[CAUT] Detuning phenomenon; was: How long to stabilize??

[Richard Brekne]

If this were true, or at least an adequate description of what was going 
on at the bridge top, then we would see similar thickness's of string 
indentation on the top face of the bridge front to back. But we don't. 
Nearly always there is a much deeper and longer indentation from the 
front bridge pin to the center of the bridge top then from the back pin 
towards the center. So either the back half of this string length does 
not (for some reason) present as much resistance to an encroaching 
bridge material pushing upwards during growth cycles (which makes 
difficult much of the reasoning behind the claim that this same 
encroachment explains how bridge pins seem to loose contact with surface 
of the bridge) or it is not directly this upward push that is involved 
at all.

Alternatively... since it is quite frequent to find more bearing front 
side then back side... any combined upwards force increase against the 
string will be resisted by the string plane accordingly...which means an 
increased strain between the sounboard/bridge and string plane at the 
front of the bridge compared to there back. Given the curved form of the 
bridge one immediately is drawn to some change in the angle between the 
top of the bridge and the string plane as a possible contributing factor.

The reason given I've heard always before as to individually tied 
strings has nothing to do with this. Rather it was related to the idea 
that there was some kind of slippage around the hitch pins. I've never 
heard of a design where back lengths were planned out to be the same 
length... and given the stress placed on so small an area of the 
plate... I have to wonder how long it took for that idea, if it ever 
came up, was dropped.

Cheers
RicB


    The bridge cap gets a large part of the blame, but it's not reacting
    more in one place than another. When the cap thickness changes with
    humidity, all the strings of a unison will change in length by
    (nominally) the same amount. That's not the same percentage, the
    same amount - like 0.001" (arbitrarily). The result will be that the
    shorter string (overall length) will change pitch more than the
    longer, with the one in the middle in between. That's it. Where they
    are at any moment that you happen to measure them depends on the
    humidity conditions at the last tuning, where the RH has been since,
    where it is now, and in what direction and at what rate it's
    changing. You're looking at a snapshot in time of a continually
    changing dynamic system, most of the dynamics of which are not on
    your notepad as you ponder what's happening.

    Pianos having individually tied strings were originally intended to
    have all the strings the same length overall to combat this effect,
    so the unisons would stay closer in tune even though the overall
    pitch would continue to change. And yes, epoxy laminated veneer caps
    do most definitely improve tuning stability.