Bridge caps

[Delwin D Fandrich]

----- Original Message -----
From: <Yardarm103669107@AOL.COM>
To: <pianotech@ptg.org>
Sent: April 02, 2001 10:46 AM
Subject: Re: Bridge caps


> My question is how to set up a laboratory experiment to test energy
transfer
> in the bridgepin/bridge contact keeping it separate from the bridge top
> contact of the string. What has been tried, Del, and what has failed here?
> PR-J

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Obviously, you don't. Ideally the string should contact both the bridge cap
surface and the bridge pin. That is the way they exist in real life and that
is the way they should be tested. I don't have a lot of time or interest in
test that don't in some way replicate real life piano situations. While they
may be of some academic interest to some research student somewhere, I find
them to be mostly counterproductive. Consider, for example, all of the
experimental work and modal analysis work that has been done using unloaded
soundboards. In one notable case the soundboard did not even have a bridge
and was simply screwed to an inner rim. What was learned? Nothing of any
practical value -- it was all just a waste of time and money.

I don't know what else has been done, but you might try something like the
following:

You set up a series of tests in which you allow only one variable -- or as
close to only one as you can. A mono-chord with a well regulated and
adjusted model action actuated by a dropping mechanism for consistency does
nicely. Then you set up a method by which you can measure the amplitude of
any side-to-side vibrations of the pin just above the string. A laser
vibrometer will do. You also mount a simple accelerometer to the top of the
bridge and another to the bottom of the soundboard assembly, both hooked up
to a reasonably accurate fft analyzer. You don't need a million-dollar lab
for this, you can rent or make everything you will need. It will still be
expensive, but more on the order of a few thousand.

As may be, you string the mono-chord and tune it to whatever pitch you are
concerned with. You set the bridge pins to some starting height -- say 10 mm
above the surface of the bridge cap -- and take a series of repeated
measurements. You'll need to take enough so that you can get a reliable
average -- of both power and sustain and pin movement. Say 10 at a minimum,
20 or 30 will be better. You carefully drop tension and remove the string so
that driving the pin down doesn't alter the coupling of the string to the
bridge top, tap the pin to 8 mm, replace the string, pull it to pitch and
repeat the experiment. You keep doing this until you are down to 2 mm and
are satisfied that your results are consistent.

Yes, there are differences from test sequence to test sequence, but they are
both exceedingly minor and statistically inconclusive. They vary from test
to test in a rather erratic way, but overall the amplitude of vibration, the
overall sustain time and the spectrum of the wave envelope will be
consistent enough so that all the numbers you have accumulated simply don't
prove a thing. They are certainly not anything I'd like to base doctrine on,
though others are free to do so if they wish.

Now, before dismantling your test mono-chord you might remove the 25 mm pin
and replace it with a 19 mm pin and repeat the test one last time. Again,
you will find no appreciable difference in the amplitude of vibration,
sustain time or the wave envelope.

No, I do not now have access to the numbers.

Del