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Klass wrote :

"...ribs stiffer than sandwich? Can you explain?..."

It's not what I wrote, or meant to express. For the same given thickness, a sandwich would be more stiff than a ribbed and otherwise non-sandwich construction (in fact the "ribs" I imagine would best be described as foam-filled cores, a sort of localized sandwich construction).

Depending on the number of ribs, their placement, the depth of their section, how many layers of laminate on either surface, etc, one could end up with a structure that is super stiff, and way easier to make than a full sandwich type construction of comparable stiffness.

What I like about this approach is the following:

1) It accommodates compound curvature!!!! This is a big deal... For example, the transition from round throat to rectangularish mouth is easily accommodated, as are bolt flanges. And, lest we forget, giving a form compound curvature is in itself one of the very best means to impart structural integrity to that form (This is one reason why chickens do not lay six-sided eggs)

2) It uses techniques I already know (ah, brainless progress!) : I.E. A big plaster core on the floor of my kitchen, onto which can be laid a thick laminanate, followed by deep ribbing.

3) It concentrates most of the material on the inner wall, which means an inner wall having more density, more mass, higher acoustic impedance, which in turn means (provided the whole of the horn doesn't start resonating!), cleaner sound coming out of the horn. If one could create a sandwich having compound curvature and a dense inner wall, it would be superior in terms of overall rigidity and in resistance to resonating as a whole structure. But when I say sandwich, I mean a sandwich having a thickness appropriate for the scale of the horn... So not a 6mm piece of foam between a few layers of fiber and resin... For a 45 Hz exponential horn (nearly 3m in length with driver) I would go with a minimum of like 75mm thick walls.

4) Purely practical... No need to transport, manipulate and store large sheets of plywood and foam (remember, I am doing this in a Paris "apartment").

5) In my case, it would probably end up being faster if adopting this process, and would result in better quality of the finished parts

Here's the stuff I would use to make the ribs (it will conform to a curve if desired... Section depth up to 200mm or 8", more depth via special order)... Scroll down a bit : http://www.compositesolutions.ch/pdf/tds/omegaf.pdf

The drawback is that the materials cost would likely triple. More layers of fiber + More liters of resin... I was shocked at the prices for Epoxy resin over here, but along with superior mechanical properties, this stuff would go unnoticed by my neighbors; it's more or less odorless. (This not being the Wild West, one cannot just kick off a few gallons of polyester resin in one's kitchen).

jd*

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