Rocking bridges

[Ron Nossaman]

>I have said from the outset and have been forced to repeat numerous 
>times, in response to suggestions that I have denied it, that the 
>movement of the bridge occurs.  By the same token the bridge in 
>rocking will obey the same physical laws.  How can you still pretend 
>that there is any argument about this.

The only persistent confusion I've seen here is a disagreement of what
moves when and what moves it, not whether or not anything moves at all.
We've already dealt with that and, I had assumed, corrected any
misunderstandings. Yes, everything still moves. If we're not talking about
the same thing here and haven't been through this whole thread, I'd like to
know that now please. If we are talking about the same thing, then we have
been far beyond that point for some time. To bring you up to date, the
current question I'm dealing with is whether a string displacement in the
speaking length of a piano can rock the bridge. On that question, on 12/19,
you wrote this. 

>>You are asking the list and me to accept that a variation in tension 
>>of a few pounds at most due to the trasverse vibrations of two 
>>strings is sufficient to cause a rocking of a long bridge firmly 
>>glued to a soundboard and held in position at its top by over ten 
>>tons force?  You'd have the bridge in smithereens before you could 
>>rock it with a crowbar let alone such a minute force.

This was up to today, as nearly as I can tell, your stated opinion. If you
had reversed this opinion prior to your most recent post, I missed it
altogether. Since you have apparently reversed it in the current post,
saying it has been obvious all along, I presume pending any contrary
revelations, that the point is conceded.


>My model was designed so that those unfortunate people without lasers 
>and mirrors could see for themselves what it is not possible to see 
>with the naked eye and furthermore carry out tests that would be 
>impossible on a strung grand without highly sophisticated measuring 
>equipment:

You must not have a cat. Anyone with a cat needs a $6 laser pointer. I
think it might be a rule. In any case, there likely aren't three people on
the planet who will actually try what I've suggested, so the potential
global hardship imposed in exotic hardware expenditure and broken mirrors
is negligible. As to your model, the less like a piano these models are,
the less use they are to this thread. Your demonstration illustrates, as
did mine, the principal that your quote above denies is even possible. So
far so good. But you then ask how it can affect tone???? By moving the
soundboard! How many times have I said this? Glue your bridge to a piece of
card stock supported by a perimeter frame and try it again. The string
moves the bridge, the bridge moves (deflects and bends) the soundboard.
Into what shape in your model? In a piano, when the bridge is either rocked
or deflected, or bent (it bends some along it's length too), the movement
affects the tensions and termination position in space of all of the
strings attached to it as well as deforming the soundboard. Once the
soundboard is moving there is obviously feedback to the strings from then
on. Don't stop now. You've almost proven my point.


Also, contrary to your observation that the bridge is held in place at the
top by over ten tons of string force, you might note that if a
conscientious tuner that carefully equalized string segment tensions on
both sides of the bridge has done his job correctly, there is only the
tension difference (resulting from attempted movement) between segments on
opposite sides of the bridge inhibiting the rocking. I don't know what this
tension difference is, but it can be approximately calculated if anyone
wants to take the time, and it isn't going to be much compared to half the
total string tension. This makes the bridge a lot easier to rock than it
might first appear. The soundboard stiffness will obviously inhibit string
deflection induced rocking too, in this case. 

Ron N