[CAUT] pre-stretching new string?

[Ron Nossaman]

>     Also, part of Darrell's thesis was to "prove" that bridge rise (caused
> by soundboard expansion - increase crown) couldn't account for pitch rise,
> using calculations. He came up with a calculated bridge rise of .5 inch to
> account for observed pitch change. There were typos, and, I think,
> miscalculations. I think the calculations based on the distance between
> tuning pin and hitch pin (based on changes in deflection) are far more
> reliable and closer to real life. My own empirical experiments showed that
> 0.1 inch rise in string deflection would yield a pitch rise of 15 - 30
> cents.

Hi Fred,
On that A-4 I posted earlier, my calculations show about 23 
cents with a 0.1" rise. That would take a 1° downbearing angle 
to 2.5°. This just doesn't happen in real pianos with positive 
bearing and strings on them, so soundboard rise and fall can't 
be the major cause of tuning instability. As far as I'm 
concerned that is conclusive.


> Yes, I understand that. And your thoughts about bridges, bridge pins,
> angles, and so forth are excellent and make a whole lot of sense. I'm just
> balking at your more "absolute" seeming statements, like
>> "Hasn't it already been determined fairly reasonably that soundboard rise, or
>> fall, isn't a significant (there's that word again - not absolute or
>> exclusive, even if measurable) factor in pitch change?"
> I'm perfectly willing to go along with a statement that probably bridge
> changes are actually more important than soundboard rise or fall, but I'm
> not willing to discard rise or fall as not being "significant" until I have
> seen that proved in some fashion. 

Then measure deflection angles from season to season and see 
if they change even 0.2°, much less 1.5°. I'm really not 
concerned if you're convinced or not. I am, until somebody 
shows me different.


>All right, "significant" is an inexact
> word, but your own calculations show that a .6 degree change in downbearing
> (soundboard rise/fall, perhaps) is equivalent to your measured and
> documented .2 mm expansion in bridge. Now, you say you have never measured
> more than .5 degree, but wouldn't that .5 degree change be "significant,"
> nearly as significant as the .2 mm bridge expansion? Maybe it is clear that
> the bridge contributes more than the soundboard, but the soundboard may
> still contribute a good bit. Could be 60/40. Or 70/30. I'd call the 40 or 30
> significant.

I never measured a 0.6° angle change, or anything anywhere 
near half that, and that's the point. As far as I know, it 
doesn't happen. When it starts showing up in pianos, I'll 
believe it, and not until. Yes, I measured a 0.2+ mm change in 
bridge pin height above the cap. That gave me the length 
changes in the string path between the pins. I also measured a 
0.56mm increase in overall bridge height, which would increase 
the bearing angle on that A-4 by 0.47° if the soundboard was 
concrete and didn't move at all. In real pianos, the bearing 
doesn't increase like that, so if anything, bridges swelling 
and getting taller are pushing soundboards *down*, or we would 
be seeing big bearing increases in the wet seasons.


>     My experience tells me the grands at UNM with dry humidistat and a 50
> plus a 25 watt rods change pitch about half or less as much as those
> without. Lids are almost always up. Air is in constant motion from the HVAC
> system. I'm real skeptical that enough moisture is pulled from the bridges
> into and through the soundboard to overcome what is being absorbed into them
> from the air. Or that the heat from the rod raises the temperature of the
> bridges through convection enough to do the same. Maybe it's true, but my
> common sense notion is that most of what I am affecting with my rods and
> humidistat is the board.

Fine, be skeptical. The math says no, and I don't have more 
detailed answers for you. Let me know when you have them for 
me and I'll ask the questions.


> Very true, and I a pretty well convinced by your writings that the reason DC
> systems work less well than expected often is that the bridges are exposed.
> I haven't got around to fooling with top covers yet. I have, so far, found
> bottom covers less effective than I would have liked to expect. Effective,
> but somewhat marginally in many cases. I'm wondering if I can work out a way
> to manufacture string covers using material similar to bottom covers without
> overmuch time and hassle. One of these days . 

Actually, I meant real covers, like the pianos I have in 
churches that are covered all week, and only opened on Sunday. 
I don't have a piano with a string cover and Dampp-Chaser to 
compare to anything, but it does seem like it would help. Then 
there's the fun of looking for or continually replacing the 
missing string covers in a school, so that might not be such a 
good idea after all.


> Thanks for your thoughts.
> Fred

One more. Again, on that A-4, a 0.025mm (0.001") change in 
string length will change pitch six cents. Cy, Do pinblocks 
change dimension? Find out. Take a 15cm (6", or narrow enough 
to get your calipers on) piece of pinblock, especially a 5 
ply, since it'll change more than a 73 ply. Cycle it slowly 
between 5%MC, and 10%MC, record the extremes in dimension, and 
estimate what some portion of that will do to the string lengths.
Ron N