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Well, those of you who are still following my plodding progress towards audio heaven will realise that nothing came of my threat to add upper bass reinforcement. Having floated the idea on the IB Cult Forum I was handed the problem of providing a suitable crossover. So the system continued unchanged while I examined my options. Having too many choices has always been my downfall.
The next crossover up the Behringer range is the CX3400. It does indeed offer 3-way stereo frequency division but the "mid/high" crossover point is at 444Hz minimum. Rolling off my floor standing main speakers (40Hz Mission 753Fs) at such a high frequency seems like a complete waste of their valuable cone area. I might as well use small bookshelf speakers for the Mains. (like my Linn Kans?) Except that I'd probably lose the distortion-free SPLs I'd paid for in the bigger Missions. So I decided that the CX3400 was not the real answer even if it was the cheapest upgrade option. I could have moved the Kans to the front (on stands) and the Missions to Rear speaker duties but decided I didn't have room at the back. The attic is rather cramped at the AV end due to the central stairwell.
My present CX2310 offers a summed Subwoofer-out option as well as providing a stereo Low/High variable frequency, active crossover at 24dB/octave. The Subwoofer-out connection has a variable low pass filter with a maximum roll off at an indicated 215Hz but of unspecified slope. This seems much more useful than a 444Hz high pass. The only (potential) downside is that the Sub-out connection is mono and unprotected by a rumble filter.
I can't feed the IB from the Subwoofer-out socket and then split the 4 x 15" OBs from the Mains in stereo. This would limit the crossover point for both OB and Mains to an unacceptable degree. Leading to exactly the same problems as using the CX3400.
So, using the present CX2310 the system begins to look like this:
The four newer AEIB15 drivers would continue their sub-bass duties up to 80Hz in a 2 x 2 opposed driver manifold cut down from the present 8 x 15" box. The IB would be driven by the EP2500 as parallel pairs for 4 Ohms per channel. Exactly as it was before I added the older drivers but now with stiffer, doubled 3/4" plywood manifold walls all round. Previously I had used a single layer of 3/4" while I decided what to do about my various options.
The four, older, vinyl coned AEIB15s would be mounted on open Baffles and driven by the Bash amp from the unused SVS16-46PCI. These drivers would be rolled off at the maximum frequency of 215Hz. This would become my upper bass reinforcement to get the missing "chest thump" and "kick" due to the present deep trough at ~160Hz. This would also return the IB enclosure to a far more favourable Total Vas multiplier.
I have tried the SVS over on the right to balance the IB. The SVS fills the trough nicely (according to REW) but adds its own muffled character to the IB sound. It also adds its unwanted very deep bass without adding the upper bass "wallop" which I feel I am missing. The SVS has always provided a steeply rising response all the way to 16Hz in my room. So trying to raise the output at higher frequencies brings an excess of the unwanted deep bass.
Four 15" drivers driven by a claimed 325 watts should be capable of moving enough air to produce the kick of a mule. Though SVS don't quote RMS figures for their Bash amps. The main problem is finding room for serious OBs. I am severely restricted for space on my attic AV stage beneath the 45 degree sloping ceiling. The narrow passband of the OBs should counter any doubts concerning the sound quality of the Bash amp at higher frequencies. The onboard filters/crossovers will be disabled leaving filter duties to the CX2310. The older AEIB15s are likely to roll off very quickly indeed below 100hz on open baffles. Their stiff suspensions already hamper them for IB duties without massive boost. (+16dB @ 20Hz)
The Mission 753Fs would continue to be driven by the Naim NAC72/Hicap/ NAP180 pre-power set from ~80hz upwards. This maximises their usefulness and dynamics while safely protecting them from really deep and powerful bass. If the 15" OB's prove really useful I could build OBs for the Mission drivers and forget about their bass output. The present crossovers could be altered to drive all the cones simultaneously instead of splitting them between bass and midrange.
Just to add to the confusion over OB progress I have a potential space for drivers in the front wall of a dormer window. The dormer is situated just to the right of the right Main speaker. This wall could easily act as a baffle for a pair or even four 15" drivers. Though it would place the drivers 3 feet behind the Main speaker baffles. Beyond the 4' high wall (below the dormer window itself) is the underside of the 45 degree sloping roof. There is a hell of a lot of rockwool out there but it is wide open to the outdoors via full length, 4" wide (netted) ventilation slots. Which suggests it might be rather leaky at high (very audible) levels and frequencies despite all the absorption materials. Not an ideal space for a subterranean IB but possibly useful as a sort of labyrinth for the upper bass.
So I'm going to try the OB options first. They don't involve any extra heat loss and should help to keep all the noise indoors at the "expense" of taking up a lot more room.
At first glance there are several possible OB sites: Each offers a different balance between appearance and acoustic desirability.
There is room for a single 15" on a small baffle on either side between the TV and nearest Main speaker. This would allow the 15" drivers to be brought flush with the Main speakers and not look too intrusive. But, then I'd need to find room for two more 15" drivers. These could be arranged outboard of the Mains on their own small baffles at the risk of looking rather clumsy.
Alternatively taller baffles, each with two 15" drivers, arranged vertically, could be pushed back in the space behind the Main speakers and LCD TV. Or, the Main speakers could be pushed further apart to make room with the baffles brought right forwards. Or, these taller baffles could be placed outboard of each Main speaker. Though they would be likely to look much more intrusive here and are likely to suffer a frequency imbalance due to corner loading only being applied to the left baffle.
A tall four x 15" driver OB could be arranged to stand outboard of the right Main speaker. This would nicely balance the offset IB in the left wall. I think I rather favour this idea over the other options with some caveats. It might lead to reaction forces but I'm hoping that limiting the low frequency extension would help to counter this problem. It might be rather heavy in appearance. (did I hear impressive?) It hardly matters in a sea of black components against a maroon curtained background. The 6' tall baffle would stand in front of the redundant SVS. Though the SVS would ideally be moved out of the way to allow the OB room to breathe. Replacing a large black, redundant column with a large black baffle is not a serious reason not to try this option.
I am still intending to try using the four older drivers in half of their present tall manifold working as a folded OB. I doubt the internal dimensions of the manifold would produce boxy colourations. Particularly given the very limited bandwidth involved. A manifold would offer four possible degrees of rotation relative to the listener in the hot seat. This driver arrangement would also cancel reaction forces. Others have folded their Open Baffles to a far greater degree than this without complaining about inferior SQ. It just seems so logical to try a manifold as an OB before dismissing the idea as unworkable. Many OBs have sides to stiffen them and to increase the acoustic path length.
A low horizontal array baffle could stand on the floor at the front of the stage below the TV. The Main speakers would need to be pushed further apart to make enough room. Otherwise the lower driver units of the Main speakers would be obstructed.
Or, finally, a canopy baffle of four 15" drivers could be hung from the rafters in the area above the TV and Main speakers. Because of the 45 degree sloping back wall this would bring the drivers flush with the main speakers. Though it might (would) look rather heavy unless raised as high as possible.
Here is a rough drawing of some of the various OB options. The LCD TV is 37" though nothing is really to scale here. The IB is represented by the wall on the left. It is surprising how many 15" drivers could (potentially) be squeezed into this space.
If only I had my previous, minimalist, attic listening room. With its acres of open, unclaimed space, life would have been so much easier for placing any number or size of OBs.
Today I have been experimenting with crossover points on the Main speakers, IB and SVS. The Mission 753Fs are a 2.5 way system which means only two of the 4" drivers, acting in association with their reflex ports, contribute to the bass. The other two 4" units handle the upper bass and midrange and the tweeter on top does the rest. The Missions sound fine with a 40Hz 24dB/octave crossover. I have measured them at 38Hz (-3dB) when fed a full range audio signal. What they lack is the killer bass of either the SVS cylinder and IB. When fed from a 444Hz crossover the bass speakers cease to operate at all as far as cone excursion is concerned. There is no sense of vibration at all. The resulting sound is devoid of any bass and accentuates vocals and drums. As I said above this high crossover point seems like a complete waste of these 3' high floor standers. With two of the 4" drivers out of the equation they cannot move as much air to reproduce the power to match the IB (or upper bass OBs for that matter)
The SVS is very powerful low down and can rattle everything completely effortlessly. However, due to its steep rise in output with falling frequency it doesn't produce much upper bass in my heavily damped room. This is no problem at all at the usual 80Hz crossover point since no upper bass is normally asked of it. However, the big cylinder does reproduce a rather dark rendition of music in association with my Missions. Even crossed over at 350Hz (internal crossover disabled) the SVS doesn't do vocals or drums. It is a good HT subwoofer however. I used to run it in parallel with my Mains on high level connections so the speakers contributed well to the final result. Perhaps the IB has spoilt my appreciation for the SVS but the big cylinder does sound dark and muffled compared with the IB. The 16-46PC is still an amazing reproducer of infrasonics for the money. Fed from the Sub-out of the CX2310 I have the gain set to barely on and everything still shakes and rattles.
The IB plays at much higher frequencies (subjectively) than the SVS using the same 350Hz crossover. Though the results are bit coloured it has much more slam than the SVS. Even at this high crossover point no bass location information drags the bass image off to the left wall where the IB is situated. I could still place the bass beats and pulses on Illegal Bass with perfect precision (within an inch or two) right across the sound stage thanks to the Main speakers steering the imagery via the harmonic content.
What I'd like from OBs is the hard kick presently missing from both subs and the speakers alone. The sort of chest thump I have felt from M&K pro monitors and Wilson Watts/Puppies. I have the freedom to adjust gain and crossover points with my present set up. So I might drag in a couple of earlier baffles just to see what can be done without spending any money on new baffle materials. I will have to seal the manifold openings as I remove the drivers because it is still very cold out in the enclosure. I'll try a couple of 15" first and then add two more after I've had a chance to experiment.
Well, having brought in the side of a former manifold with cut-outs for two 15" drivers I rediscovered exactly how large these units are in a domestic setting. Safely housed in a manifold beyond the baffle wall they seem innocuous enough. Even a vertical array could be let into the wall to take up no real room at all. Brought into the very limited space of my AV "stage" I find myself struggling to fit a single driver on a modest baffle on either side of the TV. The left side of the stage is bounded by the IB baffle wall. On the right a bookcase which protects suicidal lemmings from the end of the open stairwell. In front, the stairwell itself dictates the depth available. Behind a concealing curtain are bookcases. Above it all, a 45 degree sloping ceiling.
If I space the main speakers slightly further apart I will have room to fit a single 15" driver, on a small baffle, on each side of the TV stand. The latter also supports the Centre speaker and a few DVDs. What the baffles themselves lack in useful area will be made up by the proximity of the Main speakers, Centre speaker and TV screen. Each unit adds to the whole. To lower the roll-off point due to cancellation from out-of-phase pressure waves meeting at the edge of the undersized baffles. I do not need, nor expect very low frequency output from these open baffle drivers. Only experience and REW will suggest their actual contribution to the frequency spectrum. I shall make the baffles just tall enough to (almost) reach the underside of the TV to extend their useful area to the maximum. Once the baffles are painted black, they and the drivers they support, should become invisible.
For those of you interested in SVS products here is a snap of the inside of the Indigo 325watt Bash amp fitted to my 16-46PCI. The output tags are near the heat sink clips on the circuit board left of centre. There is a very high component count and neat workmanship. I think most of the weight is in the zinc plated, steel base plate and the big metal heat sinks. (the two upstanding steel plates at centre and right) I intend to use this amp to drive my OBs.
The 12" SVS NSD upgrade driver from the earlier PCI has a very large magnet and a steel basket. Solder tags, rather than clips or clamps, are unusual in these days of Chinese mass produced products bringing prices down to mere pence. Completely irrelevant in the context of a sealed subwoofer unit of course. Why would anyone want spring clips or terminals on something which is never opened? Hardly for show.
Even on this lowest tuned cylinder the trumpet flare on the lower end of the port is a good 6" above the amplifier cut-out. Those looking for a low tuned (16Hz) commercial sub need look no further than this provided they find the cylindrical appearance acceptable. I used to read ~108dB(C) on the RS SPL meter on the dragons wings on LOTR and similar room shaking effects with the older PCI driver. Presumably the new driver goes even louder but I haven't tried it on films since exchanging the drivers. A stunning performer considering how cheap it was when I imported it direct from the factory. This was prior to SVS setting up European distributors of course.
I'm now considering using this 12" on an open baffle to try and fill my 160Hz trough. Since I don't need low frequencies from it I thought I'd try feeding it from the 80Hz and above speaker leg of the CX. This will protect the driver from large excursions. A quick sweep with REW will tell me if it is useful for this purpose. I have no idea how high it will go without a low pass filter.
If only the CX3400 had a lower Mid/Hi crossover point I'd probably buy one. A minimum of 444hz seems much too high and a waste of my floor-standing speakers capabilities. To balance things up again I would need two large, stereo speakers to fill the gap between the speaker's new 444Hz roll-off point and the IB at 80Hz. The vinyl coned IB15s could manage it easily but take up far too much room on the stage in open baffles unless I can find another way to use them. I thought about using the stage volume to house them but it isn't really deep enough and would required considerable strengthening of the living room ceiling below the stage.
The four older IB15s could go flush in the sloping ceiling as a (sloping) horizontal array behind the speakers on a solid baffle fixed to the rafters. They could run in a horizontal line above the TV. They would be nestled into 12" of rockwool which is open behind to the entire, ventilated roof space. So at least they would be on the warm side of the insulation. They'd stay reasonably warm and suffer very little back pressure. Not much leakage of noise outside either. Provided they were rolled off below 80Hz there should not be too much of a problem with reaction forces. The suspensions are far too stiff to ever worry about cone sag.
Or, even better(?) the four older drivers could go into a long, horizontally-mounted box with its vertical baffle somewhere behind the TV and main speakers. The box would be open-backed and triangular in cross section. It would be fixed to the underside of the 45 degree rafters and sealed to a full sized cut-out in the sloping ceiling. Most importantly, the roof insulation would remain untouched. This would place the drivers securely inside the insulation but using an unused space which is usually rather dark and obscured by the TV and main speakers. Provided it was rolled off early such a system would not need to act as a true infinite baffle. There could be a slight acoustic delay of about two feet between these four big drivers and the main speakers in front of them. Though the main speakers could be easily pushed back if it helped.
Unless, of course, I lift the array box above the TV... Which would probably be rather harder to get past the Head Gardener's natural caution regarding such strange, audio-related activities. I have never considered such a driver layout seriously before due to the lack of clear space behind the drivers to act as a true IB. The thermal implications of cutting away the insulation to let (true) IB drivers breathe freely would have been horrendous! The clear space below the roof surface wasn't remotely enough for a true IB anyway. With an 80Hz, high pass roll-off the rules have changed. The drivers would fire into the open box and then filter through the undisturbed insulation. The cyclic airflow for such a limited pass band should be quite limited in comparison with the requirements of a "normal" infrasonic IB.I could hang a pelmet of matching cloth over the array to hide it from view.
Moving the older drivers away from the IB would return the enclosure to optimum volume for the four newer IB15s. So these newer drivers would play the infrasonics and sub-80Hz stuff which is what they do best. The older, vinyl-coned drivers need massive boost to play low. So both sets would be playing in their best power band with the new layout. I still have slight reservations about asking 444Hz of the vinyl-coned AEIB15s but I can easily check that out with my present crossover. Acting as upper bass drivers the output from four 15"s should be quite amazing. (even though they would need to be properly balanced with speakers and IB) The downside is the need for a new stereo amplifier to drive them properly and for a new crossover. I can't drive both sets of drivers in stereo from a single EP2500 even if I bought the CX3400. The SVS Bash amp is mono and my only other amp offers 2 x 30Watts RMS. I knew there was a catch. :-(
I could run the speakers from above 444Hz using the little A&R A60 stereo amp. Then move the vinyl coned IB15s onto the Naim NAP180 but that would only provide 90watts per channel for four 15" drivers. It doesn't sound remotely enough even for the proposed, narrow, emasculated pass band.
With Loreena McKennitt playing I have just had a listen to the older drivers with and without the speakers with a 444Hz crossover. The speakers sound thin and tinny alone. The 15s honk like hell in the manifold and sound as weak as hell higher up. Rolling off their bottom end only made matters worse! I could hardly tell they were playing higher frequencies even with 12dB boost. Returning to 80Hz balanced things up again. These older drivers till produce thunderous bass even when I turned off the 16dB boost at 20hz and engaged the 25hz high pass filter on the CX. With a 1kHz crossover they wake up to produce a very coloured and very bassy response. So I engaged the 50Hz high pass filters on DIP switches 3&8 on the EP2500 to roughly simulate an 80Hz roll off to try and kill the heavy, low bass with the 444Hz crossover.
Then I tried bringing the newer IB drivers back in unfiltered on one channel with an 80Hz crossover to the left speaker. The older IB drivers and the right speaker were still on a 444Hz crossover. The result was a bit unbalanced, of course, but drums had slightly more impact. Returning everything to normal provided by far the most satisfactory results. The warmth and depth came back. Rather inconclusive results then which is only to be expected. Would it be worth investing in another pro amp and crossover to run the older drivers in a horizontal array above or behind the TV? I really have no idea but I can easily imagine the Head Gardener's reaction to the suggestion. 8-|