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

[Fred Sturm]

On 3/7/06 7:32 PM, "ed440 at mindspring.com" <ed440 at mindspring.com> wrote:

> Fred-
> 
> We wrote about this at great length to no conclusion a few years ago.
> 
> What always strikes me is that the outer strings of the unison create a
> trapezoid with the bridge and V-bar.
> 
> If you can imagine the bridge moving a slight distance laterally relative to
> the plate, then one outer string would have increased tension, and the other
> lessened tension, as the angles and lengths of the long sides (strings) of the
> trapezoid shifted.
> 
> Can anyone imagine a humidity-induced change in the case, soundboard or plate
> that would cause a lateral movement of the bridge?  If so, then maybe we got
> it!
> 
> Ed Sutton
Hi Ed,
    Yes, this is certainly a possibility. Not lateral movement per se, but
"diagonally lateral" movement, on the assumption that the SB swells cross
grain, and there is more of it on one side of the bridge than the other.
Hence, the bridge will move relative to the plate one way or the other, but
at about an average 45 degrees relative to string direction. Still, there is
a lateral component. That geometry, together with bridge swelling/shrinking,
together with possible subtle "canting" of the bridge (what you might call
roll, but having to do with how much of the crown is in front of the bridge
versus behind it). In my own visual imaging of these effects, though, I
haven't come up with a mechanism that corresponds to the direction of pitch
differences other than what I described.
Regards,
Fred Sturm
University of New Mexico