soundboardinstal again

[Ron Nossaman]

>Ron Nossaman stated in a previous post that (if I understand
>correctly) he designs a rib set, for a rib-crowned board assembly,
>to give a certain set of ranges of deflection under different humidity or 
>dryness conditions, for a certain range of
>expected downbearing forces from one end of the bridge(s) to the other, 
>and that one of the criteria for the bottom of the ranges
>of deflection is, that the ribs should be the primary elements that make 
>it certain that the board assembly will not go into flat crown
>or negative crown over the expected humidity and dryness range,
>and will make this certain for a reasonable expectation of
>lifetime for the assembly.

Yes, almost. For crown formation and retention, I pretty much ignore the 
panel when I design a rib set, so humidity isn't a factor in rib support of 
bearing or longevity. Panel thickness and grain direction is important for 
other reasons, but not to me in determining rib stiffness. I wouldn't 
expect to see boards like this go flat in any humidity conditions, which is 
one of the primary reasons for doing this in the first place.


>Factors that can bring about change
>over lifetime--and in some designs, begin to do so during, or
>even before, installation--include various possible compression forces 
>within the elements of the assembly, as these interact with
>each other, and with the strings and case parts.

Exactly, which is why I like rib supported systems. The stresses on the 
ribs and panel are very light and dependably predictable as opposed to the 
extremely high compression levels in the panel required to both bend a 
straight rib (which resists being bent) and support string bearing as well 
in a compression crowned system.


>The reason most commonly given for designing for the prevention of flat or 
>negative crown is, that the coupling of the string to the board assembly 
>is otherwise either likely, or certain, to become
>insecure.

Regardless of reasons commonly given, coupling doesn't normally have much 
to do with it until the board is flat enough to produce negative front 
bearing on the bridge. If the soundboard assembly doesn't present enough 
stiffness to the string plane, as in the typical old flat or concave 
crowned compression crowned board, the impedance mismatch between board and 
strings is too great and the tone suffers. Percussive attack and short 
sustain in the killer octave is the classic example. A compression crowned 
board depends on panel compression at destructive levels to the material to 
provide that soundboard stiffness, thus impedance, to the proper levels. A 
rib supported board depends on rib dimensions to provide stiffness at much 
lower material stress levels for the same thing.


>But the prevention of excessive deflection in the board assembly--
>under any humidity and dryness conditions--is also the prevention
>of a certain degree of stiffness of the assembly at the coupling,
>when the string and board are at rest (zero amplitude). This is, in a 
>sense, independent of the question of the coupling's security.  The 
>assembly, as a hardening spring, must give that degree of
>resistance to the downward vertical transmissive force of the
>string when set into movement--"independently", so to speak, of its 
>downbearing force at rest--to insure, at one and the same time, the
>(grand) piano's dynamic range, sustain, and desired tonal character.

I'm not sure what you mean here, but there is certainly a difference 
between static load support and dynamic response. This again, isn't really 
coupling. It depends on the string bearing load and frequency, spring rate 
of the soundboard assembly, initial crown height, deflection under bearing 
load, mass of the assembly at that point, and back scale length. For 
instance, compare two 500mm ribs, each with 1mm remaining crown under 
identical loads of 25lb (11.34Kg) from identical string scales and 
configurations. One was 20mm wide, 28mm tall, and had a 1.74mm crown height 
before loading. The other was 17mm wide, 23mm tall, and had a 2.6mm height 
before loading. They both support the same weight with the same remaining 
crown, but I would certainly expect their dynamic response to be different 
because their spring rates are different.


>This idea of a certain "independence of function" is, I take it,
>what has often led to proposals for a different sort of coupling.

I don't understand your use of "coupling" here..


>But it has also led me to wonder about such innovations as
>Grotrian's "counter-bridge", underneath the piano, under the bridge. Or: 
>*whatever* Grotrian's own purpose may have been, why not consider having 
>one or more counterbridge as a "garantor of crown", in the same sense that 
>Ron's rib set is, and replacing a certain part of that latter function?

I don't see any connection here because bridges don't support crown. Some 
combination of ribs and panel supports crown, and the bridges serve to 
distribute load among the ribs.


>Ribs are still needed for the solid spruce planks, but - perhaps - the 
>whole assembly could then be designed without as much of a problem of the 
>effects of compression on the panel sub-assembly, i.e., especially the 
>vector of compression that runs across (perpendicular to) the plank's grain?

Which is the first purpose of rib crowning - very little compression of the 
panel across it's grain.


>My assumption in all this is, that, in many present designs, the
>planks benefit--as acoustical components--from having a longitudinal, 
>end-to-end crown, and that the with the new approach, they could keep 
>this, while otherwise suffering no damage.  Of course, I'm far from sure.

I doubt that there is any real benefit gotten from crown along the grain, 
despite repeated assumptions to that effect, but it happens to be pretty 
nearly unavoidable on a nominally flat rim with crowned ribs.


>Would this yield a sound that is too much like an early laminated
>board?

?? Too many undefined variables. Try it and see.


>And, what *did* Grotrian have in mind with a "counter-bridge"?
>
>Randy Jacob

I don't know. They might have been trying to form a crown there, or merely 
adding stiffness and mass to the bridge.

Ron N