[CAUT] Wire Stretch

[RicB]

Hi Fred.

I have never seen a piano behave like this in 30 + years of working on 
them.  There is always a major shift at each break in the scale. Most 
times one can even see a slight jump even when the scale is 
broken/widened at the front termination and not at the bridge.  I've 
watched pianos very closely for a long time relative to climatic 
changes, and what I see is a similar type of movement in each section.  
If the lowest part of the tenor is sharp relative to the rest of the 
tenor section... then you see the same thing in the tenor/treble 
break... and the treble/diskant break (up to the last highest octave)  
and in the bass section.   Same thing in reverse if the low tenor has 
become low with respect to the rest of its section.  The pattern is 
nowhere near as severe in the bass or highest treble (diskant)... but 
its there.  I dont think I have ever seen an exception to this <<rule>>. 

The only part of the piano that doesnt seem to fit into this is the 
highest octave...especially from F-G7 upwards.  Seems to do what the low 
tenor is doing and seems to have a very large reaction.

There is another problem with the idea that bridge/soundboard growth or 
shrinkage can be so very much involved.  The amount of downbearing 
change required to account for any significant pitch change is just too 
high to account for more then a small amount of vertical deflection 
change. If you start off with a say half degree string deflection in the 
tenor and graduate to 1.5 degrees in the treble as a starting point... 
and start figuring out how much vertical change is needed to account for 
pitch changes and then look at resultant downbearings.... it gets scary 
real quick.  Even with the change in length across the bridge surface 
that would happen as a result of a change in string offset angle through 
the bridge pins when the string  climbs up/down the pins.

I posted a small calculator that does shows rough results for a single 
string a while back.  If anyone wants to take a look at what one has to 
accept as <<doable>> for a soundboard in terms of vertical deflections 
for pitch changes... drop me a line.

Cheers
RicB


     > What is problematic about
     > this is that any such change in string position height to the
    bridge pin
     > is governed by the response of the surface of the bridge to climatic
     > changes, and hence quite uniform... which in turn dictates a pitch
     > change that is directly related to the length of the string. Shorter
     > strings will be more effected then longer strings

        Yes, and that does happen on some pianos (response to humidity
        change very
        large in the top octave or two). Steinway and Baldwin products
        come to mind.
        Not so on other pianos, as Yamahas for instance. I haven't a
        clue why there
        should be that difference between makes, but I do always know to
        anticipate
        that enormous pitch change in octaves 6 - 7 on Hamiltons and
        1098s (and
        Everett studios), and to a lesser extent on the grands of those
        makes. We're
        talking within 20 cents in the rest of the piano, and 30 - 50 in
        that top
        octave and a half.


    ... hence a very
     > graduated and even curve of change in pitch.... which clearly
    does not
     > happen. We see big breaks where the bridge is doglegged which
    need to be
     > accounted for amoung other things.

        Yep, lots of strange things happen, and lots of factors are
        involved. I have
        become more and more convinced that bridge growth and shrinkage
        in all
        dimensions (height, width _and_ length) are quite significant,
        sometimes
        more than soundboard. This could explain why some pianos just
        don't seem to
        want to stabilize as much as others with a Dampp-chaser (excuse
        me, ahem,
        Piano Lifesaver) system installed. A thought, anyway.
            So many factors, including lamination/solid (bridge and board),
        compression/rib crown (and does it still have crown?
        Downbearing?), rib
        cross section, age of wood (how hygroscopically active is it?),
        orientation
        of grain, fineness of grain . . .  And then you add the wire
        factors.
            We will never really get a handle on the inter-relations of
        all these
        factors. I'm not saying it can't be done, but nobody is going to
        go to that
        much trouble. We might tease out a few things, though. Ron's
        epoxy laminated
        bridge, for instance, and how much of a difference it makes.
        Regards,
        Fred Sturm
        University of New Mexico