Soundboard stiffness variances

[Jude Reveley/Absolute Piano]

Hi Ric,

I'm still here and still interested. For my part, I ran out of things to 
talk about so I'm back to the drawing board. I get to a point where I can 
only learn by getting my hands on some work and hearing the results. To that 
end, Ron N. and Terry are helping me out with a Mason A6 RC&S design. I'm 
hoping this experience raises as many questions as it answers; keeps life 
interesting.

Perhaps we should be talking about what controlled experiments we could be 
conducting  to prove, disprove or answer any questions we have. While we 
need to start with a premise, talk only goes so far, wouldn't you agree?


Best,

Jude Reveley, RPT
Absolute Piano Restoration, LLC
Lowell, Massachusetts
(978) 323-4545


----- Original Message ----- 
From: "Richard Brekne" <ricb at pianostemmer.no>
To: "Pianotech List" <pianotech at ptg.org>
Sent: Saturday, February 09, 2008 5:50 PM
Subject: Soundboard stiffness variances


> List.
>
> The discussion basically left off where it began... with many folks in 
> apparent confusion... some left with questions dangling and answers were 
> evidently not forthcoming any time soon.  Seems like some of the basics 
> are misunderstood... and a few basic comparison questions about how CC, RC 
> and RC&S boards each get their stiffness and strength degrees in different 
> areas of the soundboard have been asked several times over the years 
> without any answer being really ever given.
>
> We do have (or should have) a basic grasp of the RC&S approach.  And that 
> should suffice to allow for at least some speculative thought as to how RC 
> and CC boards do essentially the same thing. I hope by now Dean and 
> whomever else has got straightened out about the stiffness to mass ratio 
> bit in the tenor and the bass.
> A next logical question, already put to the list by a couple earlier yet 
> not answered went along the lines of how much stiffness in general does 
> the soundboard require as a minium in all areas..  and then what the 
> effect of a given amount of mass is when coupled to this stiffness in the 
> various areas.
>
> If one takes one of the basic presepts of the RC&S gang as a starting 
> point, that might help to get a few thoughts rolling.  These fellows 
> figure a scale, and assume a certain degree of downbearing, and match this 
> to a set of ribs.  One of the primary targets here is to leave the loaded 
> board in a state that anywhere between 40% to 75 % of its unloaded crown 
> remains.  Given a scale, and an assumed string deflection angle... its not 
> difficult to figure out what the overall soundboard deflection is.  A 
> scale with say 600 lbs of net downbearing force that deflects a soundboard 
> to 50 % of its unloaded crown height leaves the board (or the rib set as 
> it were) with very predictable amounts of residual upbearing force 
> remaining... which can be translated into stiffness conceptually.
>
> How does then one figure the same thing for a CC board ?  Clearly using 
> rib center cross section or average cross section... or even treating the 
> rib as a beam at all isnt going to work out right. It appears that to no 
> small degree strengths in these kinds of boards were arrived at 
> empirically.  But there should be some kind of way of translating the kind 
> of beam strength an RC rib set has to similar strength degrees in a CC 
> rib/panel set.
> If I'm not mistaken, in both cases the strength of the resulting 
> soundboard is non linear in character.  So as a CC board is loaded... it 
> gets increasingly resistive until such point that it is over loaded (ie 
> compression failure in the panel occurs). But up until that time one might 
> be able to figure how much force it takes to bend a CC rib through the use 
> of horizontal strain along the glued face as the panels face takes on 
> compression. The stiffness of the CC system at unloaded state then could 
> be calculated so as to be comparable to any given RC&S rib set.  The only 
> difference from a support perspective would be where the <<strength>> of 
> each system is based. If one succeeds in calculating CC strengths 
> accurately thus... then they become every bit as predictable as an RC&S 
> board.  The same goes for any compression reliant variant.
>
> That done... one can go on to deciphering what basic operating windows we 
> are dealing with when it comes to mass to stiffness ratios.  It has been 
> stated several times that the degree of mass in a CC rib system is not 
> essentially different then an RC&S board.  I am not sure this is the case 
> really.  While it is true a rib that is 1 meter long and 28 by 14 has the 
> same mass whether you align it one way or the other....  I have not seen 
> it shown that a rib required to give any given degree of support seen as a 
> beam will have the same amount of mass as a rib required to combine with 
> soundboard compression to yield that same given degree of support.  And if 
> that DOES turn out to be the same.... well then one has an indirect way of 
> calculated the needed dimensions right there.
>
> Just a bit of food for thought for any of you still interested in this 
> thread.
>
> Cheers
> RicB
>
> Cheers
> RicB
>
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