Rocking bridges

[Robin Hufford]

John,

     I am not disputing the obvious need for flexibility and variable
flexibility in the system but am simply taking cursory note in this post
of the implications of an overly flexible bridge and the necessity for
the string/bridge/board interaction, mechanically, at least to clamp the
string, and to effectively stabilize its termination in space so that
mutually induced changes in speaking length tension, as rocking occurs
are not such that they confuse and degrade the sound.  In this I am
continuing, as we have done heretofore, or at least have attempted, to
maintain separate the questions of mechanical transfer of energy from the
string and the subsequent acoustic behavior of the soundboard assembly.
In this context I say, conceptually, that the minimization of the process
of such rocking, and its elimination altogether, were such possible, is a
plus as this would provide the most effective termination and that
reducing the effect of such rocking insofar as it perturbs string
tension, would always be ideal, and that piano design has proceeded in
this direction.  Flexibility of the soundboard assembly in the context of
soundboard behavior and sound radiation is one thing; flexibility in the
context of the termination function applied to the string at the mutual
interface of bridge/sounboard and string is another, as you know.  It is,
among other things, the variability of solutions offered by piano makers
and designers in their approach to these questions that has resulted in
the profuse and wonderful variety of pianos and sound which we have the
pleasure to hear and observe.
Regards, Robin Hufford

John Delacour wrote:

> At 2:23 AM -0800 12/25/01, Robin Hufford wrote:
>
> >Ron,
> >
> >      Should the string be substantially, physically, moving the
> >bridge as a result of its cyclic behavior that to any significant
> >degree contributes to the sound  then one has to contemplate a
> >panorama of immense complexity  regarding the tension in the strings
> >as the bridge supposedly flexes back and forth, rocks and ripples.
> >While I think it obvious that putting pressure on the strings or
> >pulling up on them will move the board a little bit, and stipulating
> >the string does so then if your example were true would this make a
> >difference in the sound and how would you calculate it?  0.0005 of
> >an inch seems to be a rather small and very trivial amount.
>
> >I emphasize again that I believe, in fact, that any rocking,
> >rippling or flexing motion is not a significant contributor to the
> >energy level acquired by the soundboard and that there must be a
> >very critical limit that such motion, if it existed, could not be
> >allowed to exceed.  In fact, I believe, historical, that the
> >practical evolution of soundboards has been precisely in the
> >direction of minimizing and elimanating any such motion.
>
> Robin, I think there is no dispute concerning the flexibility (i.e.
> mobility) of the soundboard/rib/bridge system and that the degree of
> flexibility at different locations is critical to the efficient
> production of the sound and acoustic radiation of the various
> frequencies.  The same is true, of course, of all stringed
> instruments, though the quantities involved and the means used to
> achieve the optimal impedance are different in, say, the fiddle
> family, let alone flutes and trumpets.
>
> A piano with no ribs and a piano with too many ribs, on the same
> board, will both sound wrong in different ways because the degree of
> flexibility or mobility ( or their opposites, stiffness and rigidity)
> is wrong.  If this flexibility were totally _eliminated_, as you risk
> saying above, then the system could not function at all and all the
> energy from the vibrating string would be reflected at the bridge
> just as it is, ideally, at the stud (American = agraffe).  _Minimize_
> is also not the word, for the aim is to get the ratio of pressure to
> displacement (the acoustic impedance) just right, hence for example
> the higher stiffness of the system traditionally designed into the
> treble by various means such as thickening the soundboard and extra
> attention to the rigidity of the front termination on a grand.
>
> But none of this has any bearing on the way the sound travels, simply
> on the efficiency with which it travels.  It is not the movement of
> the bridge/soundboard that causes the superimposed frequencies of the
> sounding strings to be audible.
>
> JD