[pianotech] Measuring Crown Radius

[David Love]

Not wasted, it's allowed me to formulate more clearly many of my questions
and some of the answers.  For that I'm appreciative.  You could work on
style though FWIW.  

 

There is a practical side to this.  As I said, I believe the differences
between CC boards and RC&S boards are, on a practical level, one of degree
not of kind and the differences are characterized by where the bulk of the
load is focused.  On one far end of the continuum is a complete compression
system in which there are no ribs at all.  The crown is formed as I
mentioned by drying the panel down and gluing it into the rim and allowing
it to expand.  Any load then that's put on the board via the string scale
will be supported by nothing more than the compression of the panel.  Not
very practical for obvious reasons but that's a pure compression system.  On
the other end is the pure RC&S system which calculates the rib scale as if
the panel isn't there at all and the ribs are designed to take the entire
load of the string scale.  I've heard it expressed that way on more than one
occasion.  But there's a problem.  The panel does go there eventually and
will contribute to the load bearing properties of the system unless you do
one of two things: not include the panel (can't do that) or minimize or zero
out the load.  Because as soon as you start to load the board you start
introducing panel compression and it now becomes a factor in the overall
stiffness and impedance characteristics.  Very quickly that system will
become too stiff and will have all the acoustical characteristics that go
along with that.  The rate at which the stiffness of the increases in
stiffness may differ (you've talked about linear versus non linear systems)
but linear, in my view, is not the case.  The systems are the same.  The
rate of deflection looks something like this.  A system that starts without
compression simply starts farther left on the x axis and the initial rate of
change is greater, more linear if you will, than one that starts already
with some compression.  But I digress.

 

cid:image007.png at 01CC376C.19134270


 

So the next question is how important is loading the assembly.  I've heard
it argued that in this system it's not that important.  A very light load is
ok.  Well, I would say that a very light load is not just ok, it's
necessary, but it's not without tonal consequences.  If you believe that
loading the board is necessary for effective (acoustically effective that
is) transfer of the string load to the panel load then you would be SOL.
Further, if you believe that loading the board in order to maximize the
potential energy in the system (something that's been discussed) is
necessary you would also be SOL.  If you don't believe that or are willing
to live with the acoustic outcome of a relatively unloaded board (or a board
with very high impedance characteristics when loaded) then fine.  I'll give
that that's an esthetic choice.  For me personally, having built boards like
that, I don't prefer it but I accept it as an option.  

 

Now those are two extreme examples that in the real world don't happen
(hopefully).  In fact, for most soundboard builders (that I know) there is
some attempt to balance the amount of load bearing support of the rib scale
along with a manageable amount of compression, commonly in our discussions
referred to as a hybrid system.  My point is that in practice all systems
are hybrid systems.  But the emphasis on focusing things at one end of the
continuum rather than the other forces certain other choices that affect not
only stability and longevity, but the tonal envelope itself.  I am more and
more inclined to think that there is a somewhat narrow sweet spot in which
those factors are in balance and discussing the issue as if it's one or the
other and neither the twain shall meet neither represents the reality,
practicality or the broader acoustic esthetic that gives us some leeway to
manipulate the tone in one direction or the other with some guarantee of
success.  

 

 

David Love

www.davidlovepianos.com

 

 

-----Original Message-----
From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of Ron Nossaman
Sent: Thursday, July 14, 2011 10:13 AM
To: pianotech at ptg.org
Subject: Re: [pianotech] Measuring Crown Radius

 

On 7/14/2011 10:15 AM, David Love wrote:

> You haven't

> adequately answered the question, at least not to me and I am open to it.

 

Then the failure is mine. Since I'm obviously not able to get across to 

you one extremely simple mechanical concept on the fundamental 

difference between the function of ribs in CC and RC&S boards, I'll quit 

wasting your precious time trying and you can find for yourself a more 

capable teacher who can.

 

My apologies for your wasted efforts so far.

 

Ron N

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