Besides, it was long overdue to plank over the sloping ceilings of the IB enclosure. Which was once a gable end balcony but I had glazed over the open end to close it off from the prevailing winds and weather. It had become a handy storage area disguised by curtains over the antique French windows.
I removed all eight drivers using a rechargeable drill with hex bit in the chuck. Making sure I safely collected up the 64 furniture screws and T-nuts into a tray. You really don't want stray T-nuts getting inside the ventilation slots of the drivers. Then I laid the "coffin" on its side while I "sawed the lady in half" with an electric jigsaw. This went effortlessly apart from the physical exertion of lifting and turning the beast. With two matching halves to choose from I decided to use the top half for a four x 15" manifold. This saved me having to make a new, cosmetically acceptable top.
Previously the box had been so tall that it hit the sloping ceiling out in the IB chamber. While still being well below the 10" high, stage floor level in the listening room. I made some 4"x4" timber feet to lift the box up to its new level and sealed the remaining hole left in the partition wall above the now-shrunken manifold.
I won't dwell here on the years of struggling to match the two very different sets of four drivers. The 13Hz newer drivers, with flared paper cones, drooped very rapidly with rising frequency. While the 32Hz conical plastic cones maintained a more level output until they were rolled off electronically.
As seen in the image of raw responses at the listening position. I have matched the curves for level at the lower frequency [room induced] peak. The newer drivers roll steadily downhill. While the older drivers hump at 28Hz before commencing a much gentler roll off with rising frequency.
So in the end the complimented each other nicely. Eventually I had made them matched and obtained satisfactory sound quality and quantity. But always with that niggling lack of gut-bashing impact. The bass drum and bass guitar didn't so much kick me with a jackboot as wallop me over the head with a big, soft pillow.
The matched responses, using quite a number of BFD filters, can be seen in this image. Even so the curves of each set of drivers rolls off quite differently at the top end.
The newer, 4 Ohm drivers were soon rewired in series parallel. Then I could start my first listening test with Tzar of Instruments. Which is an excellent, well recorded CD of Russian organ music. I immediately noticed the lack of clear timbre to the great pipes. With all eight 15" drivers the tonal quality of each and every pipe is effortlessly laid bare. With only four drivers you really have to concentrate to hear the leading edges, tone and subtle detail. This has nothing whatever to do with volume and everything to do with very low distortion. Distortion masks the wonderful clarity and realism of the well recorded, pedal organ. All you get is loud, prolonged sine waves which [sort-of] stop and start at random. Without all the starting chuffs, unique tonal identity, variable sustain and realistic decay.
The output from the four, newer, AEIB15s was very obviously diminished compared with the previous eight drivers. Hardly surprising without the aid of the older drivers with their much flatter response. So I wound up the control on that channel of the Behringer EP2500. Then added a few dB on the calibration menu, test tones on the Onkyo TX-NR818 using the Galaxy 140 SPL meter set to C-weighting. There will be no REW bloopers at this stage. Meanwhile I reset the single channel of the DSP-1124 Feedback Destroyer to no filters.
The bass was now better balanced [for level] but softer and more rounded. With even less impact and the grunge now missing. "Grunge" just means the hard edge of LFE bass sounds and the effect it has on the timbre. The quality of the sound, if you like. The difference, if you will, between plucking a bass at the bridge or somewhere in the middle of the string. This all confirmed what I had noticed when I had first added the four older AE 15" drivers in one huge manifold. With the doubling of the number of drivers from the original four x 15" the IB had gone from nauseatingly powerful and impressive to effortlessly violent. All thanks to its eight huge cylinders of potential displacement brought to bear in moving lots of air.
It's all about cone acreage when you want really low distortion, dynamic bass. There is no substitute for cone area IME. Displacement achieved by single, long throw drivers is merely insurance against bottoming out. Using many more bigger drivers adds vital cone area. The cones then have to move only a millimetre, or two, to achieve deafening levels. Most of the time you see no cone movement at all. With "only" four 15" drivers cone movement becomes visible again. Every animal and object which makes loud and deep bass sounds is very large. So why try to reproduce those same sounds with a single cone? A cat is not an elephant. A violin is not a double bass. A striped bass is not a whale.
I've tried making a number of OBs using the older, plastic-coned, AEIB15s. Usually with rather mixed results, though I haven't quite given up yet. Perhaps they just aren't suitable for the job with their 32Hz free air resonance but I intend to place the bottom half of the original 6' eight driver manifold over on the other side of the stage from the IB. I will use the manifold box inverted with the four older 15" drivers in an open-backed box. Opposing the drivers in the typical IB manifold form will kill any unwanted vibrations.
An early image of my four x 15" IB with the older AEIB15 drivers. Virtually the same view when I look into the latest 4 x 15" open baffle from the wall-facing, open end. The minimum width of the box is set by the depth of the drivers when mounted externally. One pair could be reversed to reduce even-order distortion at the risk of exposing the undesirable 3rd harmonic.
There will be just enough room for the big box below the sloping attic ceiling right behind the right Main speaker. The four x 15" OB will be driven by the other channel of the EP2500. Though I shan't be boosting by +16dB at 20Hz using the DSP1124 as was so desperately necessary in the IB. That was the only way to get the older AEIB15 drivers to give any low bass, at all!
Upper bass augmentation and impact is all I ask from the new IB/OB set-up. That may require some tailoring with the BFD and roll-off point. And, if all of this messing about doesn't work, then I shall have to think of something else. I have had my eye on the possibility of using the entire stage as a large open baffle. With the drivers arranged in a row right at the back of the stage above a slot through to the room below. This will provide a 5' wide baffle x 10' long. The problem is being allowed to replace the old ceiling downstairs with new boards. A vitally necessary but very messy job given the age of our old home.
I might even hide the OB manifold inside my computer desk in its familiar dormer window. The underside of my glass and steel-framed desk is all fresh air. It could easily swallow the manifold provided I remove the desk's bottom shelf. That should bring new energy to my usually anaemic, computer speakers. I just hope all those big magnets don't affect my computer!