More on soundboard crown

[Calin Tantareanu]

Hello Del, Ron, Don & Co.

> You can't leave the ribs out of the discussion. They are integral. And
> we've been over the "arch" question numerous times, both on this list and
> in the Journal.

What happens in the case of a compression crowned board?
I think that there, the ribs tend to pull the boards down (because they are
straight to begin with, and then bent upwards by the board that forms the
crown by gaining moisture and expanding).
I would say that in this case the board works in a way resembling an arch,
because the it expands upwards due to a lateral force, not a vertical one,
or?

In a rib-crowned board, things get complicated and it seems obvious that the
ribs provide most of the stiffness.

> > How significant is the stiffness added by the crown?
>
> Very.

Sorry, I meant to ask how significant is the stiffness added by the board
(not ribs) in a rib crowned soundboard..

> >
> > My conclusion, so far (and please correct me if I'm wrong) is that:
> > 1. Stiffness is directly inflencing a soundboard's function, while the
> crown
> > does it indirectly.
>
> Crown is only significant when it is being depressed by the sting bearing.

Thus creating more stiffness. I agree.

> > 2. If a soundboard would be kept in a (hypotetical) environment without
> > humidity fluctuations and no subsequent wood expansion/contraction then
a
> > flat board with flat ribs should work very well.
>
> I don't follow your reasoning here at all. Crown provides a mechanism to
> increase the stiffness of the soundboard assembly, but this added
stiffness
> is only developed when the crown is depressed by the string deflection
over
> the bridge--i.e., string bearing.

Right. I was thinking that since you can choose the desired amount of
stiffness by rib dimensions alone, you could make a flat panel (that is
supposed to stay flat, that's why I said no humidity fluctuations) so it
doesn't add any more stiffness to the system. Let the ribs provide all the
stiffness needed.
But this also means an assumption, on my part, that a crowned panel also
adds stiffness, besides the stiffness provided by the ribs, and that a flat
one adds less.
Del said he considers the panel neutral when calculating the stiffness. Does
that mean it provides (theoretically) zero stiffness?
Do you think that a panel, wether crowned or not, behaves in the same way
regarding stiffness?


> > 3. Crown is a way for dealing with the expansion/contraction of the
> wooden
> > panel in a predictable way.
>
> Where did this idea come from? It does nothing of the kind.

I understood that compression crowned boards don't always expand upwards
only, they can also form some sort of S-shape, reverse crown, or whatever.
That's why I think that crowning the ribs would ensure that the soundboard
moves into one direction only.

> > 2. What is the difference between making a board (for the same piano)
> with:
> >  a. Thick board with thin ribs,
> >  b. Thin board with thicker ribs.
> > For both cases, let's assume that the boards are designed to withstand
> the
> > same pressure from strings and to provide the same stiffness.
>
> This is a complex question that cannot be easily answered within the scope
> of something like this list. At least not by me. In general, the thick
> soundboard assembly with thin ribs will have more mass, the thin
soundboard
> assembly with thick ribs will have less mass. You can get a general answer
> by remembering that impedance is frequency dependent and is generally
> mass-dependent above system resonance and generally spring-dependent below
> resonance. To tell where this resonance (resonances, actually) are would
> require building and measuring samples.

Any idea on how this influences the piano's tone?
Are any books on this subject available?



 Calin Tantareanu
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