Rocking bridges

[Ron Nossaman]

> Ron,
>     On Dec 15th, 2001 in the first post I offered on this subject, and in
>anticipation of the relevance of the nature of motion,  I addressed this issue
>with some implications arising from to the nature of a collision of a mosquito
>and an aircraft carrier.  You stated that the metaphor would best be expressed
>in terms of the impact of an airliner,  and this better expressed the
>relations between string and bridge.  

I said the mass relationships between the soundboard and string were closer
to the aircraft and carrier than to the mosquito and carrier. In any case,
I'd be surprised if you could find a physicist anywhere that would agree
that a collision with a mosquito would have no affect at all on the
carrier's speed. 


>Subsequently, and  I won't take the
>trouble to find and quote your post you said that the motion was
substantially
>above the molecular level, always adhering to the idea that the  behavior of
>the soundboard was a consequence of this motion.  Yet it would seem you now
>have progressively diminished you claims of motion  to the  level indicated by
>that of the mosquito, if not smaller.
>     You implied that this motion was in some way confirmed, not wishing
to put
>words in your mouth, by  a  level of deflection detected  in your test with
>weights on the plate and the rim;  measuring, if memory serves me,  a value of
>0.001 of an inch in deflection; later you find pressure on the string to move
>the board by about 0.0005; you take no notice of my repeated comments that the
>purported motion appears to decreasing in magnitude.  What is the level of
>insignificant  motion to which you wish to proceed?

I didn't imply the motion. I measured it, and got different measurements
for different degrees of applied force. However small, it is still
undeniably motion. At what displacement measurement would you consider
something to have moved? It appears to be over 0.001". 


>  And now you say that you don't know what the effect is, if any,
>on the sound and claim that "rocking" of the bridge, however small and,
>apparently, however insignificant to the sound, is the point at issue and
claim
>the point conceded.  Preposterous!  I don't think is so.  If this is what you
>reduce your argument to then you and your co-thinkers have in fact conceded,
>which, of course, I don't think you do.

My intent all through this has been to illustrate that the strings
physically move the bridge. I haven't reduced it a bit. I still think that
this is the case, and certainly haven't seen anything to the contrary so
far. When the bridge moves, the soundboard must follow. This starts the
complex feedback cycle that produces piano sound, but it is the physical
movement of the bridge by the string that starts and drives the system. And
it's true. I don't know the precise effect on the overall sound from the
bridge rocking, nor do I recall ever having claimed to. If you or JD can
produce the post in which I made this claim, I'd sure like to see it. 


>So, there is substantially more at
>issue than mere rocking of the bridge were this even so which I think not the
>case.  Were it so, it would be as I have always maintained, essentially
>insignificant and, let me now be categorical; :  this is  by no means the
>prinicipal mechanism of transfer of energy from string to soundboard.  

And who said that bridge rocking was the principal mechanism? I said, and
say that bridge movement by the string is the driving mechanism, and we
have obviously not gotten beyond that stage.  


>Should such motion  exist in any appreciable degree markedly deleterious
>effects would then be incurred in the sound.

This appears to be a statement. If so, then it is the first remotely
definitive statement I've read here, or anywhere that I can recall, as to
the affect of this rocking motion on tone. I'd like to see your evidence
that rocking doesn't occur, please, since I have demonstrated in a strung
piano, to myself at least, that it does. Then I'd like to hear your
explanation of how this rocking motion would negatively affect the sound.
The proof first please. 


>Excursion of the bridge and
>soundboard at this point is neglible if extant at all.  One could, I suppose,
>call this a ripple but this is misleading and of little significance.

Misleading to what, I'd like to know? There is a time factor here, that is
critical to forming this progressive wave, traveling wave, or ripple. The
board has mass as well as stiffness. The stiffness is not infinite, nor is
the mass. Returning to the blanket on the clothes line analogy, deflecting
the center off plane slowly doesn't make obvious waves because the slow
movement lets the stiffness of the material pull the areas adjacent to the
displacement along without obviously bending the material. The cycle time
of the displacement determines the wavelength of the "ripple". Do it more
quickly, and the material flexes as it is held back by it's own mass
adjacent to the displacement. Do it very quickly and the waves are closer
together. The range limits of frequency response are determined by the mass
and stiffness of the medium. This happens in a soundboard too, even though
the mass to stiffness ratio is considerably different, only the motion is
less severe. After the soundboard is moving, all sorts of complicated
interactions take place. The "if extant at all" comment here indicates that
we are still at day one as to whether or not the string moves the bridge.
Without this understanding, none of the rest of this is to any point. 


>     Now you appear to have reduced the entire question to the extent of the
>ripple("rocking" and that would be truly insignificant.

No, I haven't. Talk to John if you want an answer to this one. Better yet,
read the posts.


>     Both I and J Delacour  have been very explicit on numerous occasions that
>the issue was in fact transduction of the energy of the vibrating string
at the
>string/bridge interface: 

You have repeatedly stated your theory, but I haven't seen anything in what
I consider logical support of it. I showed that string movement moves the
bridge, which moves the soundboard, which was and is my claim. You may show
me that the string moves the soundboard first, which then moves the bridge,
at any time you see fit. I'm ready to see it.


> In an attempt to clarify this several  posts have
>been proffered addressing the nature of motion itself and, in  particular,
>addressing the question of suppression  by some sort of clamp or other source
>of pressure, this purported motion, and its effect on the vibrating string.

To which you received what I thought were reasonable answers that you
didn't refute. Nor did you attempt to immobilize your bridge with anything
but a finger. Did you solidly clamp it to something extremely massy and
stiff and test the result? As I've said before, the burden of proof here
isn't unilateral.  



>Examples have been offered.  All have elicited little commentary other than
>blase generalizations for the most part.  I asked what is the magnitude of
such
>motion, if it exists, and how does it contribute to the sound.  Your
answer is
>apparently, I don't know. 

That is correct. I don't know the specific tonal effect of rocking bridges.
I realize that admittedly not knowing something is an unpopular concept in
this discussion, but that's the way it is. Show me where I said I did know,
then show me that you know anything to the contrary. I'd like all your
details as to what causes what, with appropriate references as proof,
please. If this is what we're down to, let's have it.


> The relevance of the effect on the board and
>therefore sound has always been the issue in the context of how sound itself
>arises from these processes.

The relevance of anything will to a large degree depend on it's existence.
When you can offer any sort of evidence at all to the initial premise that
the soundboard moves before the bridge from a string being struck, I will
be happy to read it. 

Ron N