CA on bridge pins

[Greg Graham]

Following up on threads from earlier this summer, I
tried some CA on bridge pins with false beats, new and
old pianos, grands and verticals.  

Before using the glue, I tried just touching the tip
of a screwdriver to the pins, and, yes, the beat went
away (sort of the anti-Sonny&Cher?).  Applying the CA
sparingly to the pin/bridge joint stopped the beat. So
did gentle string seating on neighboring strings. 
I'll have to see which lasted when I go back (though I
need to take much better notes for future
experiments). 

THEORY 1 of false beats involves the pin and notch
terminating the string at different effective speaking
lengths parallel or perpendicular to the bridge. 
Generally, string cuts at the notch increase the
speaking length and lower the pitch when vibrating
perpendicular to the bridge.  String seating and
careful notching assume this is the dominant theory.
Could CA help this? Maybe?(see below)

THEORY 2 suggests the pin is loose and vibrating,
creating the false beat. 

  Theory 2-A: I'm wondering: could this be the result
of adding pin mass to the vibrating string and
lowering pitch when moving parallel to the bridge, but
not when perpendicular?
  Theory 2-B: Is the mushy pin creating the illusion
of a longer string by rocking back and forth the same
distance the string would move if terminated a short
distance beyond?  Picture playing jump-rope.  Your
hand is the bridge pin, and your shoulder is the
"virtual" termination point.  Now put your wrist on a
table.  "Longer" string and lower pitch parallel to
the table, or bridge.  A very small pin movement could
simulate a significantly longer string. 

(Theory 3, the old "kinks or defects in the string"
thing, seems to have gone the way of "the Earth is
flat".)

Observations and complications:  It is easy to hit the
pin with CA in a grand without getting any on the
string.  Visibility, accessibility, and gravity are
all favorable.  The opposite is true in a vertical. 
Any tips for hitting the D7 pins in a vertical without
a couple or few drops missing the mark and running
under the strings on the bridge?  I missed a few
times, but eventually got the CA to the bridge pin,
cured the falseness, and seemed to not hurt anything. 
Is CA under the string a horrendous no-no, or merely
an acceptable side effect with no serious
consequences?

ALSO... would the CA under the string at the pin tend
to fill in, swell up, shore up, or otherwise improve
the string cuts at the notch edge, thus improving the
tone according to Theory 1?  An alternative to
seating?  Too many negatives to consider it?

Something else to chew on:  False beating strings in
the high treble clearly show as two frequency peaks in
TuneLab's spectrum display, often several cents apart.
 Would a 1mm difference in speaking length in octave 7
create a 5 cent pitch change?  I don't have the
formula handy to do the math on that, but it would go
directly to the plausibility of either Theories 1 or
2-B.

Lastly, I did not note where resulting pure pitch of
any of these experiments ended up relative to the
upper, lower, or average of the TuneLab "twin peaks". 
I'll have to look next time.  If it goes to the higher
of the "false" pitches, Theory 2-A or 2-B could be
possible.  If to the lower, Theory 1.

Theoretically, that is.

Greg Graham
Brodheadsville, PA


		
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