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It came as a surprise to realise that somebody was actually listening when I suggested concrete lintels to mass load and stiffen an (IB) baffle wall. They certainly make for clean, strong, stiff and dense units for incorporation into a stud wall. Though certainly heavy they only have to be handled once and are not usually beyond the strength of a pair of most adults. The same could be partially said for bricks or (dense) concrete blocks for introducing mass though these offer no stiffness to the construction. It would be rather pointless to try using lightweight blocks unless they were bonded together into a complete block wall of sufficient thickness to be stiff enough in its own right.I don't intend to discuss stud, baffle walls very much because they are not usually stiff enough in normal domestic thicknesses. (4" or even 6" /100-150mm) Adding two layers of plasterboard cladding doesn't add much stiffness to such a flexible structure. Only a little extra mass. Which is neither here nor there when driven by a decent number of large IB drivers to normal levels. I might consider a stud wall using 8" x 2" studs but this raises the cost significantly. Finding suitably straight lengths of timber in this size for an entire baffle wall might prove frustrating. Not least for your supplier as you pick and choose from his twisted and wavy stock! Leaving only the unsaleable lengths for the poor devils who follow in your path. Sawn finish, structural rafters or joists may not be what you were hoping for.
It seems to me that stud walls are only really stiff in the vertical plane. They can flex fairly freely across the width of the wall despite all the noggins and the ceiling and floor plates along the top and bottom. You could think of a stud wall as corrugated cardboard set up with the corrugations vertical. Easy to roll up one way but much more difficult "across the grain."
The greatest resistance to bending is from top to bottom. Not least because the wall height is usually the smaller dimension compared with its width. The wall could still "balloon" overall under the reciprocating driver reaction forces and/or pressure effects. Remember that a large surface needs to move by only a very tiny amount to undo all the careful design of our relatively small drivers.
Such movement can be additive or subtractive from the driver's own output. In or out of phase with what the driver cone is doing makes little difference in the grand scheme of things. All baffle wall movement is bad! Not least because it usually affects the frequency response. It may also rob the reproduced sound of serious impact. Some people make quite a good living providing sound absorbers not too dissimilar to a normal stud wall in flexibility and absorption.
One answer to this flexibility problem might be to adopt sheet covered crate style boxes. A simplified, honeycomb structure raised on steroids! Laminations of sheet material over boxes are used for everything from humble flush house doors to ultra high tech aerospace materials and even the world's largest wooden structure. Search for Seville Metropol Parasol to get an idea of the possibilities. See link below:
http://inhabitat.com/metropol-parasol-the-worlds-largest-wooden-structure-opens-in-seville/metropolparasol1/?extend=1
In the case of our more humble Infinite Baffle wall you could imagine a very large, letter sorting frame. One stretching from floor to ceiling and wall to wall. Covered both sides with stiff sheet material all glued firmly in place. The box sides would be formed by using interlocking halving joints along the lengths of suitable strips of stiff material to the desired depth (thickness) of the baffle wall. I'll stick my neck out and suggest at least an 8" /20cm thick without any real mathematical basis. It just seems about right as a rough minimum to ensure success.
Links to images of halving joints:
http://www.raygirling.com/images/cab06.gif
http://www.raygirling.com/images/cab07.gif
A circular saw or router and cutting jig would greatly aid accuracy and ease of construction. A carbide tipped blade or bit would be most sensible to cope with working in abrasive sheet materials. A degree of freedom at the joint cut slots would greatly aid gluing and construction provided it was not overdone. Cutting the sheets into strips is best left to the supplier if they can be absolutely trusted to make straight lines.
MDF, plywood and OSB would all make suitable and affordable building materials for indoor use. Since the sheet material is used to build a rather thick wall its flexibility across its own thickness becomes almost secondary. High mass is still desirable though. Probably suggesting MDF, as the leading candidate. This material is more readily available in larger sheets than plywood in my own experience.
I seem to remember 10' x 6' could be obtained without too much effort. Though these sizes are not absolutely essential since sub-modules of boxes could be built up and each module glued edge to edge to produce a much larger overall area. The natural doubling at the edges of the smaller modules would add some mass and local stiffness. Building sub-modules of a manageable number of boxes into a vertical wall is far easier than raising the entire baffle wall from the floor!
Though there are affordable winches if you really fancy your luck. Naturally you will only raise the empty box structure. Cladding, mass loading and installing the heavy drivers comes later. You'd also need to study the geometry of deep rectangles as they rotate. The radius travelled by diagonal between bottom inside and top outside edges is much larger than the physical height of the wall! You are going to have to allow for this by making your structure a bit shorter. Then add ceiling packing once you have the whole thing upright.
If made to suitable sizes then the boxes could be mass loaded with size-matching bricks, paving slabs or concrete blocks. Or even very strong or multiple nested bags of dry sand. The apertures for the drivers should be left open on both sides of course. While depth is highly desirable in the sides of the boxes for both strength and stiffness the driver apertures do not want to be too enclosing. Otherwise the drivers cannot breathe easily.
Colouration is probably unlikely given the IB's frequency bandwidth and the relatively modest size of the likely box dimensions. Though it may still work against the desire to build a free-breathing array. The IB manifold is rarely attacked for its introduction of colouration. But perhaps we should set higher standards of driver clearance for an array? Or why build an array in the first place? Very thick baffles around the driver will choke off the freedom to breathe incorporated into the usually slender, cast baskets in even the largest of modern drivers.
The driver apertures do not need to be the exact module size for our intended reinforcing boxes. Though this ought to be born in mind. Through apertures could be made by leaving out (say) 2x2 box sections from the wall structure. Though I would hesitate to allow a tall empty slot in the structure for a vertical array without intervening structural "shelves" between each driver.
The really ambitious (and skilful?) builder might even incorporate curvature into the intended baffle wall. Not necessarily an overall bending but a gentle thickening at the centre to add greater stiffness or visual drama. The structure still has to be covered in plywood. Or one's alternative choice of surfacing material. Extremes of curvature would require more flexible covering materials. Stiff sheet materials do not readily lend themselves to curvature in two planes! Though smaller sheets are rather more accommodating.
The downside with a box core structure is the difficulty of adding concrete lintels. Bricks or blocks would need local fastening, wedging or bonding to avoid isolation from the overall structure. Finding and curing a rattle between bricks or blocks after the surface material was fixed in place could be a nightmare!
The box structure itself might offer decorative potential to the imaginative builder. It may be that the wall itself would be stiff enough without its dual surface sheeting. The structure could be clad on the rear only to achieve airtightness, for example. Leaving the front open to the application of translucent or transparent material. Or even completely open to be used for storage and/or lighting effects. One must hope that one's vast collection of BluRay disks and CDs do not themselves rattle when stored in such a structure!
Early aeroplanes used doped fabric to achieve remarkable stiffness. As do model makers using a variety of thin materials fixed over otherwise weak balsa or thin plywood structures. Providing the baffle wall is still reasonably airtight the possibilities are endless. One might even imagine a series or pattern of boxes being covered on only one side or only the other. Perhaps to provide storage on both sides of the wall without a continuous cover on either but still remaining airtight.
These suggestions are only for further examination in the real world of building baffle walls. I offer no guarantees (whatsoever) that any of them will work in practice. Not even with careful thought, design and construction.
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