This is a postscript to my posting about organ reed pipe voicing
[121011 MMDigest, "Reeds for Trumpets"].  Dom Bedos [*] describes
a test for determining if the resonator length is correct:

Put your open hand over the top of the resonator as if to stop the
pipe (while it is playing).  It should fly off pitch, much the same
way as if you had been tapping the tuning wire sharp enough to hit the
flip-point.

When you then remove your hand, if the pitch immediately settles back
to the main pitch, then the resonator length is correct.

If it stays at the fly-off high pitch, then the resonator is too long.
If it slowly settles to the fundamental pitch, then the resonator is
only slightly too long.

If, however, the pipe does not fly off pitch with the hand covering
the top but stays at the main pitch, then the resonator is too short.

Of course, in adjusting resonator length as may be needed, one then has
to re-tune the tongue at the tuning wire to compensate, which then
slightly changes its relation to the resonator length.  This is tricky!

Also, changing tongue curvature changes the relationship of the tongue
the resonator length: a too-flat tongue can allow one to reach a false
flip point.  But with experience one soon gets a feel for what results
one is aiming for and how much adjustment is needed in which area to
get those results.

I have found Dom Bedos' test to be the absolutely crucial benchmark
in reed voicing and tuning.  Without it, one is simply groping in the
dark.

Timothy Tikker

 [ * "L'Art du Facteur d'Orgues [The Art of the Organ-Builder]", by
 [ Dom Bedos de Celles, 1766.  English translation published 1977, ref.
 [ http://www.mmdigest.com/Archives/Digests/200312/2003.12.03.05.html 
 [
 [ See also "New Book: How to Make, Tune and Voice Reed Pipes" at
 [ http://www.mmdigest.com/Archives/Digests/201001/2010.01.24.03.html 
 [
 [ -- Robbie