Having failed to produce a pleasing response from OBs running off the Sub-out socket of the CX2310 I need to think hard about my remaining options. I still like the idea of a vertical array of four 15" drivers on the right to balance the IB on the left.
There is no realistic way to build another True IB on the right of the stage which would completely avoid massive heat loss in our cold climate. This is a compromise I dare not take merely for audio satisfaction.
Instead of using the Sub-out socket on the CX I could drive the OB straight from the right channel of the EP2500. This means I would still retain the completely independent control of the CX and BFD to smooth out any anomalies on that channel. I would also enjoy the stereo information in the sub-80Hz frequency band. And, I might quite possibly be able to push the crossover point higher for more impact. Using the huge cone area of four 15" drivers I would move a lot more air than the four dinky little, 4" drivers in the Mission floor standing speakers. If it should prove possible to raise the crossover frequency sufficiently I could even imagine the drivers from these speakers becoming OBs. There would no longer be any real need for serious bass output so the boxes would become redundant.
Those of you who haven't followed every detail of my set-up, so far, should be reminded that I enjoy stereo bass from both music and films. The LFE socket in my old Yamaha 5.1 surround processor/3.1 amp remains unconnected. I use "All bass to mains" on the DSP E800 for film watching. For music the Yamaha remains switched off and the CX2310 feeds a stereo signal to the separate sets of four 15" IB drivers via the BFD. The Yamaha supplies the Centre and Rear channels on film and provides full range, line level, Main speaker channels to be split at ~80Hz by the CX2310. The Mission 753F floor standers act as the Main speakers and the IB provides sub-80hz output.
Admittedly stereo has little real meaning in this context. The IB has two sets of four drivers. Each set being fed by their own channel but the drivers are all arranged above each other in a six feet tall, vertical box. There is no lateral separation (or information) to build a stereo soundfield across the stage area. The main speakers do all the steering of the bass imagery and do it very well.
Which is where a tall array comes in. Placed over on the right of the stage it could provide the missing sub-80Hz information in the right speaker channel. Subjectively, there is absolutely no sense of lop-sidedness in the bass with the present set up.
Whether a four x 15" vertical array needs the full power of one channel of the EP2500 is another matter. Given that these drivers are presently enjoying such power in an IB situation I doubt there is very much to worry about. The lack of back pressure with an OB is hardly different from that of an IB. I had in mind to engage the 25hz high pass filter on the CX2310 anyway to reduce cone excursion and reaction forces on this channel.
Unlike my first (IB) vertical array, ensconced in the flimsy IB baffle wall, an array to the right of the stage cold easily be stiffened with bracing. Either free standing or tied back to the rafters behind the baffle.
A tray form, with deep horizontal and vertical ribs, should help to stiffen the baffle sufficiently to be free standing if held only at the top. Made as compact as possible the baffle should sneak under 6 feet in hight. In fact I could get away with a vertical height of 68" resting on the floor. A perfect height for the situation without protruding into the dormer window area. Or affecting the position of the Right, Main/stereo speaker.
This height would still allow an inch thick rib between each pair of drivers to stiffen the baffle. I thought I'd use 2" x 4"s at each edge to stiffen the long dimension and 1" x 4"s between each driver. Another cross rib at top and bottom will help to keep it in shape and spread the loads. Fortunately a baffle with 15" drivers installed wants to lean backwards with a vengeance. So there is probably no need for a strong fixing at the top. The baffle can just lean back against the ceiling with two rafters perfectly placed at 15" centres. There is no way for the baffle to slide on the carpeted floor with all that weight almost vertically above the footprint. Fortunately there's a floor joist centred on the baffle to stop it going straight through the stage floor. Or so I hope! 8-| Folding the baffle might make it more compact but would probably introduce colouration.
As stated earlier, the removal of four drivers from the IB would return the enclosure to a far more satisfactory Vas multiplier of about 9. The newer drivers, with their much lower Fs, can easily manage the very deepest bass in the left channel without any extra help. The older, vinyl coned, 32hz drivers would do what they do best. More muscular upper bass but naturally rolling off much earlier.
Moreover, the present deep troughs at 160Hz and 320Hz in the frequency response should vanish. I believe these troughs are distance-dependent on the wide separation of the IB and the right speaker. There is a great deal of the impact on music and film in this area of the audio frequency response which I feel I am missing. I doubt that any imbalance between channels will be noticeable if IB and OB each do their own thing within reason. Provided, of course, that their respective crossover frequencies remain modestly low.
Tucked under the sloping ceiling and partially obscured by the right main speaker, I hope the array will melt into the background. A hanging curtain of dark red cloth, similar to the background material, can easily hide the array if it should really jar the senses. Or, I could simply cover the baffle in the same red cloth to lower the visual impact while leaving the flush-mounted drivers exposed. Yet clothed to their outside rims by pressing the cloth neatly into the driver cut-outs as I carefully mount drivers.
Having accidentally left a rough drawing of the baffle on my desk my wife was adamant it (the OB vertical array) wasn't going to happen! Her immediate fear was that I was going to cut holes in the ceiling or walls. A natural reaction given my past history with IB installations. A strong denial that I was never going to compromise our vital insulation helped to defuse an awkward situation.
First I have to cut the driver holes and attach the array baffle stiffeners. Then cut the present IB manifold in half, raise it by four inches and move the eight drivers. Then close off the large hole in the baffle wall. All, while the outside temperature remains well below freezing and it's blowing a gale outside. I have also run out of suitably large, clean pieces of 3/4" plywood. Ideally I should use two thickness of 3/4", glued and screwed together. The front layer can allow the drivers to be sunk-in flush. The back layer providing support for the driver baskets when clamped through their rubber gaskets.
What I will never forget is lifting my original IB array to the vertical with the drivers in place. I nearly lost the lot and it taught me an important lesson. A baffle without the support of deep box sides is a terrifyingly flimsy, wooden net surrounding very large holes. It has no more strength than the thinnest section at the edges of the drivers. The massively heavy panel which I carefully cut, glued and screwed together from a whole sheet of premium quality, birch-faced Baltic ply was really as limp as a wet blanket. A worthless, cosmetic decoration around the drivers. Nothing more.
Adding cross braces is much less useful in comparison with fixing very deep sides to the baffle panel. No doubt the driver baskets supply some of their own cross bracing though it is doubtful it amounts to much if the spacing between the drivers is rather small. It's back to the skinny, laminated wooden net surrounding big holes again! The baskets cannot provide any stiffness (at all) along the length of the baffle. It will try to fold at the hinge line across each driver and possible between them if their spacing is small.
The Achilles heel of an open baffle with large drivers are the thin sections beside each driver where the holes meet the sides of the baffle. The H-section baffle not only provides a greater acoustic path length but can greatly stiffen the baffle along its length. Provided, of course, that the side ribs of the I-beam cannot flex inwards or outwards. A baffle with rear ribs will provide only half as much stiffness as an I-beam. Or H-section as it is more commonly known in OB terms.
If one has ever worked with thin metal or plastic I-beams it is easy to see how flexure in the edge flanges produces a very limp beam. It has no real stiffness at all and will happily twist along the axis of the beam itself. This should be held firmly in mind if one is tempted to thin the side ribs too much. While the depth of the side ribs is the vitally important dimension their thickness must not be compromised for appearance or material saving. A flat panel with thin side plates can easily be twisted or flexed with the bare hands once the driver holes are cut out. Where the panel is narrow you could be trying to flex plywood with a local cross-section as little as 1" x 1.5" for a doubled 3/4" plywood baffle. Imagine if you had built your baffle out of MDF! Its strength would be almost nil at the critical point each side of the drivers without very deep side plates to reinforce these flimsy sections.
The ideal reinforcement, in addition to side plates, or rear ribs running the length of the baffle, would be a bow (or bows) of considerable depth and width to avoid flexure baffle flexure. The bow (or bows) would still need to be tied into the middle of the baffle between the drivers. Or even pressing against all the driver magnets in a tall array to stop the entire structure from bending under its own weight. Or flexing to the considerable reaction forces incurred with large drivers when playing low frequencies. Where a single bow is involved one should avoid blocking the driver pole, ventilation holes of course. As a large bow is a heavy user of expensive sheet materials it is probably simpler to brace back to the rear wall where a free standing structure is not desired.
Here's a crazy idea for a compact, small footprint, vertically stacked, four 15" driver, OB subwoofer. The black sections are half cylinders. I imagine lengths of studding with nuts and washers to rigidly support 3/4" plywood, 18" diameter, round baffles. The half cylinders would be wrapped around between pairs of baffles to increase the baffle area (and acoustic path length) and to provide a bipolar radiation pattern equivalent to a vertical OB array. All drivers would move the same way. No rocking motion like a tall array. No loss of cone movement just to accelerate the baffle(s). Better than stacked, naked drivers? Has it been done before? Who knows? :-)
Here's a safer/ more sensible arrangement of four 15" drivers. Reversed relative to each other to reduce distortion. Some reduction of reaction forces by placing them at right angles to each other. Low enough to fit under my sloping ceiling. Not too deep so they will fit partially behind the right speaker. Two side boards will support an extended "W" of smaller baffles. I had to re-register under another name to get back into the Hawthorne Audio forum to catch up on the latest building methods for open baffles. The galleries are well worth a visit for those who drool over superb woodworking. Partial sand filling and foam sealed, detached sub-baffles are the order of the day it seems. The sand provides mass resistance against driver vibration and the sub baffles press against stiff, foam strip for isolation.
I built some sand-filled and concrete baffles and lids in my misspent youth for corner reflex enclosures. Sand filling was more practical because concrete had to be cast flat and then a hideously heavy panel had to be carefully raised to the vertical. Sand could be filled in place and then the panel finally closed with the top batten. I can still remember drying the damp sand in baking trays on the kitchen cooker while I stirred gently with a stick. Concrete was far superior for deadness of the finished panel. The real fanatics built their corner baffles from house or engineering bricks with a heavy, cast concrete lid! Not a great idea for an attic listening room though.
A trial mock-up of a folded, W-form, four x 15" open baffle subwoofer. I haven't decided whether to go with full sideboards yet. The structure is just clamped together to get a sense of scale. (about 52" high x 14" deep x 21-22" wide) I didn't have time to make a new bottom baffle. This is actually the view from the back since I don't want a driver firing straight at the 45 degree sloping ceiling from close range. I'll saw out a proper, 3/4" plywood, bottom baffle tomorrow and buy a new length of batten for corner supports. I didn't have enough matching batten to do a proper job.
In the end I used 3/4" plywood triangles for stiffening instead of battens. It's all just screwed together without glue. It's rough but nothing more is needed for REW and listening tests. If it's a total disaster it hasn't cost anything but a couple of hours. If side board prove necessary they will hide a lot of the untidy workmanship with a bit of filler, some sanding and a coat of black paint.
I may use thick, hardwood, kitchen working surface board for new baffles if the listening tests go well. The added weight of four 15" AEIB15s will require a really solid structure even ignoring the reaction forces involved. Sideboards would be stronger than battens allowing much larger gluing and screwing surfaces. They would also increase the baffle area by extending the cancellation path.
Well, my OB listening tests were a complete disaster. I couldn't get any real volume out of the drivers and bass was completely absent! When I turned the pre-amp up very high the sound became all jangly and boxy. Probably from cone resonances. I got fed up after a few hours of trying everything imaginable and put the four drivers back in the IB manifold. Even then they wouldn't play properly! I unplugged everything in the CX and BFD but all the plugs and settings were exactly as before. I had previously Dymo labelled every plug for speedy reconnection. I checked the DC resistance of every driver singly and in series-parallel. Everything seemed fine.
I was suspecting that the EP2500 had gone quiet on one channel. So I swapped the IB cables and found both channels were fine on the other driver set. While I had the cables off the rear terminals of the Behringer I checked driver polarity with a 9 volt battery. Again it all seemed fine and all cones jumped the same way. I checked for stray strands on every driver terminal. I checked the push switches front and back on the CX and BFD again. Then suddenly the sound came back at full power when I reconnected the main cables to the EP2500.
A complete mystery. My hours playing with the OB had been wasted but (worse) I am none the wiser as to what went wrong. The drivers are firmly fixed back in the manifold and rewired and the OB woodwork has been carried back to the workshop. The structure flexes a little so could do with side boards or a pair of battens to stiffen it up a bit before I try again. I bought some heavy, planed softwood battens but they aren't quite long enough to cut in half to do the job properly. So it's back to the shops for more. It was the only length the DIY outlet had in that size and quality.
What is more irritating is that I hired 2012 and The Hurt Locker overnight while I had only four drivers running in the IB. The difference between four and eight 15"s is truly remarkable no matter how I turn up the gain on the four 15"s to compensate. Neither film will be making it into to my modest collection but I'm sure we missed a lot of the LFE fun.
2012 will probably go down in history as the most bizarrely overdone special effects film of all time. We just sat there laughing loudly as the world ended. When (surely?) we should really have been shocked and afraid. Just because one can make anything appear real on the screen does not mean that one should! Haven't they heard of the suspension of disbelief? They should also have a person with a scientific background on the film staff to check for the infant grade scale of science and common sense failure on dialogue and script! Just because you want something to be really, really, really true does not make it remotely so! They spend literally tens of millions making a visually believable film then throw it all away with such obviously, childish mistakes. You (really) couldn't make it up! :-))
I had another try at the 4 x 15" OB in the plywood structure. I checked the driver polarity with a 9 Volt battery and the combined (series-parallel) DC resistance at 3.4 Ohms.
Here are the relevant REW traces as I played the 4 x 15" OB alone and with speakers. Then 4IB + 4OB +/- speakers. I also tried the IB and OB together in phase and out of phase with each other. It still took "full volume" on the EP2500 to match the OB with the IB. The IB was still much more impressive in both depth, power, SQ and attack. The trough at 150Hz+ was not really filled in by both systems as I had hoped. No noticeable difference subjectively on rock music whether the OB was playing or not. Though REW told me they produced a nice flat response together. Another failed experiment. So I returned the four (older) drivers to the IB manifold yet again. Which produced an immediately noticeable improvement in both depth and SQ.
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As an aside, I videoed my IB today while it was playing some organ music. I could not believe nor recognise the awful colouration and dreadful SQ of the sound coming from my speakers on replay of the video. What prompted this test was that I had been watching quite a number of videos of audio systems on YouTube. I was surprised how poor many of these systems sounded when recorded by camera microphones. I'm not talking about the silly overdrive distortion on foolishly loud subwoofer bass here. Just the SQ of the speakers themselves in the room where they play. I've listened to lots of YT audio-related videos through my own system and with headphones and with my cheap computer speakers plus SVS cylinder. Almost universally the audio systems on these videos sound absolutely dire regardless of the system's cost or size.
It occurs to me that one could repeatedly record and then feed the last recording back through a particular speaker or subwoofer system to analyse colouration or any other fault. Each iteration would further emphasise weakness in the speaker system/room.
It should be possible to do the same with pre-amps and processors by repeatedly feeding the last recording back through the audio component and recording again at line level. Then repeating the cycle for as long as one's patience lasts. Or the SQ becomes too poor to be worth continuing. Monitoring progress could be done with ear phones.
I'm not sure how this could be done with audio amplifiers because the speakers would be the dominant weakness in the chain even if one used an anechoic chamber. Introducing a transformer to reduce the signal to line level for recording the output of each new pass might be possible. Though the load/transformer circuit could introduce its own sound to swamp any weaknesses in the audio amplifier itself.
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