Z, those speakers look awesome. A build thread would be a great read 
General question on OB stuff... how much free space does this type of speaker really need? Only a couple of weeks off building a set based on the Eminence Alpha 15A experimental OB design by M J King... these will be in my main room which is 3.5m [W] x 6m [L]. should be able to have them sat roughly 0.8m from the rear wall.
Got the amps now and will probably order the Alpha's this weekend, however, the Vifa's still haven't been shipped... Also have a pair of MA A7.3 due, wondering how these might do OB?!
Been reading about Linkwitz's LX521 OB speakers... could possibly live with those long term, but maybe a few to many drivers used in the mid to upper range.
General question on OB stuff... how much free space does this type of speaker really need? Only a couple of weeks off building a set based on the Eminence Alpha 15A experimental OB design by M J King... these will be in my main room which is 3.5m [W] x 6m [L]. should be able to have them sat roughly 0.8m from the rear wall.
Got the amps now and will probably order the Alpha's this weekend, however, the Vifa's still haven't been shipped... Also have a pair of MA A7.3 due, wondering how these might do OB?!
Been reading about Linkwitz's LX521 OB speakers... could possibly live with those long term, but maybe a few to many drivers used in the mid to upper range.
+1 on that.
Regarding space behind the OB: my experience has been that it likes more space than less. 0.8m can probably work but I think it may sound better about 1.2m out. I would love to own a pair of the LX521's, now with a 2x8 miniDSP one can make that rather simple without having to build the analog processor. I agree that a 3-way is more complicated but you can't beat them for complex high dynamic range music like a symphony. The Alpha 15A's sound like the ideal OB woofers - you can't go wrong. I will continue down the road of the slot loaded OB woofer though as I like the slim baffle it provides. My cardboard test case, and now seconded by Z's build gives me confidence to continue down this path. I just have to wait until I have a real workshop to cut some wood.
Having a work shop would be nice, or even just a big shed... I do just about all the wood working in my apartments hallway. I don't mind it getting messy, plus its the easiest area to clean afterwards.
Might have to move things around a little, but should be able to give the OB's 1~1.2m... Decided on gloss red colour
Seeing how well the multiple bass units perform, plus the smaller baffle sizing etc. it does make the slot-loaded OB design quite appealing. Falcon Acoustic here in the UK do the full LX521 speaker kit... with the plan-set & wood costs, it shouldn't be much over £1000 to build a pair
Might have to move things around a little, but should be able to give the OB's 1~1.2m... Decided on gloss red colour
Seeing how well the multiple bass units perform, plus the smaller baffle sizing etc. it does make the slot-loaded OB design quite appealing. Falcon Acoustic here in the UK do the full LX521 speaker kit... with the plan-set & wood costs, it shouldn't be much over £1000 to build a pair
Great minds think alike...
Thanks for all of the kind words. I have been working diligently on OB designs and this is my 4th build (not to mention countless test baffles). Whenever I think about publishing, I get too involved in the next project to properly document things. But I won't be doing any serious work until the spring when I hope to start on a constrained layer damped, perforated sheet metal baffle.
I hope to post a thread on the design and the construction process this weekend. But a few quick answers:
The cutouts in the baffle are an idea I have experimented with from the beginning. It's to keep the directivity like a true dipole as the driver gets smaller without having to produce the ever narrowing baffle that you see in Linkwitz's and John K's designs. This allows you to mount the HF drivers under MF drivers if you like, and also makes the speaker look like a panel, which I prefer. (Don't know if anyone noticed it, but the groove around the back of the baffle was cut for screen bead so you can stretch fabric over the front baffle and secure it cleanly without staples or glue, etc.) The cutouts underneath the mids also minimize of the near-feild ground bounce because of the dipole null underneath. (I think this is why the new Linkwitz 521 design has the baffle not connecting with the woofers.)
As for how close you can place dipoles to the walls, I usually listen to these a meter or more from the rear and side walls, but my son sometimes steals them and hauls them up to his room (since he can plug his iPod right in). He has placed them in all sorts of positions and they still sound amazing. I think this is because the directivity is so uniform the overall tonal balance does not shift, even though the precise imaging may suffer from the early reflections. In any case, once you hear OBs it's really hard to go back to "boxes" (which generally sound much improved out from the walls anyway).
FYI- I have heard the Alpha 15A in an OB and it has great bass at moderate levels, but these slot loaded woofers sound much more dynamic with the right EQ, IMHO. I have them EQed flat to 38Hz. They have the ability to move much more air volume (about as much as a 20-inch woofer) and the box, although shallow, reduces the cancellation at low frequencies. In my construction you essentially feel no bass vibration at all, the speaker is dead still because of the woofer force cancellation. I hope I can find the outdoor directivity plots I took, as I remember them they looked like a dipole all the way to the bottom.
Thanks for all of the kind words. I have been working diligently on OB designs and this is my 4th build (not to mention countless test baffles). Whenever I think about publishing, I get too involved in the next project to properly document things. But I won't be doing any serious work until the spring when I hope to start on a constrained layer damped, perforated sheet metal baffle
.I hope to post a thread on the design and the construction process this weekend. But a few quick answers:
The cutouts in the baffle are an idea I have experimented with from the beginning. It's to keep the directivity like a true dipole as the driver gets smaller without having to produce the ever narrowing baffle that you see in Linkwitz's and John K's designs. This allows you to mount the HF drivers under MF drivers if you like, and also makes the speaker look like a panel, which I prefer. (Don't know if anyone noticed it, but the groove around the back of the baffle was cut for screen bead so you can stretch fabric over the front baffle and secure it cleanly without staples or glue, etc.) The cutouts underneath the mids also minimize of the near-feild ground bounce because of the dipole null underneath. (I think this is why the new Linkwitz 521 design has the baffle not connecting with the woofers.)
As for how close you can place dipoles to the walls, I usually listen to these a meter or more from the rear and side walls, but my son sometimes steals them and hauls them up to his room (since he can plug his iPod right in). He has placed them in all sorts of positions and they still sound amazing. I think this is because the directivity is so uniform the overall tonal balance does not shift, even though the precise imaging may suffer from the early reflections. In any case, once you hear OBs it's really hard to go back to "boxes" (which generally sound much improved out from the walls anyway).
FYI- I have heard the Alpha 15A in an OB and it has great bass at moderate levels, but these slot loaded woofers sound much more dynamic with the right EQ, IMHO. I have them EQed flat to 38Hz. They have the ability to move much more air volume (about as much as a 20-inch woofer) and the box, although shallow, reduces the cancellation at low frequencies. In my construction you essentially feel no bass vibration at all, the speaker is dead still because of the woofer force cancellation. I hope I can find the outdoor directivity plots I took, as I remember them they looked like a dipole all the way to the bottom.
Z,
Thanks for the details. Awesome design you have there. Neat idea about the cutout to avoid floor bounce. I saw the groove in the back and was not sure what that was for. I noticed you rear mounted the Vifa's so they are sunken. That may affect the HF. Did you use any modeling or simulation to help with setting the dimensions of the slot OB chamber and slot widths? I used AkAbak and found it was helpful. I agree that EQing the bass flat is the way to go, the slot loaded woofers are very easy to do that with. Looking forward to even more details if you get around to it.
X
Thanks for the details. Awesome design you have there. Neat idea about the cutout to avoid floor bounce. I saw the groove in the back and was not sure what that was for. I noticed you rear mounted the Vifa's so they are sunken. That may affect the HF. Did you use any modeling or simulation to help with setting the dimensions of the slot OB chamber and slot widths? I used AkAbak and found it was helpful. I agree that EQing the bass flat is the way to go, the slot loaded woofers are very easy to do that with. Looking forward to even more details if you get around to it.
X
As promised, posted more details about the CNC cut design in a new post here: http://www.diyaudio.com/forums/mult...ffle-construction-techniques.html#post3848686
slot loaded OB, force cancelation?
Hi there Z and X: Could you explain the force cancelation aspect of this OB system and how are the drivers wired (with drivers positioned facing magnet#1 to cone #2)? Are the drivers bolted to each other with spacers or does the slot chamber structure transmit the frame forces between the drivers? It seems curious that pressure in the slot does not cause vibration in the slot structure. ...regards, Michael
Hi there Z and X: Could you explain the force cancelation aspect of this OB system and how are the drivers wired (with drivers positioned facing magnet#1 to cone #2)? Are the drivers bolted to each other with spacers or does the slot chamber structure transmit the frame forces between the drivers? It seems curious that pressure in the slot does not cause vibration in the slot structure. ...regards, Michael
The drivers are wired so that they move in opposite directions to produce dipole bass. The cones either squeeze or suck the volume behind the slot. Because the motion is diametrically opposed, the net reaction force (Newton's law) is zero for identical masses moving in opposite directions. The drivers are mounted with magnet inside cavity on one side and outside on the other is to cancel hysteresis induced harmonic distortion. There is still pressure on the slot and that can make baffle move due jet forces.
Wiring the slot woofers
The pressure in the slot must indeed cause vibration, but that's low compared to the inertial energy from the cone movement, which is cancelled. Because the slots are a laminated structure of solid MDF (but not in the cardboard implementation) it's like a monolithic block with very thick walls and I can't feel this vibrating at all. I just posted some more on the baffle/slot construction here: http://www.diyaudio.com/forums/multi-way/252593-novel-open-baffle-construction-techniques-2.html
I am not certain about the exact series/parallel wiring scheme I used, but it did end up with 4 Ohms for sure. I think it's wired like this:
The pressure in the slot must indeed cause vibration, but that's low compared to the inertial energy from the cone movement, which is cancelled. Because the slots are a laminated structure of solid MDF (but not in the cardboard implementation) it's like a monolithic block with very thick walls and I can't feel this vibrating at all. I just posted some more on the baffle/slot construction here: http://www.diyaudio.com/forums/multi-way/252593-novel-open-baffle-construction-techniques-2.html
I am not certain about the exact series/parallel wiring scheme I used, but it did end up with 4 Ohms for sure. I think it's wired like this:
Nice explanation of your build in the other thread. I would love to hear how these sound but I think I may have a clue from mine - yours would just be that much better with the excellent damping and no panel resonances/vibrations due to using cardboard.
What did it cost to have the boards CNC'd - it may be a very cost effective alternative to buying a lot of shop tools for some of us who have Solid Works but no tools.
What did it cost to have the boards CNC'd - it may be a very cost effective alternative to buying a lot of shop tools for some of us who have Solid Works but no tools.
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slot loaded OB force cancelation & wiring
Hi there X and Z: Many thanks for the informative replies. In another thread covering slot loaded subs, (from my note book, DJK commented "plenum size, mostly plenum depth controls high frequency response", since this was a ported box he further stated "T/S parameters and box size control bass response"). Could the slot depth be investigated with a computer program? I'm not an AKABK user, so I can not do this calculation. .....regards, Michael
Hi there X and Z: Many thanks for the informative replies. In another thread covering slot loaded subs, (from my note book, DJK commented "plenum size, mostly plenum depth controls high frequency response", since this was a ported box he further stated "T/S parameters and box size control bass response"). Could the slot depth be investigated with a computer program? I'm not an AKABK user, so I can not do this calculation. .....regards, Michael
Michael,
Yes, I use AkAbak to help refine the driver plenum and slot size. However, I find that making it big enough to hold the drivers and make the slot about 20% of the cross sectional area of Sd gets close. The rest you can EQ flat with DSP - which really is how I would recommend you run this. Cannot do it with passive XO - too complicated. Also use drivers with fairly high Qts and large xmax. That will get deeper bass. Tell me what driver you want to use and maybe I can run a sim.
Yes, I use AkAbak to help refine the driver plenum and slot size. However, I find that making it big enough to hold the drivers and make the slot about 20% of the cross sectional area of Sd gets close. The rest you can EQ flat with DSP - which really is how I would recommend you run this. Cannot do it with passive XO - too complicated. Also use drivers with fairly high Qts and large xmax. That will get deeper bass. Tell me what driver you want to use and maybe I can run a sim.
It's last spring, so I don't recall exactly, but I think it was something like: $25 for the CAD/CAM conversion, $75 for the setup and $80 to cut each sheet (including the material). About $260 total the first pair ($160 each for any additional pairs cut at the same time). I could never have come up with the same quality and would have worked for about $2/hr if I had fabricated them myself. (That's what I made as a clerk in a hardware store in 1975
.)If you ask for parts from different thickness sheets, and especially if you cut from both sides (a lot more fixturing and handling) it gets more expensive. If you can stay with a design that cuts everything from one sheet, cut all from one side using a number of tools that all fit in the tool changer, than it's going to be this sort of cost. Then, once the machine is set up- it's slap down a sheet, turn on the vacuum hold-down, let-er-rip and then remove the finished pieces.
UPS ground cost for shipping was an annoying part of the cost, added about $65 more for the pair. MDF is heavy. It would be nice to find a local shop, but I have to say that my experience with AllRout CNC Routing Service was so good that I would like to continue with them. Perhaps I can find something lighter than MDF that has good properties? Thinner Phenolic? PVC sheet?
No, I have not tried the dual Vifa's in a 1.5 config as the tops for the slot-loaded OB. I have been having too much fun listening to my single Vifa DCR speaker. It sounds really good - and I won't even qualify it with that it is only a single 3.5 in driver running full range. There is good bass down to 50 Hz with distortion of only -35dB when listening to usual levels of 75 dB. I may try this on my Nautaloss II speakers though. I don't have a ready supply of inductors so will have to order them. Right now I just made my first hardwired BSC with a 1 mH + 10R + 0.47uF and it sounds really good.
Buildmesomething,
I saw your dual blue TPA3116D2 amp in the class D forum. Pretty nice. I have been thinking of how active biamp can be made easier and cheaper for the masses. What we need is a 1x2 miniDSP that is the tied to a stereo amp but used vertically one channel for bass and one channel for tweeter. This way, the DSP and amp reside at the speaker - they are tiny and take up less room than a 20 mH coil and some caps (probably cost about the same if they can make a $40 1x2 miniDSP. On second thought maybe they should make a 1x4 plugin and then you can have a 4 way active speaker with 2 power amps. Cost would be still same as regular miniDSP. Is there a plugin that does 1x4 XO?
X
I saw your dual blue TPA3116D2 amp in the class D forum. Pretty nice. I have been thinking of how active biamp can be made easier and cheaper for the masses. What we need is a 1x2 miniDSP that is the tied to a stereo amp but used vertically one channel for bass and one channel for tweeter. This way, the DSP and amp reside at the speaker - they are tiny and take up less room than a 20 mH coil and some caps (probably cost about the same if they can make a $40 1x2 miniDSP. On second thought maybe they should make a 1x4 plugin and then you can have a 4 way active speaker with 2 power amps. Cost would be still same as regular miniDSP. Is there a plugin that does 1x4 XO?
X