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

[Ron Nossaman]

> The only difference I can see between the three strings is the
> placement of the bridge pins (nearness to front edge) and the differences
> in the backlengths. As a totally uneducated guess, I'd choose the backlengths
> as the culprit, especially in the capo sections.
> 
> Does anyone know about this, instead of just speculating?
> 
> Susan Kline


Know, as in proving beyond a doubt to everyone on the planet - 
no, no one does. From available evidence though: You have 
overall string length, front bridge pin to tuning pin length, 
and various friction points. With humidity changes, speaking 
length pitch changes from bridge and pinblock movements 
reflect front bridge pin to tuning pin length differences 
first. Back scale and overall string length come in somewhat 
later because the friction at the bridge pins is greater than 
that at the capo (or whatever), and segment tension 
equalization lags behind there. Meanwhile, before the system 
stabilizes, the humidity is changing in the other direction, 
and it's going back to or past where it just was, confusing 
the neat and  tidy indicators you thought you had by listening 
to the speaking length pitches. Bottom line is that until you 
have a way of measuring every variable in the system, both 
what they were then, and what they are currently, it can't be 
explained simplistically.
Ron N