Soundboards/stress

[Richard Moody]

----------
> From: Kirk Beasley <Kirk_Beasley@msn.com>
> To: pianotech@ptg.org
> Subject: RE: Soundboards/stress
> Date: Saturday, June 28, 1997 11:20 AM
> 
> Jim,
> 
> I've been intermittently following this thread (as I have been
moving) but 
> there are a couple of minor details that I would like to mention. 
First, In 
> 1978-9, I built (per Harold) a prototype router jig for the SD-10
and the 
> SF-10.  It's purpose was to allow the underside of the treble
bridge to be 
> contoured to the soundboard to "optimize fit."  While the jig did
its job 
> well, I remember the result was insignificant in terms of the sound
of the 
> piano.  Second, if one were to do a contour map of a given grand
soundboard 
> the non-symmetrical, asymptotic nature of the "curvature" would be
apparent.  
> Soundboards are not like the tops of violins or guitars because
they are under 
> so much more bearing force by the 200+ strings.  I think that
issues like 
> impedance or "stiffness" are central to this sort of discussion,
unless I have 
> misunderstand the direction of the questions posed.  
> 
> Kirk


Interesting point about the contour of a piano sound board.  The
bridge can't ride on top of the curve or arch or crown, at least not
all of it.  The crown puts tension in the wood, that tension somehow
amplyfies sound, not quite scientific words, but the sound board is
given a crown because a crowned soundboard gives a better sound.  
Another consideration of adding tension to wood is the undercut
bridge as seen in the top treble of some pianos.  The string pressure
here will tend to "tilt" the bridge, this "tilt" as long as the glue
holds certainly must put more tension on the wood of the sb.  Why it
is undercut toward the tail of the string instead of the speaking
length, is something to ask during a factory tour.   

Richard The Curious 

>