>Please explain: > >"compression crowned, rather than rib crowned, soundboards. " > >We made a pneumatic soundboard press, it works well but which of the >above processes matches our method? > >Roger Hayden, RPT >Clarks Summit, PA > Hi Roger, Drying a soundboard panel down to around 4% equilibrium moisture content (EMC), and gluing flat ribs to it will result in, for want of a more universally accepted term, a compression crowned soundboard when the panel re-absorbs moisture and swells. A rib crowned board is made by machining a curve, or crown, in the rib, and drying the panel down to a less extreme degree (around 6% EMC), before gluing the ribs to the panel. In a compression crowned assembly, the straight ribs actually resist the formation of crown, so the panel must not only bend the ribs, but hold up the string downbearing as well. Beyond certain limits of high humidity and bearing load (depending on the panel thickness and rib scale) the internal compression of the panel exceeds the (average) 580psi limit that spruce can support, and the softer growth rings are crushed first. That's the compression ridge, and this type of soundboard can go flat very quickly. In a rib crowned board, most of the crown is formed and supported by the ribs, with the less severe panel compression only adding a small amount of support. Since the panel isn't under nearly the compression with this system, it is less likely that the 580 psi limit will be exceeded in the panel with high humidity swings, and compression ridging will typically be less. A rib crowned board will maintain a crown much better, and much more predictably, than a compression crowned one. You can assemble either type of system with your pneumatic press, but machine crowning the ribs, and clamping panel and rib into a negatively crowned caul to match the rib crown, is a far better way to make a soundboard... in my opinion. Hope that answers the question. Read Del's Journal articles for a more thorough and detailed description. It's well worth your time. Ron N
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