Seeing the massively thick IB manifolds illustrated on the IB Cult forum has finally nudged me into action. I bored two holes and fitted a piece of studding as a cross brace between the two sides of my 3/4" plywood manifold. The studding (or screwed rod) is 1/2" in diameter. (12mm) With very large, square, thick, galvanised washers (inside and out) to spread the tension and compression loads from the driver reaction forces in the side panels. I had carefully chosen a brace position which lay midway between the drivers and the open side of the box.
I carefully tightened the nuts hard without distorting the box and put on a favorite organ CD to judge the effect. The improvement seemed well worthwhile on the subjective level. With plenty of detail in the deep bass beautifully exposed. However, a quick check with REW produced zero difference between the nearfield response plots whether the nuts were tightened hard or completely loosened. Absolutely no change could be seen between output levels or response curves. Well, at least I made the effort and it took only a few minutes to gather the tools and materials to make the modification.
A standard of 2 x 18mm (2 x 3/4") layers for a laminated box manifold is still probably a very good idea for most IB installations. Ensuring a solid but very heavy box well able to resist reaction forces between the opposed drivers. Giving them the best chance of moving air rather than "flopping about" uselessly. (like my array)
My own manifold used an entire (metric) 8' x 4' sheet of 18mm (3/4") high quality plywood with very little left over. This gives a pretty good idea what the bare manifold weighs. Add the weight of another sheet to bring the manifold up to proper 1.5" thickness. Then add the weight of the four drivers and one should plan an IB installation very carefully. Fortunately my manifold sits on the floor and has easy access both inside and out. In a ceiling installation one should make certain that the joists can support the load without sagging.
When I think about it; the back, top and bottom of my manifold add plenty of stiffness on three sides of the driver panels. The open sides of the manifold are also being reinforced by the wall studs between which the manifold fits. The surface covering on the adjoining walls adds further resistance to bowing at the manifold mouth provided the manifold is well screwed to the studs.
If one doesn't like the look of a naked threaded cross brace then a pipe or even a length of opaque hose could be cut to fit between the inner nuts. Then slid over the screwed rod as it is assembled in the box. Perhaps a plywood "window" brace might be better. Though if the brace is glued in the drivers become more inaccessible. It is usually so dark in my own manifold that appearance hardly matters and the performance is still superb. After a few days there still seems to be greater weight and detail in the bass I'm still not sure whether it is a purely psychological improvement, or not.
Here's the simple studding brace in place showing the big, load-spreading washers to resist crushing forces on the plywood.
Having had time to think about it I now realise that REW is a rising sinewave sweep at a fixed level. The reaction forces of the drivers when subjected to sudden acceleration from severe transients is hardly the same thing. So it is hardly suprising that I saw no difference in the REW response plots.
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