more heresy

[Richard Moody]

If I may pose this senerio, no I don't mean "foist"; a sound board is
designed to resist all of the tendencies Robert has listed below. 
Think of arched systems  under compression with as uniform load as
possible as elastic medium, and one that must vibrate as freely as
possible with out the distortion of "rolls", "valleys", "bulges", and
"waves".  In other words it must resist these effects which by
deduction would intefere with the soundboards ability to vibrate
freely, to produce musical tone,  or at least vibrate according to
best intentions of craftsmen and engineers.  True a soundboard may
exhibit some or all of these complexities but they are encoumberances
rather than enhancements, something the soundboard is designed to
resist, rather than accomodate. 
	I think (guess) it is the idea of front bearing on a bridge that
causes some sb to exhibit a roll which is an indication of failure at
least in the eyes of the designers. But it is this additional tension
put on the wood of the sound board that further "amplifies"the sound
just as the bending of wood to attain crown also "increases" sound.
It should tread the thin line of producing tension, but not producing
a "roll". 
	I know the word "amplify" is objectionable to some, but when a
tuning fork is placed on a straight piece of wood and sounded, and
then that piece of wood is "bent" (crowned) and then the tuning fork
sounds louder, I don't know what else to call the phenomenon.  

Richard Who's on Foist

----------
> From: Robert W.Hohf <rhohf@idcnet.com>
> To: 'pianotech@ptg.org'
> Subject: RE: more heresy
> Date: Tuesday, August 19, 1997 6:46 AM
> 
> 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.  This is not the case in
pianos.  If we 
> consider a cross-section including rib, board, and bridge, the load
applied to 
> the bridge is a _point source_.  This type of loading does not
produce 
> simple compression.  It causes the bridge to sink into a valley
with bulges on 
> both sides.  The greater the loading, the deeper the valley and the
more 
> pronounced the bulges.
> 
> Of course, real soundboards are not cross-sections, but have an
extremely 
> complex shape in 3-dimensions.  Real soundboards can have bulges on
either 
> side or both, and the side of the bulging can change in different
areas of the 
> same piano.  This sounds a lot like "bridge roll", doesn't it?  Do
we need 
> another explanation of bridge roll?
> 
> This valley/bulge creates a "wave"  shape in the board.  The
resulting pattern of 
> tension and compression is complex.  Sometimes it can cause a
charateristic 
> pattern of  compression failure and cracks.  It can cause
paradoxical cracks on 
> the bottom of a soundboard.  It can also make measuring the crown
on the 
> bottom of a board inaccurate.
> 
> Bob Hohf
> Wisconsin
> 
>