Rocking bridges

[John Delacour]

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