Hi baoli liu I kind of get the feeling that both the soundboards vibrational modes as you point out, and also the condition of the bridge gets into the picture. How springy or massy the impedance characteristics are at the bridge, even before the soundboard modes get into the picture may come into play. For that matter the condition of the bridge pin (whether its tight or loose in the bridge) might play some role. In any case, I cant say that I have been able to observe that string coupling results exclusively in pitch drop. Actually I am not sure that this pitch drop happens significantly more often then other things. On the side, it seems more and more to me that both pitch decay and and whether the pitch rises or falls in coupling also have something to do with coincident partials to the note being played. I find what looks very much like increased sustain especially in the high treble when I use the perfect 12ths tuning. And it also seems to be that when this "extra" sustain is most apparent, the unison displays a slight rise in pitch as as it decays. Weird. But like I say, I believe the whole subject matter is not completely understood, and that like our understanding of just how the soundboard vibrates and other such subject matter, lots more hard research and experimentation is needed. RicB
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