IB Slimming course.

Some of  the last few chapters of this journey of exploration suggest a disenchantment with the IB8. This is not remotely true but something was definitely missing since I added four more drivers. The missing link to audio heaven was the very deep bass. I always had power aplenty. Clarity and brutal realism to die for. But there was always a shyness in the infrasonics. It was there but masked by the higher frequencies.

It matters not whether an organ is growling at 100dB at 40Hz if it lacks weight. Just sounding deep is never enough. It must be underpinned by subharmonics and infrasonics to sound real. To convey that staggering immensity of a real pipe organ. Without the massive foundations of the extra octave, or two, propping up the fundamental one might as well listen to random sinewaves. Listening to an organ with ordinary floor standing loudspeakers is an exercise in folly. Even large subwoofers, like my SVS 16-46 with its solid 15hz capability, just aren't good enough for the task of rendering a pipe organ as a believable instrument at home. It needs far greater resources than mere domestic subwoofers. Arguably it needs more than a 4 x15" IB if you want true realism. My smaller 4 x15" IBs were wonderful organ impersonators. Only with the addition of four more drivers were the lesser devices exposed as merely clever mimics.

My previous subwoofers had always enjoyed a ski ramp into the dark depths of the infrasonics. The big SVS cylinder more than most. Though the 4 x 15" iterations of my IB were not too far behind. Even with a 16dB BFD boost at 20Hz to my older drivers they were still dragging down the roll off slope. My previous attempts to match the drivers were remarkably successful but still left a hump in the mid bass. Each new attempt to tame the hump brought greater pleasure and satisfaction but I was simply being too gentle. Too afraid to wreck what I already had. I needed to be much more brutal. The problem can be seen in the REW graph below:

The top violet curve is the last effort to reduce the belly of the IB8 and includes the main speakers. The clarity was absolutely stunning. The power truly effortless. It just lacked the very bottom end. The area beyond the inaudible barrier at 20-odd Hz. The mood changing frequencies. The floor shaking stuff. The range which shreds the air and makes one afraid of the unknown. The touchy-feely area of low frequency reproduction.

The red curve is the new combined response of all 8 drivers in their two very different sets. The two lower curves are the matched sets of four AEIB15s. One set of old drivers. One set of new. I cut hard at 50hz by -6-7dB with a broad stroke and had little else to do beyond that. Except to test whether what I had done allowed both sets of drivers to contribute equally. I owe it all to the BFD and REW. Recently rechristened "Room Exercise Wizard" on account of  the miles I have to travel in an hour of REW testing. All the running backwards and forwards between the computer and the BFD. Changing and saving filters is only half of it. I have to switch back and forth between sets of drivers and polarity on the CX2310 at intervals. Then back to the computer to run another test sweep. Probably delete some earlier graphs then back to the BFD to make another small incremental change by a couple of dB or a change of bandwidth.

It is too early to say whether I am getting closer to delirious satisfaction. Messiaen sounds truly wonderful on my favourite track 7 of the Helikon CD. You would not believe the detail and clarity of the gentle breath across the great pipes. As clear and realistic as if it were in the midrange instead of on the verge of inaudibility. The range of deep voices is always a delight but it is shocking how each improvement to the IB exposes detail which went completely unheard before. As I have said before the recording on this CD is state of the art. Nothing else I have heard comes even close on pure sound quality in the very deep bass. One can practically peer down into the big pipes themselves and smell the dust and the old wood and metal. Sadly the music will not appeal to all listeners.

Just imagine what eight, new model AEIB15s would do! With all drivers matched and all playing on the same page the deep stuff would pour straight out of the box and into the manifold. Each would add to the whole in perfect unison to provide gut wrenching power well beyond 10Hz. No silliness with unmatched sets and different impedances and responses so completely divorced from each other. They would thunder unhindered by their weaker, older brethren. Perhaps one day. Meanwhile I can still enjoy Guillou's Franck in all its splendour.

By the way I have also added abbreviated 3/4" plywood shelves in the middle of each set of drivers. The full shelf remains to separate the two different sets acoustically. Also to block imbalances in the pressure waves emanating from each set. (if any) Airtight barriers are probably unnecessary between matching drivers where only bracing is required. So I just sawed one of my unused shelves in half and pinched them tight inside the manifold with the screwed rod braces. (studding) Sadly I broke my own rules and failed to measure any difference before proceeding with lifting the IB8's bottom end by giving it greater exposure.

As soon as the Head Gardener goes off to make dinner I shall have another listen to Messiaen. Just to see if I have lost or gained anything important. My hope is that the shelves will improve dynamic range and expose the lower frequencies more. On steady state sinewaves one would not expect too much from their addition. Though the last shelf proved to have quite remarkable properties even well outside the pass band of the IB.  I have had to roll back the attenuators to 10 o'clock on the EP2500 power amplifier as the bass is now subjectively much heavier. Though it is creeping back up again as I try other organ CDs and find a nice balance between bass and speakers. These things take time and only experience will fix their relative levels.

I have a plan to add a small but powerful woofer somewhere to fill the nasty trough between the main speakers and the subwoofer. I still have the original driver from my SVS cylinder following the upgrade to the NSD. I'm fairly sure that my wife will block my ideas for 2 x 15" manifolds in the dormer ceilings. So a sealed compact box with the 12" SVS driver is far easier to get past the local planning officer. It need not reach too deep given its intended purpose in the 150-300Hz region. I'll have to do some homework first into a suitable box size to match the completely unknown driver parameters. Somebody is bound to have discussed using this driver on one forum or another. As there must have been plenty PCI drivers knocking about cheaply for various projects after the upgrade offer. I could use the cylinder as a filler but I really don't need its deep bass and doubt it will perform very cleanly at higher frequencies due to its design. Perhaps an open baffle with the 12" is all I need? Tomorrow it is then. :-)

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