Read into that what you will:
My present 8 x 15" driver IB manifold sits in a corner a couple of feet to the left of the left Main speaker. I would much rather the manifold was somewhere between the speakers. Being so far off axis it badly affects the frequency response by cancellation with the far [R] Main speaker. Leaving a severe frequency trough between the IB output and that of the Main speakers. This trough coincides with the maximum energy area of the upper bass spectrum. Precisely where one normally feels a nice thump in the chest from loud rock music, drums and bass guitars.
The present 6' tall manifold IB can easily lay waste over a large area but lacks this desirable wallop. It is insanely powerful and clean in the infrasonic and lower bass. It just lacks that something on rock music. This area is usually covered by powerful speakers. My Mission 753F floor standing speakers have been measured repeatedly at 110dB(C) but just can't do chest thumping bass.
The sort of bass you hear regularly at Hifi and AV shows. The Wilson Watt Puppy and M&K pro speakers excelled at this sort of thing. As can many others of course. I am not in the market for better speakers. I like what they do. Many speakers strike me as too hard and too bright. Certainly impressive, but difficult to live with in the longer term. I hardly need to mention the high cost of such transducers. Their size alone makes them difficult to house in my attic AV space.
Which brings me nicely back to the real world. Extending the manifold before it joins the large IB enclosure volume would be such an easy option for many of us. Yet the received wisdom is that manifolds be kept as short as possible. The exact opposite of my plans.
Such a strange layout as this would not be dissimilar to a huge reflex enclosure but with the driver[s] effectively closing the rather large and ridiculously long port. One could even build a large tunnel straight out from the speaker wall. Then arrange the drivers on the closing baffle. Or placed either side in an opposed driver manifold. Perhaps to bring them into much closer proximity to the main speakers to avoid phase issues.
Since multiple drivers would/should normally be applied to any IB the foolishly long "port" of this hybrid IB suggests they [the drivers] could easily be staggered. Arranged along the length, in a linear array provides a number of advantages. The deliberate spread of the drivers might well confuse the sharp tuning of this port as a closed 8' organ pipe. [@64Hz] Any likely pipe resonances should be killed by the multiple driving points along its length. Each driver being fed the same signal simultaneously should help to break up any fundamental or early harmonic pipe resonances.
The main problem is likely to be the extremely complex pressure waves in the "pipe". Each driver will see a constantly changing level of back pressure depending on the phase and frequency of the wave fronts at any particular moment. The rear pressure wave output from the drivers should hopefully be lost in the adjoining and heavily damped enclosure through the pipe's open mouth.
Though there is likely to be a reflection of each pressure wave from the exit opening back up the pipe. The problem is the sheer complexity of these pressure waves. Would the pressure waves/pulses effectively turn each driver into an ABR? [Auxiliary Bass Radiator] Albeit ABRs which are under the active control of the drivers' power amplifier. Rather than imitating the floppy cones of passive ABRs.
In my own case I would have the option of simply halving an 8'x4' sheet to produce a long, horizontal baffle. Each half could be glued and screwed to the other half to effectively double the baffle for extra mass and stiffness. The upper edge of the vertical baffle would rest against the 45 degree sloping ceiling of my attic AV space.
A simple batten could be fixed along the floor to stop the baffle from sliding outwards and further improve its sealing against the floor. The top of the baffle would rest firmly against the ceiling thanks to the weight of the four 15" drivers. Possibly requiring no mechanical help or fixing.
A good seal is readily ensured using simple weather stripping foam, draught excluder. The cost of the whole project is as low as obtaining a cheap 8' x 4' x 3/4" sheet of suitable material. Or rather its metric equivalent. [2440 x 1220 x 18mm]
The drivers can be spaced along the baffle entirely at the builder's own whim. Allowing for plenty of batten, or even concrete, reinforcement between the drivers.
Such an arrangement would offer a very small cross sectional area to the 'pipe.' In narrow musical instruments such as an oboe or saxophone, a narrow pipe produces a strongly nasal tone. This is due to the spread of the many, quite dominant harmonics. [Though the reed also helps of course] The fundamental is not nearly as strong in a narrow pipe compared with a much larger area, organ, flue pipe.
Is all this a good or a bad thing as far as an IB is concerned? Probably all bad. I have been warned by those far more expert that it will not work. An ideal IB offers zero back pressure and no reflecting surfaces anywhere near the driver cones. I am deliberately going against this advice and building one to put myself beyond any further doubt in the matter.
How will it perform in practice? Well, it should have much lower back pressure than any box small enough to fit on my stage. So it won't need massive power to reach maximum cone displacement. I have never heard of a reflex enclosure or hybrid where the drivers are fixed in the port. If it were a pure reflex then it would be tuned incredibly low! Single figures of Hz is a very likely tune with 600 cu feet working against an 8' long port.
It certainly isn't a tapped horn because no tapering is involved. What about a bandpass box? It has vague similarities to the 4th order bandpass. Except that the drivers are not sitting in an internal baffle and the enclosure/port is overlong and a continuation of the same cross sectional area. So the likely frequency of the pass band is a complete unknown. It isn't a quarter wave anything because the drivers are staggered along the length.. Nor is it a labyrinth. My hope is merely that it will offer the drivers a smooth, low pressure, resonance free environment to do their thing within a fairly narrow, upper bass, pass band.
Fluffy insulation could be laid along the floor of the pipe below the drivers and fixed to the sloping ceiling above. This would help to reduce direct reflections from passing straight back out through the cones. If enough of fluffy stuff were used it might also soften any remaining pipe resonances. There is no problem in filling the entire tubular space if it helps to smooth things out.
The proof of the pudding is easily achieved by obtaining any suitable 8' x 4' sheet of plywood, chipboard or MDF to give the idea a try. If it performs as an upper bass augmenter then I shall be happy. I really don't care about an extreme bandwidth provided it adds some serious upper bass wallop.
The most obvious mechanical downside is the reaction forces of having four 15" drivers facing forwards on a simple baffle. This arrangement of drivers has become a horizontal line array. Albeit one housed in a seriously stiff and heavy part of the roof structure. Fixing the baffle to the rafters might prove necessary if the baffle vibrates badly in use. Fortunately appearance hardly matters in my case. The baffle will lie along the floor in the darkness, behind the concealing curtains of the overhanging ceiling area behind the LCD TV.
The existing BFD can be used to flatten the response electronically. Though I have no plans to boost the low bass as has been the norm with the older, 32Hz AEIB15s when housed in the manifold. This should help to reduce the reaction forces by rolling them off much earlier.
The open end of the baffle/pipe will exit into the existing IB enclosure. A 600 cu ft storage area. The other end will be sealed from the AV room with a triangular plate of plywood or MDF. I shall begin with the substandard, vinyl coned, 32 Hz AEIB15s. I have no need that these drivers dig very deep since the infrasonic area is already taken care of by the later 13Hz AEIB15 drivers. Which are presently sitting in the lower half of the manifold. The box will be halved in height if the experimental layout works well.
My personal IB battle has always been the lack of a space above or below the speaker/TV platform. There isn't any useful IB enclosure space above. If there was it would chill the room through the very thin, speaker cones. I rebuilt the roof myself just to put 40cm /16" of rock wool up there. So I am not about to make large holes in it now!
And, if the experts are proved right, all I have lost is the cost of the materials. Plus the fun time wasted in cutting the holes and fitting the drivers. This daft idea has been nagging me for years now. Unless I try it I will never know how badly it performs. These older, vinyl coned, AEIB drivers are [probably] unsuitable for smaller boxes and were rather disappointing in open baffles. If this idea works better than expected I could always swap the drivers for the newer ones for more infrasonic bass. Though the infrasonic performance may be limited by the very design I intend to build.