[CAUT] pre-stretching new string?

[Fred Sturm]

On Jun 7, 2007, at 10:15 AM, Ron Nossaman wrote:

> Bridges are in the Dampp-Chaser environment as well as soundboards.

Hi Ron,
	Thanks for the specifics. They are certainly persuasive on the  
surface, though there are a couple issues I'd have to sort through.  
It isn't necessarily just length change in terms of how far the  
string goes from tuning pin to hitch. There is also a speaking length  
component. The two are related, but one without the other (for  
purposes of analysis) is incomplete. Where the bridge pin ends up  
relative to the front termination is an open question, not one easily  
measured. One of the Fandrich's (I forget which, but think Darrell)  
wrote about pitch change due to elongation of the bridge toward the  
tenor end about ten years ago, citing speaking length change as a  
major factor. I didn't entirely buy the argument at the time, as it  
seemed that increasing the distance from tuning pin to bridge pin was  
causing a rise in pitch (setting aside the tension component). Longer  
speaking length means lower pitch to me, other things being equal (if  
they ever are). Still, the general principle of changes in speaking  
length caused by wood swelling and shrinking makes sense as a factor  
to consider.
	Soundboard movement in response to humidity change isn't just up/ 
down. The position of the bridge relative to the front termination  
may change due to whatever is happening to the board. A bit sideways,  
a bit back or forth, a bit of change in cant. It certainly isn't as  
simple as up and down, increasing/decreasing bearing (and, hence,  
distance from a to b, and, hence, tension).
	You really think the moisture transfer from the bottom of the  
soundboard into (and from) the bridges of a grand is enough to  
stabilize the bridges to a significant degree? That's where my mind  
can't get away from the soundboard being a fairly large contributing  
factor in humidity induced pitch change - the degree to which a full  
Dampp-chaser system, or just a heating rod (or two, with enough  
wattage) and humidistat really take a large portion of the pitch  
change away. I've measured RH on top of the board many times, between  
plate and board (as far under, in as undisturbed area as I can reach  
the instrument), and found a miniscule effect from the system beneath  
(maybe 2% above/below ambiant, rarely as much as 5%). I would think  
there would have to be more moisture transfer to and from the  
relatively static air above the board if it was true that significant  
moisture would migrate through to/from the bridge, and particularly  
to the top of the bridge, where, presumably, most of the effects you  
describe are taking place. That's got me stumped. Maybe you have  
worked through it. It's something that could be set up as an  
experiment and measured, given the right measuring tools.
	
Regards,
Fred Sturm
University of New Mexico
fssturm at unm.edu


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