Check out Piano Soundboard Movements

[Richard Brekne]

The article is actually quite interesting, and tho I'll need a couple 
more reads to properly digest it there are a couple relevant points 
right off worth mentioning.  It turns out, according to these folks mind 
you, that the soundboard deflects under string bearing in patterns quite 
identical to the those of the lower modes of panel vibration.  They 
claim to be able to identify changes in mode shapes and resonant 
frequencies for changes in string bearing amounts. Further,  they claim 
they are able to measure driving point impedance of the assembled piano 
with relative ease and without an impedance head that simultaneously 
measures the applied force and the acceleration. They use a kind of 
electronic version of the Chladni method which does not require the 
soundboard in a perfectly horizontal orientation. I'll include the 
conclusions paragraphs for your edification:  The whole article is 
definitely worth a read.
Cheers
RicB

Conclusions:

    We have described a method of electronic speckle pattern
    interferometry that not only works with moderate decorrelation
    of the speckle pattern, but demands it. We have
    shown theoretically and experimentally that this arrangement
    can be used to determine the deflection shapes of an object
    that is normally too unstable to observe interferometrically,
    and applied it to the study of a piano soundboard in situ.
    Using this interferometer we have investigated the dynamics
    of the soundboard of a piano and have compared the
    results to a simple closed-form theory, as well as a finiteelement
    model. Comparison of the deflection shapes of the
    piano to those predicted by these models demonstrates that
    the pressure exerted by the strings on the soundboard can
    make significant changes in mode shapes and resonant frequencies.
    The presence of this pressure has a significant effect
    on the lowest modes, but appears not to be important in
    determining the shapes and frequencies of the higher modes.
    We have also shown that this interferometer can be used
    determine resonance curves and driving-point impedance.
    We have presented the resonance curves for the lowest three
    modes of a soundboard and shown that they do not overlap
    significantly.
    We close by noting that the applications of this interferometric
    technique are not restricted only to the investigation
    of piano soundboards. Harmonic vibrations of any unstable
    object that meets the requirements outlined in Sec. II can be
    observed using this technique. Additionally, the theory can
    be applied outside of the approximations if the value of  e in
    Eq. (10) is known.

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