ETDs/unisons/vowels

[Mark Davidson]

If, as one person put it, when tuning a unison, we are 
changing the "vowel" sound, what does this really mean?  

Differences between vowels are caused by different strengths
of various partials, rather than by differences in inharmonicity.  So how
do you change the strength of a partial by making small tension
adjustments?  The only thing I can think of is that there is a range
of tensions where the fundamentals will stay in phase due to
sympathetic vibrations, and within this range, other partials may
be aligned or misaligned, and this alignment/misalignment affects the
net strength of the partial by causing it to be reinforced or not.

I have tuned unisons using ETD so that the fundamentals are well-
matched, and the resulting sound to me is "hot" (particularly if done
across the whole piano), like someone turned the treble knob all 
the way up, emphasizing some high partials (think tape hiss). 

Aurally tuned unisons do not have this characteristic.

So is the well-tuned unison designed to de-emphasize particular 
partials? This would seem to be the case.  

Experiment: tune 2 strings with ETD so fundamentals are in 
phase.  Measure each partial (inharmonicity is as good as 
any, just need some relative pitch in cents). Retune by ear 
and remeasure.  Which partials align? Which don't?  What 
does the spectral display show for the strength of the partials 
when the strings are played together?  In particular, which 
partials decreased in strength?

-Mark