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Ron -
Seriously, thank you for taking the time to respond and for taking my
questions seriously, mostly. It's so easy to get lost in semantics while
just trying to put a few words together in a sentence. Some of what
follows might reiterate some of what Richard Brekne discussed in an earlier
post, but I thought it would keep it simpler if I made my own comments
here. Please note that I don't expect you, Ron, to take on the
responsibility of answering all the questions I raise. In some cases the
answers may be available from original sources, with a little more work on
my part. In others, the real hard data may simply not yet be available.
At 07:22 AM 2/24/2004 -0600, you wrote:
>>How would you define "coupling", in this case?
>
>The pin angle combined with the string tension and offset angle forces the
>string against the bridge top. It's a clamp system.
This is a static definition. What I was trying to ask about was "dynamic"
coupling, that is, the interaction of the parts (string, pin, bridge) in
motion. In fact, I probably DON'T want to ask about this right now, as it
would likely bring us around to that discussion about how energy gets from
string to soundboard. I'm not ready to take that on.
>>Does the front edge of the bridge play any role in defining the string
>>termination?
>
>Yes. It's what the pin angle, string tension, and offset angle clamp the
>string TO, ideally.
I meant, does the front edge of the bridge have any effect on the way in
which the various modes of string vibration evolve & decay that might be
measurably altered by its absence?
>>If so, how is that function affected by no contact (string climbing pin)
>
>Strings don't climb pins unless something is severely wrong,
You say "unless something is severely wrong". What condition would qualify
for that description? What amount of negative force would be needed to
overcome the design parameters you have quoted..."10 degrees side, 20
degrees pin slant"? What if those parameters have been ignored,
intentionally or otherwise? The initial impact wave might have enough
force to move an inadequately "clamped" string.
>but the termination edge is crushed by cyclic wood dimensional changes
>with humidity swings pushing the string up and down the pin. It's very
>possible for a string to be resting on the bridge top and not be touching
>the notch edge, but it hasn't climbed the pin. It's crushed the cap.
This could be the case if the original downbearing was properly set and the
pitch/tension was allowed to elevate with the increased humidity. Here are
some sub-questions:
- What amount of pressure is required to crush maple cap material?
- In the normal stringing process, how much force is initially
applied to the front edge as the tension is applied?
I recall some discussion about the bridge surface itself expanding upward
(apart from the soundboard's upward excursion) and, in the process, pushing
the string up the pins. If this occurred, and then, in the dry season, the
bridge contracted, if the ratio of friction to force was high enough, the
string could remain elevated from the bridge surface, even without
attributing the cause to wood crushing or vibrational prodding.
>At that point, the loose pin flexes and flagpoles and the false beat happens.
I wonder at your conclusion, that he pin becomes loose and flagpoles,
causing false beats. I can't see that the pin necessarily becomes loose or
that false beats are the resulting symptom. My guess would be that the
lack of bridge edge support allows a degree of movement between string and
pin that would otherwise not occur. At some point, in an instrument that
has had a fair amount of use, that movement is taking place against an
abraded pin surface, which could contribute to both the unwanted string
noise and the string's ability to remain elevated.
>> In any case, it's hard for me to believe that you have not encountered
>> innumerable examples of strings which visibly settle downward when
>> tapped (gently).
>
>Of course. But it's because you're inducing a curve into the string to
>force it down to the crushed bridge edge by tapping, not because the
>string has climbed the pin.
Sorry, no. I have, with a magnifier and strong lighting, watched the
bridge-segment string set to the bridge surface from almost all the way
back to the rear pin. It can't be all about wood fiber crushing. Of
course, as I get older, and my eyes get worse, I can reasonably suppose
that I'll be seeing less of this. That's one solution!
>>It's the "clamping" concept I'm having trouble with. If there is
>>positive downbearing, the need for further clamping would seem somewhat
>>redundant.
>
>Except that's backward. That clamp provides considerably more coupling of
>string to bridge than does downbearing. Try to pull a string up off a
>bridge. Now pull the bridge pins, tune it back up to pitch and try again.
>Without the pins, there's not much keeping the string on the bridge.
Maybe. Again, it's the confusing of clamping and coupling that bothers
me. The example of trying to pull the string off the bridge would seem to
suggest that the string in vibratory motion is trying to do something
similar...that the clamping action is causing the bridge to follow the
motion of the string. Clamping TO THE BRIDGE SURFACE would appear to have
a very useful purpose in the case of zero or negative downbearing.
>> So, what are your concerns with negative downbearing?
>
>Overall downbearing in a conventional design: The soundboard isn't
>compressed, so there will be an impedance mismatch between the board and
>string scale (killer octave). The coupling between the bridge is
>compromised. Not eliminated, but less positive. The piano typically sounds
>lousy there.
>
>Ron N
From the rebuilder's perspective, and not just Ron N, : Are you striving
for positive downbearing (with positive front bearing) across the scale?
If so, do you know (by measuring) how often you do or do not achieve this
goal? If you are willing to accept some areas of negative downbearing, are
there locations on the board where its presence would be more likely to
have a deleterious effect?
It's been fun - truly!
David Skolnik RPT, DBH
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