RC vs CC again

[Delwin D Fandrich]

----- Original Message ----- 
From: "Richard Brekne" <Richard.Brekne@grieg.uib.no>
To: "Pianotech" <pianotech@ptg.org>
Sent: October 01, 2003 2:11 PM
Subject: Re: RC vs CC again


>
>
> I suspected something along these lines Del. Thanks. It's has been
> bothering me ever since it came up a couple weeks back. So the notion of
> two soundboards...one CC and the other RC,  both with exactly the same
> mass, parts dimensions, and crown isnt really going to result in the
> same kind of sound at all... because the CC board will have more
> stiffness... Is that about right ?... and same downbearing applied to
> both will result in a different match of impedances.

I should hope so. It's been written in about 47 different ways. No, the two
soundboard cannot, by definition, have exactly the same everything and
still sound the same. There will be design differences.

This does not mean that the sound produced by two soundboards, one
compression-crowned and one rib-crowned, cannot sound essentially
identical. At least at some point of the pianos life cycle. The rib-crowned
soundboard can be engineered to sound pretty much the way you want it to
sound. The compression-crowned soundboard is a moving target. (Well, both
of them are to some extent, the compression-crowned soundboard considerably
more so as compression-set alters the physical shape of the wood fibers.)

Nor does this mean the compression-crowned soundboard will have more
stiffness. In fact, usually quite the opposite is true. Especially after
the two have aged a decade or two and the compression-crowned soundboard .

Now, repeat after me -- "The ribs used with a rib-crowned soundboard system
can be made as stiff or as flexible as the designer/builder wants them to
be. And the ribs used with a rib-crowned soundboard system can be a light
or as massive as the designer/builder wants them to be. The cross-section
can be varied to suit the designer/builder's idea of what the piano should
sound like. They can be tall and narrow or they can be short and wide. Each
will give a soundboard (hence, the piano) its own unique sound. It is a
choice made by the designer/builder."


>
>
> Ok... so what is this telling me.... that in order to get two panels,
> one a CC
> and the other an RC, to have the same stiffness, then something else is
> going to have to be significantly different about them. How are you
> going to be able to end up with the similiar stiffness to mass ratios
> without either having significantly different crown and / or rib
> dimensions ?

You can't. The ribs are generally going to be somewhat taller and narrower.
Picture the ribs used with a typical compression-crowned soundboard system
turned on their sides and crowned. They may still have about the same mass
but, considered as structural beams, they will be some stiffer. This is
compensated for by having less (if any) compression within the soundboard
panel, hence no system stiffness contributed by any stress-interface
between the panel and the ribs. Even if there are some technical
differences between the two, their characteristics will be close. At least
they can be. But what is it you are aiming for? You say you'd like to
duplicate a compression-crowned soundboard system? But, as I say, this is a
moving target. Nor is it a particularly desirable target in all respects.

For starters, how much crown does that compression-crowned soundboard have?
On which day? What is the relative humidity? Over which section of the
soundboard are you going to measure crown? By nature these things tend to
distort quite a lot. All of this changes through the year and over time.
Even with the best of quality controls no two are going to be alike. Wood
is not a perfectly uniform material, every panel is going to react just a
bit differently. The is going to be a range, that is, the crown may never
start out as more than something or less than something, but it is going to
vary considerably within that range. Then, how long after assembly are you
going to measure that crown? One week? One month? A year? It's going to
change considerably over these time spans. The rib-crowned soundboard
system will remain much more stable over the same period of time.

Del