Gilding the lily

After months of procrastination and searching for new, potential IB sites in and around my AV room I have finally decided to try a tall, 8 x 15" driver manifold. This will be placed in the original IB enclosure. Four new and four old AEIB15s will be arranged in mechanical opposition to each other. The driver magnets will be brought inside the box to avoid the metalwork sweating in the rapidly rising temperatures which my unheated IB enclosure is subject to. The drivers will be wired to ensure all cones move towards the manifold centre simultaneously. This will make the open mouth of the manifold into one huge compression driver equivalent in power and area to all 8 x 15" drivers. The mouth of the manifold will closely match the total Sd for the eight drivers. (Sd = effective cone area for a single driver) Regardless of magnet position, opposing the drivers on each side of the box will kill the driver reaction forces which can otherwise cause severe structural vibration. All these features are standard design principles for a successful IB as recommended on the IB Cult forum. (and as per the truly encyclopaedic IB Cult FAQs)

The reason I haven't gone ahead with this rather obvious driver arrangement before now is the limited size of my IB enclosure. It started off at 600 cu.ft. for a total Vas multiplier of between 9 and 10 but the enclosure has become a rather handy storage area. Adding four more drivers means I am completely ignoring the total Vas multiplier rules. Even if the enclosure space were completely empty I would be down to somewhere around 4.5 as the total Vas multiplier. It is difficult to calculate the loss of volume to storage but it must have some effect. The upside is that the enclosure is very leaky and lined with at least 12" of Rockwool on the sloping surfaces. It will be fascinating to see whether the Fs of the drivers is raised by cramping their style in this way. Somehow I doubt it, but I am about to find out. IBs seem to be very forgiving in other aspects of their design so I am now probing how they cope with undersized enclosures.

The new and old AEIB15 drivers are very different from each other. The earlier drivers have an Fs of 32 Hz. Double that of the newer drivers. The old drivers have conical poly cones. While the new drivers have heavily flared paper cones The intention is to drive each set of four drivers with one channel of the EP2500. Using series-parallel wiring the old set will provide a 4 Ohm combined load. The newer drivers are 8 Ohms each and there is no way to wire four of them to achieve a 4 Ohm combined load. So they are very likely to end up as a nominal 8 Ohms. It will be interesting to see if their outputs can be matched using the crossover gain settings and/or the control knobs on the EP2500. If balancing the sets proves difficult it might be worthwhile wiring the newer drivers in parallel for a 2 Ohms load and greater output. This is a lower impedance than is desirable but there are members of the IB cult running their systems at 2 Ohms without obvious problems.

I would like to achieve a smooth response from 10Hz upwards. Hopefully the older drivers will provide just a little more oomph between 30 and 80Hz. Whether this is possible has yet to be discovered. It is usually a very bad idea to mix drivers in the same IB system. However I am pinning my hopes on isolating them on separate amplifier channels to minimise any problems. Normally it is difficult to share the loads equally between different kinds of drivers. So one driver might take more than its fair share of the load and be over-driven. While others just loaf along providing little output.

The new manifold will be approximately 6 feet high x 23" deep x 17" wide externally. This is the minimum box size which will hold all eight 15" drivers with as little space around them as possible but leaving room for "window" shelf braces between each pair. These will provide extra stiffness across the box where the major reactive loads are concentrated. Three inches of the 23" depth will be lost between the existing wall studs. The inner layer of the box is extended forwards for this purpose. The outer layer will abut against the wall surface on the enclosure side.

I have quite severe clearance problems out in the enclosure with the sloping ceiling and the adjacent double doors. This limits me to the space between the two wall studs where the original array and the present four driver manifold have previously been positioned. I will want to raise the mouth of the manifold as much as possible to match the 10" difference in height between the enclosure floor level and the stage indoors. This will help to maximise the area of the manifold opening. Naturally the box can only be raised until it touches the sloping ceiling and no more.

The manifold will consist of two layers of good quality 3/4" multi-layer plywood glued and screwed together for strength and stiffness. The open side of the box will face into the AV room as is usual with an IB.

Construction is well under way using some 3/4" flooring plywood which I just happened to have left over from building my workshop. I also recycled the original array baffle by trimming it slightly. Then I used it as the pattern to mark out the other side. It seemed a shame not to re-use the high quality plywood of the array baffle as it formed a perfect right hand side for the intended double height manifold. I had designed the array baffle to bring the drivers close enough together to avoid the top of the baffle hitting the sloping ceiling. I also used this panel to check clearances out in the IB enclosure. The missing triangle at the top (in the images) is where the original array baffle met the sloping ceiling. I shall have to glue on a triangle of similar plywood to make up the missing section.

Here's a quick mock up of two sides for scale. It's actually slightly taller than I am! Now I need more plywood to finish the job properly... The single thickness new side is in the foreground while the double side from the original, line array baffle is in the background. The visible rebate on the edge of the old baffle will have to be filled with more plywood to avoid having the back, top and bottom panels all of different widths.

At the timber merchant I was offered a choice between two qualities of similar plywood. One weighed probably twice as much as the other and the surfaces were of higher quality. The price was nearly the same so I took an 8' x 4' sheet of the heavier board. A quick prior check on paper proved that I could cut out two backs, the second layer of one side and three tops and bottoms from one sheet. I already had enough material for another top or bottom so I had just enough material for my full complement of pieces to make the double thickness, manifold box. Half an hour later I was back at home and had them all cut out with my cheapo circular saw. I don't do enough woodwork these days to warrant the investment in anything better. It gets the job done but the blade usually rusts between bouts of use. Quite honestly, I prefer a carpenter's, hard tooth, hand saw to the infernal racket of a circular saw!

Rain intervened before I could cut out the four driver circles for the last side panel. I am using an electric jigsaw for the circles instead of a router since I have no need to countersink the drivers. In fact I want to separate them as much as possible where the magnets come together. I have taken my own advice and narrowed the manifold very slightly to avoid struggling to fit this new box between the old and twisted wall studs. Doubling the plywood will also add enough extra width to afford extra clearance between the magnets. As this manifold is almost twice the height of the present one the risks of a tight fit are even greater.

I also used the original baffle array panel as a pattern to drill out the driver fixing holes in the other panels after clamping them together. I'll wait until tomorrow now to finish the cutting and sanding to avoid messing up my workshop with fine sawdust. The forecast is full sun tomorrow so I can work in comfort outside with plenty of room to manoeuvre.

Making progress. Another hasty mock-up clamped together and posed for the camera. The single thickness strip at the front of the side boards will fit between the wall studs. The double thickness edge will then seal against the wall surface leaving a neat edge.

All the parts are cut out but nothing is glued together as yet. However tempting it might be to glue it all together I'd have the problem of getting it upstairs into the IB enclosure. Being double the thickness and much taller than the last manifold the new box weighs considerably more. Without bracing shelves the bare box weighs 10 UK stone or 140lbs! That is much too heavy for me to lift comfortably. The eight 15" drivers will add a further 150lbs to the finished structure. Welcome to the 300lb IB gorilla!

Here's the new, nearside manifold panel being clamped up after gluing with PVA. You can never have enough clamps. Nor find your missing clamps when you need them! The cheapo sliding clamps with the red handles are actually very useful thanks of their deep jaws. G-cramps or C-clamps (depending on your geography) are rarely deep enough in the gape to clamp large panels effectively. Fortunately the driver cut-out holes allow greater flexibility in clamp positioning. The eagle-eyed might notice the work stands have been brought quite close together. This was to compensate for the bow in the new sheet of plywood. Sighting repeatedly along the edges of the panel as I clamped up ensured they were dead flat as the glue dried. The panels relaxed across the work benches until they were flat. I suppose I could have deliberately overdone it to ensure they dried dead straight but they look good enough. In theory one should laminate three pieces of ply together to ensure an odd number of laminations. Even numbers may tend to warp over time. Though I'm not paranoid enough to lose sleep over this.

Yet another mock-up. I wasn't certain how I wanted the box to appear from the outside. So I made the back, top and bottom panels over long. Now I think about it the box is so tall I shan't ever see the top. So the back panel should be full height for neatness and the top and bottom inset.

I still can't make my mind up whether I want the finished box to slide up a ladder and in through the window like the last one. It would be so much easier to finish the box outside, sand it and (perhaps) even give it a coat of paint. If I finish building it indoors I'm stuck with the finish I get. I can't tidy up the edges of the panels where they meet without making a terrible mess in the enclosure. I can use a block and tackle if need be to help the lift up the ladder. I just need to align a strong beam across the AV room doorway at the correct height for a nice, straight pull through the open window. The windows are nearly 4' square so there's plenty of room to get the box through the opening.

I should make it clear that I don't need any more drivers in my IB. The IB is already everything one could ask. It is just that the older drivers are sitting in their boxes and going to waste. Which seems like a perfect opportunity to test an IB's real life performance under abnormal enclosure volume limitations. The old AE drivers sounded fine when they were in use in the manifold. That is until the new drivers arrived and offered more bottom end output without EQ boost. My hope is that the extra cone area and displacement will add more of what I have already. The goal is increased dynamic range with even better realism. Not much to ask for the work involved in replacing the manifold. Cost to date is just a single extra sheet of 3/4" plywood. If the new manifold isn't an improvement then the old manifold only needs to be slid back into place and the newer drivers refitted.

Well, the old manifold has now been sacrificed to recycling though the extra panels do not appear in this image of the glued and screwed manifold. I intend to paint the new box but haven't a clue which colour to choose. The manifold is visible through the glazed doors to the enclosure and I became rather tired of the tatty appearance of the unpainted plywood of the last one. Black would be most appropriate in best "2001 monolith" style. Almost everything in my system is black already but I'm not sure I want this thing black.

Black it is. Hardwood kitchen worktop lacquer in black with a low sheen. Two coats applied by SWMBO. It looks quite like "Black Ash" speakers in the flesh. The picture really doesn't do it justice. It must be the odd, late evening sunlight. I might try photographing it again tomorrow before it is too late. The bracing shelves have been removed for lightness while I try to get get the box upstairs.

The "hole in the wall" has been enlarged to its former [vertical array] glory but I doubt I can manage a full six feet of mouth opening before the box hits the 45 degree, sloping ceiling. The enclosure door frame has been beefed up with a new 4"x 2" solid timber stud thanks to the slightly slimmer manifold. It must have been 100 degrees out there in the enclosure in late afternoon sunshine! The fresh paint still smells a bit strong so the manifold is having a rest outside overnight. Tomorrow the big lift begins. The ladder route has been placed off-limits by SWMBO. So two triple pulleys and miles of rope will assist the manifold on its way upstairs. But first I must get the huge and heavy box to the bottom of the stairs!

This chapter is likely to be heavily modified and corrected as I progress. So you may like to reload the page occasionally to ensure you see the latest version.

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