Pitch Raising

[Dean L. Reyburn, RPT]

Alan Leigh writes:
>My experience has been that tuning the base strings has a greater
>effect on the rest of the piano than does tuning other sections.
>I've measured the effect on my temperament of tuning the base vs.
>tuning the rest of the piano, and the base changes the temp. by about
>30% while the rest of the piano has very little effect on the temp.
>(one nice reason to have an electronic tuner is so one can make
>objective measurements like this).  Thus, by tuning unisons as we go
>and doing the base first, we get this "greater effect" over at the beginning,
>and the rest of the tunings are more stable.

Yes Allan!  There is a name for this effect, coined by Dr. Sanderson in
his classes on pitch raising: "Pre-Drop".  There is also an effect he
coined "Post-Drop".

Post-drop is the effect we all know, where the notes we just raised a
minute ago go flat as we raise neighboring notes.  Pre-drop is the same
thing _except_ it is  happening to the notes not yet tuned.  When you
pitch raise tuning unisons-as-you-go, the pre-drop is maximized.  When
you tune up the scale chromatically, the notes just ahead of you that
haven't been tuned yet, drop in pitch even before you get to them.  If
you remeasure and re-calculated your overpull often, you will
automatically compensate for the pre-drop.

If anyone doubts this, try this experiment:  on a piano that is say 50
cents flat or, measure the pitch of middle C (C4) with a electronic
device.  Now pull the strings up to pitch starting with the lowest A up
to the B below middle C (B3).  Use whatever overpull you would normally
use, or use none if you want.  When you get to B3, and before you tune
C4, measure C4 again.  You will find it to be MUCH lower in pitch than it
was when you started pitch raising.

But the question is, why do pianos go flat when we pitch raise them?
Some have pointed to bridge roll.  Dr. Sanderson set up an experiment
that would detect bridge roll if it occured, but the results showed no
significant bridge roll at all, or was inconclusive.

In my opinion, the real reason is that the whole piano compresses under
the increased tension, which in turn releases some of the tension that
the tuner just added.  I don't know what compresses more, the plate, the
piano back, the rim, the sound board crown or some other part.  I suspect
all of these contribute to the effect.

In any case, I have noticed that pianos with weak looking plates and/or
support structures generally need more overpull to end up at A440.  Or to
put it another way, if that spinet is 100 cents flat, I will probably
have to make two passes at pitch raising it to avoid over-pulling too
much.

I have talked to several skilled RPT's recently who will not pull bass
strings or any wound string above pitch, thus minimizing string breakage.
 I will pull wound strings above pitch on newer pianos, especially ones
on dealer showrooms where they come in real flat, but I usually limit the
overpull to 10 cents (per pass) maximum even then.  For plain strings, I
limit the overpull to 25 cents above A440 for older pianos, and 30 cents
for newer pianos.

If the piano needs more overpull than the above, I find I am probably
better off just pulling it to A440 in the bass, and A442 (+ 8 cents) in
the treble for the first pass and then customizing the overpull on the
second pass.  (Assuming the third pass is the final tuning)

I welcome comments as to what other tuners consider the maximum safe
overpull, or what overpull percentage they use in the different sections
of the piano.

Dean


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Dean L. Reyburn, RPT                    Sanderson Accu-Tuner
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