[CAUT] IRe: Unison drift (was Reading low humidity/seasonal SB failure)

[Fred Sturm]

Hi Greg,
    When I first noticed this, I suspected tuning technique, sequence,
whatever. I experimented in many ways. Initially I tuned completely aurally,
completely strip muted. I pulled unisons in different ways, sometimes all
right strings from the top (pulling the mute one at a time), then left
strings from the bottom. And other permutations, particularly when I
switched to SAT about ten years ago (and was able to quantify it). I have
tuned left string first, right string first, middle string first. Doesn't
matter. I have convinced myself over the years that I can actually do a
solid tuning, but that severe RH change will affect my unisons.
    Now the extreme case I mentioned, on Yamaha G-2s, happens in the late
summer, after a rise of a good 40%, with the average pitch of the piano at
+25 - 40 cents. Far more common are more subtle changes. But it was the G-2s
that really got me thinking about it, making notes to myself, documenting.
    I now use RCT, and it is very convenient for observing this phenomenon.
In pitch raise mode, simply read left and right strings alternately, unison
by unison, as you tune upward (or whatever direction). Then go back and read
those numbers, and see if you have a saw tooth pattern for the most part.
Sometimes it's subtle, just a cent or two on average, other times it is
quite striking. 
    Again, I don't claim to have reliable answers to why, but I do have a
pretty good wealth of years of observation. I'll throw out another couple
things I have observed, having to do with pitch change in response to RH
along the treble bridge:
1) On most pianos, the typical pattern has a large change at the bottom of
the treble bridge, lowest plain wires, which decreases up to the treble
break, then just above the break there is a big leap of pitch change.
Strangely, Hamiltons do something different. Same large change at the bottom
of the bridge, tapering to about midway towards the break, but then pitch
change increases again approaching the break. Above the break, pitch change
is suddenly less.
2) High treble varies a lot from model to model. On Hamiltons and some
Steinways (especially 1098s) the top couple octaves tend to be the furthest
off pitch, maybe 40 cents sharp where the tenor was 25 cents. Yamahas tend
to have more movement elsewhere (tenor, just above treble break), but the
high treble is closer to pitch than other areas.
    I suppose there must be reasons, but I haven't found them. But I know
what to expect, and it makes my life seem a little more rational, less
random, I guess.

Regards,
Fred Sturm
University of New Mexico


On 3/7/06 8:29 PM, "Greg Newell" <gnewell at ameritech.net> wrote:

> Fred,
>        First I'd like to thank both you and Ron
> Nossaman for your patience in explaining why you
> fret as much over the humidity changes as you do.
> It does seem that you can make rather astute
> predictions in the resultant outcome of a
> seemingly minute climactic change. Admirable!
>        I have to wonder about why I don't see
> anywhere near the left, center, right string
> discrepancies that you do. While the pianos in
> the university that I serve do go rather
> drastically out of tune with the change of
> seasons (abominable building) I still don't see
> the unison problems that you do. Those kinds of
> differences would make the pianos sound rather
> hideous wouldn't they? I may get that much rise
> and fall but it's mostly together over the three strings of the unison.
>        I cannot dispute your observations about
> the bridge pins but I also have to wonder if
> tuning sequence plays some part in it or not. Do
> you use any electronics? If so how do you use
> them. If the piano is in a practice room would
> you ever feel comfortable enough tuning from the
> bottom up by machine and then checking the end
> result aurally? Seems I remember that Al
> Sanderson maintains that better stability is
> achieved this way. Also, how are these strings
> rendering across bearing points? If you tuned
> with a temperment strip and then tuned in the
> outside strings after setting bearings then you
> could be setting up different tensions in the
> wires even though they sounded fine when you were
> done. This could be especially prevelant if the
> strings weren't rendering all that well.
>        I'll quit while I'm behind here and
> patiently allow corrections from the list. Thanks fellas (and ladies too).
> 
> best,
> Greg
> 
> 
> 
>