Seasonal pitch change:

[RicB]

Andrew, Les... whomever else

Many folks have observed that unisons seem to show a pitch change 
pattern depending on the season. The left string is either highest pitch 
or lowest and the right the opposite with the middle somewhere in 
between. Sometimes it looks like either the left or the right string is 
more affected then the other two that seem close to the same pitch, with 
the pattern being otherwise the same.  I believe this happens when a 
piano has experienced a season change after having been given a very 
stable tuning. The most stable tuners I know see this kind of thing all 
the time.

The fact that this happens points me in the direction of the bridge 
itself tilting along its curve.  Given the kind of increase in 
downbearing pressure any attempt by the soundboard to rise vertically 
results in, it seems likely to me that the bridge would tilt in the 
direction of the edge with most downbearing pressure on it during 
periods of increasing humidity. That could account for some of the 
uneven change in unisons that fit this pattern. It could also contribute 
to the overall pitch change. A 40 mm tall bridge tilting 1 degree would 
alter the string length by 0.7 mm without altering tension.  Thats going 
to affect pitch big time in short strings... not much in longer strings.

In the long tenor strings, where we see at the end of the bridge usually 
the largest pitch change neither speaking length change or deflection 
change seems likely.  Tension has to change somehow. To get 50 cents 
just from tension change you need about a 10 lb change in tension.

I havent looked yet at how much tension change would be caused by a 
change in the relative position of end points (hitch pin and tuning 
pin).  sigh... more to look into :)

Cheers
RicB


    Les,
    I'd have tended to agree but the pattern repeats in all different
    types of pianos and in the same way for different people.  I think
    there is a smoking gun here.
    Andrew

        At 10:00 PM 3/7/2007, Les wrote:
         >Do people notice tiny discrepencies of lever pressure on tuning
         >pins?  We never can get all of them perfect. My suspicion is that
         >some of the unison problem is simply the fact we are never
        perfect
         >tuners, and as the piano "flexes", when it settles back those
         >discrepencies "come to the surface".......  There are so many
         >factors at work in tuning 3 strings together..... It's a
        miracle, to
         >me, anything stays.
         >les bartlett

            Andrew wrote:
             >I've been suspicious of soundboard deflection being entirely
             >responsible too, for different reasons.  Thanks to Ric we
            know we
             >need to look elsewhere.  I've also noticed a pattern to
            seasonal
             >pitch changes that involves the unisons, especially the
            three string
             >unisons.  It usually don't go out entirely the same way. 
            Sometimes
             >in opposite directions.  Think about it.  There's not a
            whole lot of
             >board and rib to push up or sag down, but there is a lot
            of board in
             >the horizontal positions.  I wonder how much bridge
            movement back
             >and forth we get with hydration and dehydration?  By the
            way, the
             >pattern is the same on uprights too.