I finally had to do something about my lack of enthusiasm for the new 8 driver manifold. I had managed to match the outputs of both old and new sets of drivers despite their differences. However I had neutered the newer drivers ability to play low and loud. The result was amazingly fast but lacked the tactile effects through the floor and air. The weight (or authority) which I had become so attached to since building my first IB was absent. There were no more asteroids in collision. No jumbling icebergs of infrasonics underpinning my favourite organ music pieces.
I had plenty of time to think about the problem subconsciously while I became engrossed in another project completely unrelated to audio. We had watched a couple of films in the interim but I had hardly listened to any music to speak of. It was the bottom end which was missing of course. It suddenly struck me that I had no need to protect the drivers since I had lots of experience using them as individual sets. I had used +16dB boost at 20Hz on the old drivers to get anything out of them below 30hz. That was the price of having over-stiff suspensions and an Fs of 32Hz. Why shouldn't I boost them again? The newer drivers with their 13Hz Fs could take care of themselves but had rather weak shoulders in comparison with the more muscular, older drivers. As individual sets both lots of drivers had seemed almost (LFE) bombproof. Together they could rule the world!
A spare hour or two presented themselves and I went to work with the BFD. Raising the older drivers back to +16dB boost @ 20Hz was the starting point. I wore the carpet out for over an hour running backwards and forwards between my computer and the BFD in the rack. A ten inch high step and sharp right angle turn intervenes in every journey but I survived without mishap. Finally I had two sets of drivers with closely matching response curves except for the extreme bottom end. The combined curve didn't look too bad either and can be smoothed further if necessary. A quick check suggested that the speakers want to be out of phase with the subwoofer and I needed to raise the crossover frequency a little to get the combined response shoulder to match the nominal REW roll off curve with the main speakers playing.
Close enough? In case there is any doubt the overlapping (lower) curves represent the two different sets of drivers measured at the listening position. I no longer have the previous rising ramp into the infrasonics but that may not be a bad thing. The high energy area between 25 and 60Hz is well to the fore for real impact on films. The sub-20hz infrasonics are not too shy of the rest of the curve so should be able to get the floor moving on films. Organ already sounds much more interesting. The weight is back even on the most fleeting of pedal touches. The timbre of the great pipes has returned with a vengeance. Time will tell whether I need to fine tune what I have obtained so far.
Interestingly, I have gained considerably in output. Before I rolled back the IB's gain on the Behringer crossover by -6dB I was almost 15dB above average speaker level with flat settings on the active crossover! I believe low crossover points rob the IB of a great deal of output. The balance sounds about right at the moment on organ at fairly modest levels. I have now backed off the EP2500 controls by 6dB each to tame the IB on music. On films I shall watch carefully for clipping and excessive cone excursion as I gently raise the subwoofer output to produce suitably "exciting" levels. I seriously doubt it will need much more than present levels. :-)
Next day: Well I put 4dB back on the IB at the crossover and enjoyed the Matrix Trilogy on DVDs back to back. There were some nice moments where the floor and even the room seemed to pulse silently but I doubt I was even matching the performance of my old SVS 16-46 cylinder. In fact I hit only 105dB(C) max hold on the Galaxy SPL meter when I turned it on out of curiosity towards the end of Revolutions. Only the newer drivers showed any excursion (at all) and then only about 1/2" total movement. (12mm). There is obviously lots more room to run the IB hotter for action films. The EP2500 controls still have plenty of reserve with the CX2310 level controls now set flat.
Given the massive 20Hz boost on the older drivers they really ought to be trying harder by now. The obvious thing would be to remove them and fit closer-matching, newer drivers. That just isn't going to happen. Not unless I win the lottery. So it's really a matter of maximising what I have already.
It might be thought that I am not really enjoying my present IB set-up. Then you haven't felt the incredible vibrations when I played some organ music without turning down the EP2500 controls.
I am the victim of my own ambition to use the spare drivers. Nobody made me do it. The 8 x 15" is better than 4 x 15" of either vintage. 8 x 15" has the speed and effortlessly brutality which 4 x 15" simply cannot manage.
At my first attempt I ran into trouble while trying to match the very different sets by strangling the drivers which really could play low and loud without the help of the BFD. Lowering the crossover point was also bad news because it robbed the drivers of their best bandwidth. Just because the curves match closely does not mean that the drivers are physically matched at different frequencies. They still want to do their own thing. The surprise is that they do it so well together despite being in a grossly undersized enclosure! I have lost at least 5hz of infrasonic extension by insisting on using all 8 drivers. IB drivers aren't supposed to be used in undersized boxes because they don't have the motor power to overcome self-induced back pressure. I broke almost all of the IB rules and had to work to rescue the situation. I'm still working at it when the mood takes me. I'm still learning (slowly). Most people would be happy with four drivers. Most would be very happy indeed with my grossly mismatched 2 x 4 driver IB. If it ain't broke don't mend it. My 4 x 15" was a superb subwoofer.
Why would anyone be daft enough to copy my example? Anybody sensible would use eight identical drivers and reap the rewards straight out of the box. Low Fs drivers would ensure that lower bass is truly prodigious without any boost. Cone excursion would remain modest as all drivers summed their perfectly matching responses at all frequencies. The enclosure should match the Total Vas x 10 minimum requirements. Thereby ensuring the drivers enjoy a suitable environment for best performance without raising their natural resonant frequency with unwanted back pressure. So no early roll-off. The walls of the enclosure would not be subject to flexure because the pressure would remain low at all frequencies and at all output levels. So no pumping losses either.
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