[CAUT] Wire Stretch

[Fred Sturm]

On May 3, 2007, at 11:00 AM, RicB wrote:

> 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.

Hi Ric,
	Well, I have observed this repeatedly on the Hamilton 243s, Everett  
studios, and a couple Steinway 1098s I service regularly. There is a  
gradual increase in the pitch change from about C6 (which is at about  
"high average" pitch), rising logarithmically to C8. I have found as  
much as 75 cents on the top few notes, with tenor change being in the  
25-35 cent range. Of course, as I wrote earlier, this seems to be  
"model specific" to a large extent. I don't see it at all in Yamahas,  
for instance, where often areas in the high or mid high treble can be  
sharp when the rest of the piano is flat and vice versa.
	There's also a quirk to the 243s at the treble break. The pitch  
change accelerates as you approach it from below, then there is a  
sharp jog of less change for the first few strings above the break. I  
attribute it to something to do with scaling and tension, as a guess.  
Virtually every other piano behaves as you describe above, with  
mirroring jogs at each break.
	In observing this kind of thing over the years (getting to be  
decades, now), I have found that the data can often be contradictory  
and confusing (especially if you don't know intimately what has  
happened to RH in the interim between tunings). I have often intended  
to start documenting (as in actually writing things down), but the  
times when the data would be most useful tend to be the times I am  
utterly swamped and can't spare the time. So I rely on memory, and  
make short notes in my work log. For instance, I have recently taken  
to writing "RP 5-25 cents, -50 cents HT" to show that I raised pitch  
in most of the piano 5-25 cents, but the high treble was as much as  
50 cents flat. I have found myself doing this notation a good bit the  
past few years, and wonder why I didn't notice it years earlier. (I  
used to distinguish the bass, but stopped bothering. Actually,  
though, there can be a good bit of variety there as well, from not  
much change at all to quite significant - again, probably model  
specific).
	Until a large range of data is collected, very specific as to model,  
humidity history between tunings, and with samples at a good  
representation of notes, we will be making stabs in the dark. And one  
would also need to know at least something about bearing and crown to  
make it really worthwhile. I'm sure my own particular climate affects  
what I observe, as does yours. I don't get quite the extremes Mark  
Cramer and others in the frigid north get, but I do get single digits  
regularly, and up to 80% RH. 10 - 60% annual swing is pretty average.  
I suspect 40 - 90% would give markedly different results, for instance.
Regards,
Fred Sturm
University of New Mexico
fssturm at unm.edu



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