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Old 15th April 2010, 12:19 AM   #31
gedlee is offline gedlee  United States
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Yes construction and size make a big difference. My home theater is as sealed as I believe possible and I don't find much room gain. Theye is a huge resonance at about 20 Hz - the first longitudinal mode and below that it drops like a stone. Don't see much there at all.

We saw room gain in spades in cars however.
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Old 15th April 2010, 05:14 PM   #32
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Something that I think was already mentioned, but I've also thought about is creating a sort of hybrid 4th order bandpass and infinite baffle. The way I envisioned this was that the rear "sealed" chamber would be the very large open area the back wave fires into. The attic, the basement, etc. The front tuned chamber would then be designed specifically with a tuning in mind. I would imagine that these chambers have a tuning to them anyway, but that people rarely take it into account. It would not be for broad band purposes, it would only be for the ultra low frequency. Also, it would likely only work for people who have a room the back wave can fire into and not disturb others. I'd imagine an attic could be possible, and a basement only if the basement wasn't serving other purposes, and assuming the basement was sealed off from the upstairs well enough to keep the bass from traveling back into other rooms.

My thinking as to why this is advantageous over other modes of ULF reproduction is that it has the same benefits of lower distortion in the passband, with a much lower passband than is typically possible in a room. Bandpass subs capable of 115+db's below 50hz tend to be huge, so being able to move this into a space outside the room would seem to be a good thing. Whenever I've modeled a ULF bandpass capable of the output and efficiency I want, i end up with rear sealed chambers in excess of 10 cubic feet, and a front chamber often in the 5-10 cubic foot range. This means a 20+ cubic feet is perfectly reasonable, which is just too big for me, and probably most people.

Another option I thought about was a Line Array subwoofer that goes floor to ceiling, sealed, fed with excessive amounts of power, and eqed to have bass down into the very low range. It's still a pretty inefficient option, unlike the above mentioned option, but I would think would achieve the desired results very well. I think this would work best with two of these units, one in each front corner, and the the BB subs distributed around the room as before.
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Old 23rd April 2010, 12:41 AM   #33
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So I was having trouble getting the bandpass subs balanced out in my room. I tried something counter-intuitive that actually helped a lot. The problem was that around their pass band and below the response was rising too much, but lowering their level caused too much loss a the top of their pass band.

I applied a shelf filter and first order high pass filter. The highpass had a corner at 55hz and the shelf started a bit higher, around 70hz. Combined these two seemed to really flatten the response. Interesting the filters that will fix room modes. I was trying to use tons of parametric eq's to no avail.
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Old 23rd April 2010, 01:42 AM   #34
gedlee is offline gedlee  United States
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I've always said that if you are using tones of EQ then something is wrong. Thats true at LFs and HFs as well. EQ is a bandaid - when your whole body is one big bandage then you are in serious trouble.
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Old 19th August 2010, 10:07 AM   #35
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Sorry for digging out old thread, but it seems to me more appropriate than other which shifted out of theme.
I'm living in old multi-storey block of flats with thick brick walls and relatively thin brick partitions (7''). Before I moved in I had to made nearly complete reconstruction. I was short in time and money. Nevertheless when I had heard neighbour's TV-set I let to make a double-layered drywall with constrained layer made of glue (as per E.G.), but particularly as a sound insulation and one single layered on the perpendicular wall. (It turned out later, that situation was not so bad, because neighbour is deaf as a gunnery horse and TV-set was on very loud) Unfortunately the workers made it stiffer than I had asked.
I have made several quick spatial measurements of my room (using Nathan in the corner) and then identified modes and their bandwidths. BW of modes vary around 5-6Hz (below 50Hz) and around 9Hz (below 100Hz). So the absorbtion coefficient is 30% or 40% respectively. There are 9 cases where the modal BW is not more than twice the modal spacing. It seems to be a problem to me, which is not going to be eliminated even with multi-sub. (No attempts already made)
What are your experiences? Does my damping work at all? What is your room LF damping?
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Last edited by MethMan; 19th August 2010 at 10:14 AM.
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Old 19th August 2010, 11:34 PM   #36
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While I hate to speak for Dr. Geddes, I think I know the answer for him, and myself. His LF dampening consists of the CLD walls over an air pocket, created by the studs. I don't know if there is insulation in the cavities, but I would assume so. Essentially a wall can be looked at as a giant LF energy converter. If the wall has a resonance, which it does, then when that resonance is excited, the energy taken to excite it will converted from vibrational energy to heat. The efficiency of this is largely dictated by size I believe. This is why in my opinion (and many others I believe) many bass traps are a waste of money, they lack the size to be effective at such low frequencies.

My own LF dampening is nill right now. In my old place I had setup some DIY bass traps which were essentially the same CLD type. I had an 8" cavity, a large CLD surface, and fill inside. The resonance of the panel dictated the center frequency of the trap. The fill and panel dampening changed the Q. These are as effective as it can get, but to equal was the wall can do in the first place, they have to be as big as the wall, and at that point, why not just turn the wall into the bass trap.

I think if your CLD is mounted directly to the Brick with no air pocket, it won't be offering much of any LF dampening. If thats the case, then you would need to add studs to space the panel from the wall, it should be spaced as much as is reasonable, I would suggest 6-8 inches. The panel cavity would benefit from insulation of some sort, with some being a bit better at lf's due to their higher mass.

I think there is benefit to multiple subs for you too. I'm not sure I understand why you believe they won't help. I'm not sure I understand why the BW or Q of the modes matters in terms of the effectiveness of the multisub approach.
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Old 20th August 2010, 01:42 AM   #37
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Multiple subs will always be a benifit, but so will good LF damping. Sounds like they made the CLD wall too stiff. It really has to be quite flimsy to work right - think or a hanging sheet of cloth - very limp. Most often that I have seen people try this technique they make it far too stiff and it kills the effect.
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Old 22nd August 2010, 10:41 PM   #38
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Given that its made of Drywall, won't it have a certain stiffness you can't avoid? What is the thickness of the pieces you laminate together? I figured you used 1/2" or 5/8" gypsum board, with your urethane mixture as the dampening layer between them. How thick do you try and get the urethane layer? Do you mix anything with the Urethane to improve the dampening properties?

What about the attachment to the walls? Is it the typical screws or nails method, do you use more or less of them, decoupling of any sort, etc? I recall you said you used resiliant channel or RSIC or something along those lines?
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Old 23rd August 2010, 01:52 AM   #39
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I use 1/2" drywall and historically used Liquid nails. My polyurethene mixture would work better, but I have not used that on a wall structure. the Liquid Nails (for Subfloors) is spread about 1/4" with a flooring trowl. I attach the first layer to RC1 resilient channel, which is attached to the studs, then the second layer is just glued on the first. You have to hold the layers together with screws until set, then just remove the screws.
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Old 23rd August 2010, 02:00 AM   #40
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Ah makes sense.

I thought Liquid nails dried hard? Is the subfloor formula stay softer? My experience with liquid nails, such as using it to adhere drywall to firewalls has been that it dries out to a hard as rock material.

While quite expensive even compared to the urethane mixture you use, Green Glue is purported to offer the best performance. They have a lot of good independent testing to support it, but I suspect you could come close to equaling the performance with urethane and some filler. The MSDS indicates its Urethane and some stabilizers, but MSDS doesn't give precise formula descriptions. I'm guessing there are additives to improve the dampening. Plus, not all urethane is identical, different formulas. I've wondered about using the soft High density urethane foams as a possible material instead
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