More heresy

[Robert W.Hohf]

I said: 
> As long as we are debunking dogma, how about this:  loading a soundboard
> does not simply put soundboard into compression, as has been implied in
> several recent posts.  I am no mechanical engineer, but I have questioned
> several over the years on this topic in order to understand some observations I
> have made on pianos.
> 
> In order to put an arched system into compression, the load must be
> _uniformly distributed_ across the arch.


      DDF said:
Not at all. It can be rippled like a potato chip and still be under
compression. Remember it started out under compression. It was
compression that formed the arch in the first place. That compression
doesn't go away just because the panel warps in funny ways.


My reply:
It sounds like you are not disputing my description of the way downbearing
 changes the shape of a crowned soundboard, but rather the way that the force 
of downbearing effects the stresses that are built into an unloaded soundboard.  
I wouldn't compare a soundboard to a rippled potato chip since the shape of the 
chip is made with a rippled cutter.  Even though it may take on crown during 
cooking, there is no stress involved in maintaining the ripples.  Most people 
seem to agree that the equilibrium shape of an unloaded soundboard is 
maintained by a balance of the stresses between the ribs and soundboard 
panel.  A rib has a gradient of stress through its thickness which goes from 
compression on the bottom, through zero stress somewhere in the middle, and 
to tension on the top.  The soundboard panel also has a gradient of stress from 
greater compression on the bottom to (perhaps) lesser compression on the top.  
I have expressed the belief before that the stress gradient in the panel may go, 
at least in some cases, from compression on the bottom, through zero, to 
tension on top.

In any case, I believe that a change in shape represents a change in the 
distribution of the stresses:  the stresses in the unloaded soundboard are not 
uniformly distributed, and loading the board changes the stresses into a pattern 
which is less uniform and more localized.  The stresses don't "go away", but 
they are redistributed by the downward force applied to the bridge.  I suppose it 
is possible that somewhere there is a  a "rippled" soundboard, where the panel 
is all under varying degrees of compression.  But I think it is more likely that 
this shape contains localized areas of tension.  Again, cracks mean tension.  
And not all cracks result from compression failure.

DDF:
Yes, the movement of the piano soundboard is a highly complex subject.
Much to complex to go into here. In case you, or anyone else on the list
is interested, most of what I have to say on this subject appears in two
separate articles that were published in the February, 1995 and the
February, 1996 issues of the PTJ.

Reply:
I read and enjoyed your articles.  But I didn't intend to refer to a vibrating 
soundboard here.  I was referring to the static stresses and static shape of a 
soundboard at rest.

Bob Hohf
Wisconsin