THanks for your answer, but Del, most rib crowned soundboards are glued in low humidity, so there is also an amount of comppression crowning involved
To meet the loose definition of “compression-crowned” the soundboard panel must be dried to a moisture content of 4% or less. Typically they are dried to 4% during the summer months and around 3.8% during the (drier) winter months.
Soundboards built up with crowned ribs are usually dried to something between 6% and 6.5%. This may not sound like much difference but the long-term effect is significant.
Yes, there is at least the potential for some perpendicular-to-grain compression in soundboard systems that are designed and built with crowned ribs, but it is considerably less than the amount of perpendicular-to-grain compression developed in a compression-crowned soundboard system.
But crown by itself
is not the critical factor; it is the combination of system mass and system stiffness in the final construction that is going to determine how energy is transferred from the stings to the soundboard assembly.
In general soundboard systems depending on panel compression for both crown and the added stiffness this compression contributes to the system use relatively flexible ribs. Those depending more on the rib structure for their overall stiffness tend to use stiffer ribs. (Yes, I know there are exceptions on both sides.)
All so-called “solid-spruce” soundboard panels absorb and desorb moisture through the normal cycles of humidity change. The difference is that the compression-crowned soundboard panel is subjected to levels of perpendicular-to-grain compression that causes permanent damage to the cellular structure of the wood. This (usually) does not happen in soundboard systems that are designed as rib-crowned systems.
I am not a wood specialist, I understand the wood may be resisting up to some point, but its resiliency may be changing with time, most old soundboards have very long basses without a well defined attack , old panels have a particularly non dynamic tone that reduce the voicing possibilities (sooner saturation)
There are a lot of other factors involved as well. Hammers also change with time (whether the piano is played or not). And, to a lessor extent, so do strings.
I also wonder if the intense vibration the assembly is subjected to will not change the wood inner structure.
I asked a wood technologist at the U.S. Wood Products Lab
about this a few years back. His answer was that compared to what wood members endure in a structure like a railroad trestle bridge for decades and, in a few cases, for centuries, the vibrations in a soundboard panel are insignificant. Yes, they vibrate but the range of motion is very small.
I also noticed that adding downbearing on some panels, you only have a decent tone for a few years.
Downbearing play a role in tone indeed but crown is just a necessity, apparently just structural.
I’m not at all convinced crown is necessary. What we are really after is some amount of stiffness. That stiffness can be achieved in many ways. One of them is build up a system that depends on a high amount of perpendicular-to-grain compression within the soundboard panel. Soundboard crown is a natural result of this method. But, as has been discussed at great length on this forum, this compression dissipates over time—years, decades, centuries—and along with it goes the amount of stiffness that was added to the system by that compression. When the perpendicular-to-grain compression has dissipated there may still be enough stiffness in the system for the piano to produce acceptable tone.
Or there may not. And this is the issue that causes so much confusion to so many; the uncertainty and the lack of hard and fast rules. We want rules that are clear and unambiguous: Compression-crowned soundboard systems always
act this way and rib-crowned soundboard systems always
act that way. The problem is that piano soundboard systems are rarely that clearly defined or constructed. Nor are the results always that predictable. I’ve looked at pianos built with compression-crowned soundboard systems that had failed while still on the showroom floor. I’ve looked and—indeed, I’ve rebuilt—pianos that I know were originally built with compression-crowned soundboard systems a century back in which the original soundboards were in excellent condition; at least based on their sound. Replacing these soundboard panels would not have made the completed piano sound any better. A bit different, perhaps, but not necessarily better.
Boards are also glued in a crowned setup, with flat ribs as with crowned ones. A good part of the resiliency may well be due to the glue joint bewteen the ribs and the panel
In a compression-crowned soundboard systems the stress-interface between the top of the ribs and the soundboard panel is critical. The glue used must not creep
; hence the use of glues that form a absolutely rigid bond. This is less critical in soundboard systems that are rib-crowned; here any modern woodworking adhesive including the so-called PVAs (or polyvinyl acetate emulsions) can be used.
About Rippen, I am unsure they used that setup for long, most of the Rippen I have seen have traditional panels, some of then hold nicely in time.
We sold Rippen pianos during the 1970s and this is how all of them were built. I liked the system; for their type and size they could be made to sound quite nice in spite of their erratic stringing scales. And they were very stable in a variety of climates. They would have been even better if their scaling had been any good.