on in the adjacent room, and extend the port through the wall into the theater. Just put the box under a bench or cabinet, creatively cover the port with some fabric (which will change the port compensation), and measure - adjust port, repeat. I've previously found people building these out of large tubes, but they're not "tube traps" and I can't remember the name that google can find. But the more I read about that, the more I'm reminded how much trouble people have with them.
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post #7 of 7 Old 12-05-2012, 06:39 AM
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Every time I see this thread, it reminds me of Tony123's experience with cabinets. After he bought his second DTS, he later put in cabinetry in the rear of his room. His posts afterwards made it sound like the difference in bass response was significant. The first time I read it, I immediately thought that he had inadvertently added panel absorbers to his room that were smoothing his bass response. Which brings to mind the question, was he just really lucky that his cabinets (and probably the cabinet doors) were tuned to the correct frequency, or were they acting over a larger frequency band than one might have otherwise expected? If only I had a finished room that I could start playing with this stuff smile.gif

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