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Old 30th July 2008, 05:02 PM   #1
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Default box acoustic compliance experiment with Iron Lawbreakers

I recently modified my Iron Lawbreakers (the world's largest bookshelf speakers, approx 3.5 cu ft) to increase the acoustic compliance of the enclosure. No, I didn't make them larger. I used an approach similar to what KEF describes as their ACE technology where adsorption of activated charcoal is used to accomplish this.

My first reaction is that 'it works'. However, I probably need to refine the way I introduced approximately 20lb of activated charcoal into each enclosure since I may not be reaping the full benefit of it. My modeling shows that it should result in a 3db or so boost near the BR resonance peak, something that the 2220B's currently in the Iron Lawbreakers could use.

I'd say at this point that these speakers subjectively go down to 30 hz or so useable response, but there is still a response step down of a db or two below 100hz. I've got something to try to go for the whole ball of wax where I plan to use an air core transformer to give a compensating boost in this range. Of course I'll have to wind it myself, but I've already done a trial run (for a different project) where I was able to achieve probably close to a 50% coupling (from memory) by bobbin winding the secondary over the primary. So, I'm thinking if I pick my values right, I can perhaps finagle the db or two boost below 100 hz to cutoff while also using the leakage inductance of the 'secondary' as a LP filter choke as part of the 2220B xover (a twofer, if you will). My xover point is about 500hz here.
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Old 4th August 2008, 10:24 PM   #2
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Update: I improved the air flow to the activated charcoal and the overall bass to lower midrange presentation improved greatly. These Iron Lawbreakers now do 3 dimensional imaging with a vengeance with very involving inner detail. I think some of this comes from the HF horn loading and the LF driver which is about the most efficient (non MI) driver that JBL ever did. With such efficiency, there is less energy available to wiggle sound degrading resonances in general. Plus it's making that much more sound to cover up the resonances that do exist

The Iron Lawbreakers are still a bit weak from 50-80 hz or so, but I'm going to see what I can do with an air-core transformer boost in this region. Another advantage of going air core here if I can pull it off is that it would get rid of all saturating magnetics in the xover. The 2220B recones have DCR's around 15-16 ohm, so I believe I can squeeze out up to 2db of boost in this range without dropping them below about 12 ohms minimum here, so that they still would qualify as '16 ohm' systems.

They're pretty efficient, too. The altec 288G-16's (with new diaphragms and recently remagged) into a 40 x 90 degree horn only need to be attenuated 6db midband for the 2220B's to keep up with them, essentially down to 40 hz, thanks to BR loading, some xover tricks I've already used which have given me up to nearly 10db extra peak boost around rolloff by transforming the BR impedance peaks into augmented amplitude response, and the activated carbon.

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Old 7th August 2008, 03:34 AM   #3
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Another update:

I found the air core transformer proto I wound a while back and ran some tests on it. It appears that the coupling between windings is about 33% which is lousy for most transformer applications but close to what I'm looking for here, since the same device may be configurable as both a mild step up device ~2db and a series filter inductance in one Also, I might add, it would be the inductive component of the resonant network that converts BR impedance peaks into augmented amplitude response at those frequencies. A potential 'three-fer', if you will.

However, if inductor winding calculations are a bit imprecise and incomplete, there is even less on the coupling that may be expected from air core transformer construction, so to some extent, if I ultimately go this route, I'll have to wing it.

Hey, it's all part of what makes the Iron Lawbreakers one of a kind loudspeakers.
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Old 7th August 2008, 04:40 AM   #4
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Rather than messing with xformers, I think EQ would be a much more sensible expenditure of energy and $, as well as addressing the room mode problems most of us have.
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Old 15th August 2008, 06:34 AM   #5
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Hi, Noah -

I've reluctantly decided against an air core transformer (iron core instead) for this particular project for reasons I'll describe below, but I'll stay with a passive xover because I don't want to introduce the colorations a mass market active xover would introduce.

For ceramic magnet speakers, a good (not Behringer) active xover might be worth considering, but I'm using alnico, so that restricts the active xover options considerably more.
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Old 15th August 2008, 06:42 AM   #6
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As I posted above, I've decided against an air core transformer for this project due to the fact that there might be more time needed to reach the proper configuration, and because of the possible resulting wastage, and also because the physical size of the transformer would be greater than for an at least partly iron core implementation, and the Iron Lawbreakers are somewhat size constrained.

As it is, I've ordered about 22 pounds worth of square 12AWG magnet wire to wind two ~10 mH air core series chokes. This is to lower resistive losses to an absolute minimum - the total series DCR is expected to be around 0.5 ohms. Since this is a 16 ohm system, the loss will be a little more than 0.25 decibel, which isn't very much.

The Iron Lawbreakers really sound good in the bass, detailed, punchy and very deep - with a bit more boost in the 40-70 hz region (which I plan to supply soon), there would be little to criticize in its LF response, IMO. Especially since it is a 100db/w/m system with useable response down to below 30 hz (port resonance and passive boost both centered around 25 hz) without EQ in only a 3.5 cu ft cabinet. That's why I call them "Iron Lawbreakers".
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Old 15th August 2008, 11:06 AM   #7
AJinFLA is offline AJinFLA  United States
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Wouldn't "Measurement Fugitives" be a more appropriate name?

cheers,

AJ

posted at 6:05am
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Old 15th August 2008, 04:14 PM   #8
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Well, I must admit that the Iron Lawbreakers are meant to be close to a back wall, so there is that much bass reinforcement due to placement. That's why I also call them the 'World's Largest Bookshelf Speakers'
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