The imbalance between the two sets of drivers had bothered me. So having overcome my tunnel vision regarding bonding the channels of the BFD I set to work to match the two sets of very different drivers.
The results of half an hour wearing out the carpet between the computer and the BFD resulted in the following graphs:
1/3 octave smoothing applied. I have boosted the input to the newer drivers to lift the curve bodily to better match the older drivers. The newer drivers have some cut at 20hz and the older driver are boosted at 20Hz. From long experience I know the older drivers can take + 16dB boost at 20Hz using 120 B/W.
The idea was also to flatten the overall curve with less emphasis on the infrasonic as well as ensuring both sets were working equally hard at all frequencies. This would hopefully increase dynamic range and lower distortion from the doubling of the active cone area. To have maintained the frequency response imbalance would have meant a lot of the potential gain could be have been lost. Only a single set of drivers would be contributing in full at any particular moment. The downside of employing two sets of very different drivers has hopefully now been minimised. Of course I cannot overcome the impedance imbalance between the sets but the 2dB boost to the newer drivers will help to overcome their differences.
I went out into the enclosure and touched the driver surrounds lightly while some "heavy" organ music was playing. Both sets now seem to be working together. I was unable to differentiate between the vibrations of each set on loud organ bass. Cone excursion on organ bass is so low that only the surrounds registered any sense of cone vibration. None at all was visible. Bass levels out in the enclosure seemed very low indeed compared with the room itself.
Then I put on Bass Outlaw's Extreme Woofer Test/Stereo Bass/Track 12 from the Illegal Bass CD to confirm my previous findings on organ music.
http://www.youtube.com/watch?v=WJTDLxdfY7o
Unfortunately this YouTube video is not a good example as it plays at rather a low level and has a number of HF artefacts as well as seeming to lack the bottom end power of the original.
I was averaging only 98dB(C)-Slow on the galaxy SPL meter inside the enclosure but peaking around 118dB(C)-(Max hold) at the manifold mouth. Zero visible cone movement at these levels but some in-room rattles around the manifold were a bit annoying. The decorative plywood cladding really needs to be bonded to the new 3/4" plywood backing. Or replaced with yet more thick plywood! Even 95dB(C) while sitting at the computer, probably 12 feet away, is on the verge of being unpleasant to experience with this track. I have never found another track like it for massive low frequency content. A complete absence of inane vocals or screeching high frequencies just adds to the fun.
BTW: For the curious: 8 x 15" drivers is roughly equivalent to a single 36" cone but without serious cone flexure and all the other annoying little problems of housing such a huge thing. Not to mention resisting the vicious reaction loads of single, large driver. Interstingly 4 x 18" drivers have a similar cone area to 8 x 15". Though displacement may be different depending on the Xmax of the individual drivers.
My IB's displacement is now 22.6 litres (4 x 2.65) + (4 x 3) litres = 10.6 + 12 litres. Hardly worthy of dicussion when cone excursion is now limited to invisibility. The vital thing is the huge cone area. Not how far the individual cones move in an emergency before they all hit the end stops! I have the immediacy of a large electrostatic panel moving a fraction of a millimetre with the potential stroke of a huge dynamic driver to ensure massive infrasonic bass levels when called for.
Power per driver has halved despite each set being wired in series-parallel to keep total impedances an easy load for the EP2500.
The 4 Ohms set 650w/4 = 162.5w per driver.
The 8 Ohms set 450w/4 = 112.5w per driver.
With the previous manifold housing 4 x 8 Ohm drivers in parallel pairs per channel they were seeing 325 watts each. This may explain why I feel the dynamic range has dropped on films. The IB will go very loud indeed on sinewaves but seems to have lost some SPL impact on films. Despite having such a huge subwoofer in the side wall there is still absolutely no sense of the bass being lop-sided. The speakers do all the steering of LF Effects in the soundfield. I am quite tempted to rewire all the newer 8 Ohm drivers in parallel for a 2 Ohms load on one channel. The newer 8 Ohm drivers need a little more input to match the older 4 Ohm drivers. This could be the answer to providing more dynamic range since I can't safely bridge the amp across two sets of very different drivers. It would be very difficult to predict how each driver type would react to being driven together in the same circuit.
Okay, the newer driver set has been wired in parallel for 2 Ohms load. I'm seeing a considerable gain but have backed off the EP2500 control on that channel to match perceived cone excursion. There is a new, effortless quality about the bass on the car-audio tracks.
The odd thing is the massive difference between my SPL readings out in the enclosure compared with the AV room. The difference is now averaging around 10dB(C) @ 1 metre from the manifold! I have repeated the exercise on various tracks and the 10dB holds true on all these bassy tracks. It hardly feels as if the manifold is working out there with the door closed. When I come back into the room and close the door again I immediately gain 10dB on the SPL meter. In the room the bass is thundering and pounding at 95dB(C)+. Out in the enclosure it's a complete non-event. This can only be the effect of reducing the Total Vas multiplier. The amplifier seems quite happy. The orange lights are flickering on and off with around 4 bars showing on the BFD on bass hits. No sign of clipping at all. This is great fun!
I'll have to check relative levels with REW but I'm enjoying the sheer energy in the music too much for testing. :-)
Satriani's "Engines of Creation" CD went down well at a steady 95-100dB(C) on the meter. Now I'm back to organ music with Bowyer's "For Weddings" on the organ at Chichester Cathedral for a bit of a rest. The end of the Vidor Toccata: Track 23 from the 5th Symphony was magnificent at only 85dB(C). By comparison the Boêllmann Toccata from Suite Gothique by Bertalan Hock on "Wedding Music" almost ponderously superb. I'm a complete sucker for quiet organ pieces underpinned by deep bass. It would take a real Philistine to screw up the volume but I can always pretend I am profoundly deaf. Besides, I alone am listening.
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