I thought that was a strange question, so what exactly are you asking? As for drywall, it might work but I never tried that. Not sure it would flex enough. But maybe. Why do so many people, when presented with plans for bass traps that are known to work, then ask about modifying them? :) --EthanI know what depth is, thanks :-)
Panel Absorbers
Originally posted at johnlsayers.com, topic 9538.
I don't call ME an expert, that's the very reason I can't answer your question :lol:this means you'll be providing lots of expert advice here now, yes?
Well, if you're starting an acoustic forum new signature contest... you're far ahead everybody :lol:an acoustics buffoon not to be taken seriously
Good point. My acoustics teacher told me once that there was a problem with a drywall wall that turned into a membrane and projected into the room. So it can flex, but I don't know if at 120x60cm. I'll use closed cell foam as a spacer between it and the frame (glue it to the frame and to the panel, and won't tight the screws more then needed for a seal), and get the 9.5mm drywall boards, and increase depth to compensate. I hope that the foam won't hurt the "attack" of the panel somehow. Any thoughts?Not sure it would flex enough. But maybe.
1.Because a 120x240 drywall panel costs about a quarter then a 12mm particle board. 2.I have a small car, and live in an apartment I might leave. Staying with 120x60 size is portable and universal if I decide to sell my stuff and go meditate for 10 years in india. 3.Ease of installation 4.Trying something new!Why do so many people, when presented with plans for bass traps that are known to work, then ask about modifying them? :) --Ethan
I meant, how does the whole thing work? :-) The cubic meter weight of the stuff determines its resonance, how does this and the depth (I don't want to say "volume", because the size of the panel doesn't matter, and therefor, there's no function of "volume", just the distance between the panel and the wall) correspond to each other to achive the result?I thought that was a strange question, so what exactly are you asking?I know what depth is, thanks :-)
Volume is actually the correct term. It is changed to "depth" for simplification of analysis/design. It works as a mass and spring. The panel itself is the mass and the sealed air chamber behind the panel. At one level it is presented as we commonly design it (ie. F=c/sqrt(mass x distance)). At a more detailed levels it incudes panel stiffness and tension. If you are looking for more detailed information, search the the web. It is a level of detail beyond the scope of this forum. Detailed: AndreI meant, how does the whole thing work? :-) The cubic meter weight of the stuff determines its resonance, how does this and the depth (I don't want to say "volume", because the size of the panel doesn't matter, and therefor, there's no function of "volume", just the distance between the panel and the wall) correspond to each other to achive the result?
The panel absorbers have been discussed a lot on this forum already.
I remember a thread with a lot of interesting info, including this message from Bob Gold :
http://www.johnlsayers.com/phpBB2/viewtopic.php?p=49797#49797
As long as I remember :
Panel absorber don't have to be sealed. The non sealed ones are more difficult to predict, but the common formula for sealed ones often fails because the stiffness of the panel and the way it is mounted on the frame should be taken into the equation.
I've seen an interesting article by Thierry Prevost in a French audiophile magazine where he uses a 10cm rubber seal between the frame and the panel to improve the efficiency of a big bass tuned panel.
Also the shape of the panel has an effect on the final result : a square one won't vibrate in the same way as a rectangular one.
In that case you'll probably do better with 4-inch thick fiberglass panels. --Ethanlive in an apartment I might leave. Staying with 120x60 size is portable and universal if I decide to sell my stuff and go meditate for 10 years in india.
Are you sure about that Andre? I'd think the depth determines the springiness of the air trapped inside, independent of width and height. If the width and height vary, then the panel size and mass vary in proportion too keeping the same center frequency. If I'm missing something, please let me know. --EthanVolume is actually the correct term. It is changed to "depth" for simplification of analysis/design.
What wrote is entirley true for such shaped air chambers. A (I'm trying to use an every day object) greek urn with lid on it is a panel absorber,but hte shape is straight in all dimensions, so the version of the equation that we use is noy appropiate. In architecture the vast majority of the applicaions are with a rectangular space, so the simplified version of the equation is used. My post was prompted by his writing that "volume" is not the correct word. Not by any desire to pedantic. With great volume: AndreAre you sure about that Andre? I'd think the depth determines the springiness of the air trapped inside, independent of width and height. If the width and height vary, then the panel size and mass vary in proportion too keeping the same center frequency. If I'm missing something, please let me know. --EthanVolume is actually the correct term. It is changed to "depth" for simplification of analysis/design.
Actually Andre you gave a very nice and physically correct description in the other thread. Yet you went confused yourself afterwards, by not directly noticing that the average depth equals the equivalent depth. Your sentence could/should be: Calculate the equivalent depth for the same volume, which in this case equals 1/2 depth to the corner, or average depth. That first part is valid for any shape. hence for a flat on the wall trap: Calculate the equivalent depth for the same volume, which in this case equals 1/1 depth to the wall (or simply cavity depth). And for both: Calculate the equivalent depth for the same volume, which equals the average depth. :) I liked the equivalent depth part (which clearly relates it to volume).Calculate the equivalent depth for the same volume, not the average depth.Hi, I assume that if placed in a corner, I should calculate average depth from wall, is this precise?
Is this because of the portability or the size? Anyway, I found a nice original way of making it portable, yet the drywall panel is attached with a thin layer of closed cell foam- so even if it doesn't flex as much as wood, the foam has some springy qualities- the concept of these panels forms mainly around the mass of the panel and not flexibility anyway- but the drywall I've got certainely flexed to a certain amount, at least like 17mm MDF. Hopefully the usage of foam won't change the center frequency by much, anyway, it's a very elegant, beautiful velvet covered panel, which is very easy to install and fix into place, I made one today :P BTW, is there a domestic way of measuring the result? (hmmmm MLS?)In that case you'll probably do better with 4-inch thick fiberglass panels. --Ethan
without a standardized testing environment and calibrated tools, about the only thing you could do is to measure the room before and after putting the treatment into place without altering the mic placement or other objects in the room (including people).
Both, and also because the back needs to be rigid and against the wall. So by the time you do all that, you've made a very heavy device.Is this because of the portability or the size?
You bet. See this: www.ethanwiner.com/density.html --EthanBTW, is there a domestic way of measuring the result? (hmmmm MLS?)
I have a question.
Suppose you measure your listening position and some boundry point in the room, let's say, the middle of the side wall. If you see a peak in the boundry and a corresponding peak/null in the listening position then you know that you can choose which trap (low bass/high bass) to put in there.
So far so good, right?
But what if you see a null in the boundry that is a peak/null in the listening position?
Should you look for the place in the room which this problem frequency peaks?
I'm looking at the room mode application in here: http://www.johnlsayers.com/phpBB2/viewtopic.php?t=2441
In that simulation, you can only see the frequencies that are peaking at the boundries. I suppose it makes sense in regard to resonators, which respond to maximum pressure and not velocity.
So, should I ignore nulls in the boundry measurements, in regard to bass trapping? How will I decide what IS best to put in that spot?
Ethan, I know your way of doing it is to cover as much surface as possible, but in my case, It's not that possible (budget), so I'm going for critical spots.
Can't see the problem. My traps are 120x60cm each with a sealed metal frame, it's much lighter then a wooden frame, and has a 0.5cm closed cell foam layer on the back, so all I have to do is to drill the wall and fasten the thing with aestheic looking 90deg angles. If I move out, I only have to take the screws out. I assure you it's the fastest, cheapest and most elegant way of doing it :-) Check out the attachment, the blue is the foam. I also add wide cellotape to ensure the seal. About the flexibility issue discussed- the drywall doesn't flex as well as a 1.2cm chipboard, but sure better then a 1.7cm board. What do you think about that? Another issue I had in mind is that it's probably best to make the panels square, as they will act best as membranes that way, isn't it so? I had another interesing thought of combining the panel technique with a helmholtz resonator. When I say that, of coarse I don't meen the sealed version of panel traps. However, when on free air, these panels (from any material) don't vibrate from low frequencies that much, right? Helmholtz formula is based on entirely rigid box, however I think that any material I would choose for a panel absorber, from 1.2cm thick and up, will not hurt performance by much- but the panel will vibrate according to the material's physical properties, therefore, creating a more broadband trap with both methods combined. I have done some ameteurish experiments with Helmholtz resonator in the middle of my living room, and tried different lengths and diamters for the port, and found that although theoretically the shorter the port, the wider the Q- but practically it doesn't really change much as you go below 2cm long. It stays with a (relatively) narrow Q as with the 2cm. This meens that you can take 1.2 cm drywall or wood that flexes well enough for a panel trap, and maybe still enjoy a focused trapping of the helmholtz effect. Another option is to put the hole in the frame, so the panel stays complete with no hole- would probably be better when I think of it. But still, if what I said about the Q staying narrow is correct, then I can just drill this super thin metal frame- and it will still work. Darn, I don't have the time for failing experiements (-: I measured the Helmholtz box by just putting a measurement mic outside the hole one time and another one in the middle of the box, to verify what I see in each. I know it ain't professional or reliable, but I was interested in the relative response of different ports, I believe that it was a worthwhile check. Of coarse all of this coming from a noob's brainstorming, so I'm waiting to hear some thoughts about this idea..Both, and also because the back needs to be rigid and against the wall. So by the time you do all that, you've made a very heavy device.Is this because of the portability or the size?
you could try this across a corner. polycylindrical diffuser with strong bass absorption and Helmholtz effect based on percentage of perforations. 8) just use 2 strips of wood on each side to hold it. seal it top and bottom.
How can it absorb a lot of bass? The perforations usually won't allow going very low, if I'm not mistaken...?
depends on the perforations... you have to design it to meet your needs. and being across the corner it will trap bass at least as much as it will placed on a wall...
Well I guess it can be done, but not as easily as the few panel traps I've made (still not mounted)
When you can't put traps everywhere, here's my solution for finding the best places: http://www.realtraps.com/lf-noise.htm --EthanEthan, I know your way of doing it is to cover as much surface as possible, but in my case, It's not that possible (budget), so I'm going for critical spots.
I was thinking more about this. For a given mass and spring, the frequency will stay the same as long as both are scaled together. So let's imagine a theoretical wood panel type bass trap that does not have edges that prevent the panel from vibrating. Something along the lines of a rigid cement plate attached at the outer edges to a cement box using neoprene rubber (or even a bead of green glue) or some such, so it can vibrate as a single piston. If the total surface of the cement panel and box are 1 square foot, or 100 square feet, won't the center frequency be the same? If not, what am I missing? I'm not arguing with you Andre! I just want to be sure I understand this myself. --EthanWhat wrote is entirley true for such shaped air chambers.
That's exactly what I did with the foam, isn't it? :) Please tell me Ethan, to what degree do you feel the panels vibrate when bass signals are played? It's sure easier to believe that lots of insulation would work better... But it's false :) I don't know if I should really put insulation in there..maybe a thin layer (well there are only 3.5cm left for the upper bass traps.. Another assumption I hope you guys an approve, is that it's more efficient to create a panel trap for a certain center frequency from lighter material and greater depth then a heavier material and lesser depth? I think it would start to vibrate faster, therefor improving the "attack" of the panel.I was thinking more about this. For a given mass and spring, the frequency will stay the same as long as both are scaled together. So let's imagine a theoretical wood panel type bass trap that does not have edges that prevent the panel from vibrating. Something along the lines of a rigid cement plate attached at the outer edges to a cement box using neoprene rubber (or even a bead of green glue) or some such, so it can vibrate as a single piston.What wrote is entirley true for such shaped air chambers.
our example is correct. I'm to think of a good example. Maybe a fifth of JD as compared to a Texas mickey? AndreIf the total surface of the cement panel and box are 1 square foot, or 100 square feet, won't the center frequency be the same? If not, what am I missing? I'm not arguing with you Andre! I just want to be sure I understand this myself. --Ethan
can we compute the absorption characteristics of these 2 panels?
one is 4'x8' and the other is 2'x2'.
both use 2x6 frames and have the back of the trap lined with 4" of 703 insulation (leaving an air gap between the insulation and panel of about 1 1/2"). the panel is 1/2" plywood.
assume they are sealed, constructed, mounted, mass of all components, etc is all the same (the diagram is a cut-away to show the construction but in practice the panel covers the entire front of the frame, and the insulation covers the entire back).
"Attack" is the wrong concept, and is wrong in the same way that some people believe one subwoofer can be "faster" than another. I see people argue about subwoofer "speed" all the time in audiophile forums, but it's not the right concept for subs or panel traps. I can't tell you the best ratio of mass versus spring, but I can say for sure that there is an ideal to aim for. In electrical circuits you can tune an inductor and capacitor to the same center frequency using many different values. But the efficiency of the tuned circuit falls off as you get away from the ideal balance of values. A tiny capacitor and huge inductor is inefficient, and vice versa. In an electrical circuit the ideal balance is achieved when both components have the same impedance at the center frequency. I'm certain the ideal balance between mass and spring can be calculated too! And I'm sure Eric Desart knows how to calculate that. Alas, I don't know how to do the math - I know only that an ideal balance exists. But it's related to impedance, not the notion that a lighter panel is better because it seems like it would start vibrating more easily. --EthanAnother assumption I hope you guys an approve, is that it's more efficient to create a panel trap for a certain center frequency from lighter material and greater depth then a heavier material and lesser depth? I think it would start to vibrate faster, therefor improving the "attack" of the panel.