Unison Flatter than each Individual string?

[baoli liu]

The reason comes from the character of soundboard.

Usually,the string try to move the board via bridge
and play a absolutely dominant role for the vibration
system(the string,the board,the case,etc').at the same
time the board has a feedback to the strings.Tha's why
we tuners tune the string-tension, instead of
chiseling the soundboard(G).

Because three strings have more power to shock the
board(if one tune the union very well,so the strings
would cooperate together) than a individule string,the
pitch have the tendency to go flat if three string
vibrate together in the same (or very close ) phrase.I
tried to detune union and hope to see pitch raise(or
less pitch drop).I am not very good at fine
measurement,it seems the pitch will drop less after I
detuned some unions by a few beats.

Now let's think about why pitch could go sharp
occasionally.like string,the board has his own
"partials" which are called vibration modes.the modes
of the board could have a strong feedback influence if
the modal frequency of the board is very close to a
string partial,this could cause a very noticeable
"pitch drop", or "pitch raise" depends if the modal
frequency of the board is a little higher or lower
than the equivalent string partial.

I believe the right explanation should be approved by
experiment.with out any experiment,here is my
assumption for the phenomone:
one modal frenquency of the board is just below a 
string partial you are measuring,one string has the
tendency to raise the soundboard modal frenquency
because both have to vibrate together,but one string
is weaker that two/three strings.so,if three string
vibrate together,the string will force the board
vibrate a little quicker than it's original modal
frequency.

if you shift to another partial(meaning you measure a
different partial of the same note)the rare phomonene
will probably disapear.Or if you lower the pitch a
little bit the phomomee should disapera also.



It's my assumeption only.

Baoli

--- Joseph Garrett <joegarrett@earthlink.net> wrote:
> I have seen the phenomenon of two/three strings
> being "flatter" then each,
> individually. And, I accept Dr. Sanderson's
> explanation of the reason for
> the effect, more or less. However, I have also seen
> a reverse phenomenon of
> the unison being "sharper" then the individual
> strings! Would someone like
> to advance a theory on this????
> Best Regards,
> Joe Garrett, RPT, (Oregon)
> 
> I'm never lost, because everyone is telling me where
> to go. <G>
> 


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