More on soundboard crown

[Delwin D Fandrich]

----- Original Message -----
From: "Calin Tantareanu" <dnu@fx.ro>
To: <pianotech@ptg.org>
Sent: August 13, 2003 9:46 AM
Subject: More on soundboard crown


>
> By the way, how do laminated soundboards behave when exposed to humidity?
> Do they expand less than solid wood ones?

Basically they are stable. They change dimension very little with climate
and/or humidity changes. It's one of their advangages.


>
> >From all the replies, I understood that the crown is, basically, a means
of
> controlling the expansion of the board (that is, making it to move in
> predictable ways), and that it also adds stiffness.

Combined with string bearing it is part of a mechanism to add stiffness to
the assembly. I don't see that it does much of anything to control the
expansion of the board.


>
> But for that to happen (and leaving the ribs out of discussion for
> now) the board forms sort of a very wide arch that is supposed to counter
> the pressure of the strings too, besides the ribs.

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.


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

Very.


>
> In other words, in a rib-crowned board, how much stiffness is provided by
> the ribs (beams) and how much by the board (arch).

In a rib-crowned the crown and stiffness comes from the design and
construction of the ribs. At least in my calculations (and, hopefully, in
the completed board) the soundboard panel is neutral. It adds mass but does
not contribute significantly to the assemblies stiffness.


>
> I suspect that most of the stiffness is provided by the ribs (no evidence
> though) so please share your experiences.

Yes.


>
> 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.


>
> 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.


>
> 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.


>
> More questions:
>
> 1. What are the thicknesses of the sounboards of pianos produced
nowadays?

It varies. Typically 8 to 9 mm. Some go up to 11 mm. Some are tapered
around all or part of their parameter. This taper can go down to 4 or 5 mm.


>
> 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.

Del