[impedance]

[Marc Damashek]

Don <drose@dlcwest.com> wrote:
> Hi all,
> 
> I think I can safely say a sound board is *none* of the below. *grin*
> 
> impedance (from http://www.oup.co.uk/isbn/1467-3894/ )
> 
...

Don:

Ouch -- yes, if the string coupling to the soundboard changes significantly,
then the *amplitudes* of the various partials will change also. I doubt that
their *frequencies* would change significantly (apart from second-order
loading effects that change resonant frequencies due to changes in damping),
because the frequencies of the partials are essentially determined by the
string itself -- normal small-amplitude vibrations of the soundboard are not
big enough to give rise to spurious frequency components (although buzzes,
etc. are nonlinear effects that do; let's leave the basket cases aside for
now). This would affect the weighting scheme that I outlined, and it remains
to be seen experimentally how important it turns out to be.

Apart from the electrical applications, impedance is also a legitimate 
mechanical and acoustic characteristic (refs. include Morse and Ingard
(Theoretical Acoustics) and Beranek (Acoustics)). It refers to the frequency-
and position-dependent ratio of pressure to velocity in the system under
study. Audiologists often measure the acoustic impedance of the eardrum
directly with a special instrument, and use the results to diagnose various
conditions, including fluid in the middle ear and more serious abnormalities.

-- Marc Damashek
Hampstead, MD

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