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Open Baffle Epiphany!

1.6Baffle said:
Hadn't thought about over-excursion. Active crossover it is then.

Thanks for the links, I've read the papers, understanding some of what Martin King says, hopefully the parts that matter. I'll go with the Alphas in the H-frame. Is it correct that the internal dimensions of the frame should be 16" x 16" x 16" with the board holding the Alphas centered equidistant front and back?
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The H-frame is 15" deep (total) with the Alpha 15a mid-way (7.5"). Technical reasons are given in MJK's papers. 16" x 16" internal sounds right; should be just big enough for the Alpha 15a to fit in. Because the Alpha 15a is recessed into the H-frame, you must limit the high freqs coming from it. Think how bad a mid-range would sound if it were down in a tube! So pay attention to MJK's recommendation for XO. Again, the electrical XO isn't the same as the acoustic XO-- due to the effect of the H-frame. Again details are in MJK's papers.

Cheers, Jim
 
Jim Shearer said:
The H-frame is 15" deep (total) with the Alpha 15a mid-way (7.5"). Technical reasons are given in MJK's papers. 16" x 16" internal sounds right; should be just big enough for the Alpha 15a to fit in. Because the Alpha 15a is recessed into the H-frame, you must limit the high freqs coming from it. Think how bad a mid-range would sound if it were down in a tube! So pay attention to MJK's recommendation for XO. Again, the electrical XO isn't the same as the acoustic XO-- due to the effect of the H-frame. Again details are in MJK's papers.

Cheers, Jim
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Thanks Jim, I just placed the order for the Alphas, and will cut the wood to those dimensions this weekend. Also have to cut some new baffles for the Fostex drivers since they need to be off-center.

Reading back over the MJK paper on the H-frame it looks like the low pass XO should be set at 125Hz, which because of the effects of the H-frame will take the actual Hz to 200. Maybe that's what Jim meant about the difference in electrical and acoustic. Please let me know if 125Hz isn't right, I didn't understand some of the numbers on the graphs, and certainly not the math involved.

Would 200 Hz then be a good place to set the high pass XO using the FE126En in the open baffle and with the Alpha15" in the H-frame?

(Edited out some wrong assumptions in this post after reading some more about both XO and up-sampling.)
 
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Here's a quick update on my OB experiment. I recently traded some components for these Eminence crossovers. A little more sophisticated than the previous filter and cap. Spent a little more time on the wiring and terminations.


I also reused a set of CSW surround speaker stands as bases for the baffle itself! Turns out to be a pretty great solution because I can vary the angle of the baffle by adjusting the "mast." And they have that industrial look that I like.


New birch plywood boards will be cut soon to replace the OBX and then I can experiment with the shape of the baffle as well as any "wings" used to contour the bass.


I also just picked up a calibration mic so I can take some measurements when I have things "living room ready."


Fun stuff!


9a3ena5e.jpg
 
Ran into a snag on my project. One of the Morel woofers developed some voice coil issues. In the process of "buying back" the two that I sold on BT last month. Oops!
 
I recently rotated a couple other pairs of speakers through my system, while the OB's took a break. In the meantime I was able to re-do the support bracing, and put 12" lengths of Sonotube around the woofers. The new bracing totally eliminated the instability I was concerned about with the old bracing, and the sonotubes give the bass backwaves a longer path to interact with the front waves. I haven't empirically tested them, but I can tell there is more bass extension than before.

I'm also running them from a high-end AVR, and it's internal DSP. The jury's out as to whether this will be a permanent arrangement, but the bass management and EQ seem to have been positive so far.
 
OhioEric said:
updates, anyone? or are we taking the summer off?:smoke:
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I stuck some Acoustic Elegance IB15's into some H-frames that were once occupied by Alpha 15a's. Certainly an improvement in most every area, most notably sheer output capability. The IB15's probably weigh 3x as much as the Alphas, so I think my H-frame "enclosures" need to be bolstered and the magnets supported to optimize the performance of the big bad driver.
 
I'm still in a holding pattern. I got so frustrated with my electronic issues I decided to take a break. Eventually I get things sorted out. Luckily there is no shortage of nice speakers around here for me to listen too.

cubdog
 
tomlinmgt said:
I stuck some Acoustic Elegance IB15's into some H-frames that were once occupied by Alpha 15a's. Certainly an improvement in most every area, most notably sheer output capability. The IB15's probably weigh 3x as much as the Alphas, so I think my H-frame "enclosures" need to be bolstered and the magnets supported to optimize the performance of the big bad driver.
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No doubt. They would be the single biggest upgrade that someone into OBs can make, IMO.
 
Poultrygeist said:
Kinda shows you aren't into OB's
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Yes, clearly. Why would any open baffle fan be worried about lower frequency extension, lower distortion more output and subjective reproduction that is almost unanimously preferred. No REAL OB fan would seek those attributes. You've figured me out, PG.
 
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