Soundboard Deflection and Pitch Change / was Downbearing

[Ric Brekne]

Hi JD...

I have a couple 1850's straight strung grands in my care. A Bluthner and 
a Bechstein. Both have the grain angle at about 15 degrees from parallel 
to the long side of the instrument.. roughly perpendicular to the 
bridge. Ribs are roughly parallel to the bridge. In both cases the 
curved edge of the soundboard is screwed into the rim, not glued. And 
there is about 3 mm of space between the edge of the soundboard and the 
rim there. I've supposed this was because of concerns about the 
soundboard swelling cross grain. But upon removal of the Bluthner 
soundboard the screw holes showed no real signs of any elongation.

As far as why a pianos tuning is so effected by climate. I have to admit 
that I'm a bit baffled at this point. One can easily see 50 cents of 
pitch change in some places.  Thats a rather large change.  For a string 
roughly equivalent A4 frequency goes up to around 452. If the string had 
originally 160 lbs of tension at 440Hz, a string deflection angle 1.2 
degrees and was 350 mm long and 1.1 mm in diameter... that would 
increase tension to around 168 lbs. Downward tension would go up by 
about just around 0.2 pounds.  This assumes that the soundboard stayed 
at exactly the same height. But if the soundboard deflected just 0.05 mm 
in response, then the strings downwards force would go back to its where 
it started off, with  the new tension and frequency remaining virtually 
unchanged at 168 lbs and 452Hz... but what caused the change to begin 
with ? And why then does the lower end of the treble behave so 
differently then the upper end of the tenor.. and likewise with the 
lower end of the tenor vs upper end of the bass.

Something sure as hell has to be stretching the strings during the 
summer... and relaxing them during the winter... at it doesnt look like 
it has anything to do with the soundboard moving up and down.

Cheers
RicB


At 9:18 am +0200 17/8/06, Ric Brekne wrote:

 >... I've been under the impression that soundboard deflection
 >changes substantially enough due to climatic changes to account for
 >at least some significant amount of the seasonal pitch changes and
 >tuning instability.  At this point... I'm not so sure about that at
 >all any more. In fact it <<looks>> like the contribution to pitch
 >change from soundboard deflection is nearly insignificant... almost
 >negligible !

Many 19th century straight-strung grands had the grain of the belly
parallel to the lock-rail.  I presume the general adoption of the
board with the grain roughly parallel to the long bridge was due to
the greater stability it gave under variations in humidity.  The
extent to which wood swells _across_the_grain_ with increased
humidity is quite astonishing, and this happens to some degree to
every wooden part of the piano, so that the change in pitch with
varying humidity is due to the sum of all sorts and directions of
movement in the structure, not simply to any minute change in the
angle over the bridge.