RC vs CC again

[Richard Brekne]

Delwin D Fandrich wrote:
> 
> 
> >
> > 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.
> 

It appears like they cant have all the same physical characteristics
period. Somethings always got to be different, simply because ones got
compression forceing crown, and the other has ribs doing it.

> 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.)

I'll buy the short period of time bit. But like you say.. the cc board
is a moving target... there is something else that causes me one of my
headaches.. grin.. but I havent got it exactly clear yet what it is.. so
I'll hold off on that..

> 
> 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 .

Well.. not in the case of identical panel thickness, identical rib
dimensions, same mass, and identical crown.  The CC board simply has to
have more stiffness as long as all that remains the same... yes ?

Beyond that... its open season as it were...

> 
> 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."

I got that bit... thats been clear for about a year now.... it was just
this identical bit that came up... seemed to different things said by
different people... chalk it up to slight problems in communication :):)
 
> >
> >
> > 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. 

Ok.. thats what I thought. Makes perfect sense too.

> 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? 

No ... not want I want.. just wanted to know what could and couldnt be
done in this regard and howcome as it were. 

> 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.

Yes.. but we all know this, and its not what I was after... indeed gets
into other interesting and important aspects of the general theme. I was
just zooming in for a bit on the identical dimensions bit, as it relates
to the slightly more general question of duplicating the sound
characteristics of a CC board with an RC board. 

Thanks for your thoughts Del.

> 
> Del
> 

Cheers
RicB

-- 
Richard Brekne
RPT, N.P.T.F.
UiB, Bergen, Norway
mailto:rbrekne@broadpark.no
http://home.broadpark.no/~rbrekne/ricmain.html
http://www.hf.uib.no/grieg/personer/cv_RB.html