soundboardinstal again

[Ron Nossaman]

>Right, bridges serve to distribute load among the ribs, and don't
>support crown, and I was asking whether or not a counter-bridge or "sister 
>bridge" had ever been intended, at least partly, to support crown.

You'd have to ask the designer about the intent. Measuring crown 
(everywhere you can reach, and NOT just on the longest rib) in pianos that 
have this feature will tell you whether the crown has been supported by 
anything at all. Unfortunately, you can't tell if any detectable remaining 
crown is due to rib shaping, panel compression, or bridge produced support.


>Or if, without adding way too much stiffness and mass, a set of 
>counterbridges (or better, quasi-ribs parallel to the grain of the panel) 
>*could* be designed to support crown, so that, *assuming* there was 
>anything to be gained from it, ribs could be designed without having to 
>have all of the crown support as one of their functions.

I've heard many many times that the crown in the bridge is the most 
important crown in the soundboard, and we discussed this on the list at 
excruciating length earlier this year. When I build a new long bridge, I 
make it flat on the bottom, and since it is curved, it still conforms quite 
nicely to my high crowned boards. As long as the bridge is curved like 
that, it will not support crown. It simply rotates and goes along with the 
soundboard. Also, it is geometrically impossible for a soundboard that is 
crowned along the ribs and glued to a plane rim, to NOT have crown along 
the long grain perpendicular to the ribs.


>First, I probably used the term "coupling" too generally.
>I did mean what Del calls the "mechanical coupling" that is gained from 
>the combination of bridge pin angle and side bearing. (I
>think I've seen the term "string coupling" in the sense of the
>acoustical "coupling" interaction between the three strings of
>the same trichord (reinforcement, gradual increase of vertical phase 
>cancellation from unavoidable lack of perfect unison,
>etc.), but I had thought that a string could be said to be
>acoustically coupled to a board assembly at the bridge, and that
>the term could then be used more generally to include mechanical
>coupling.)

String coupling to the bridge has very little to do with soundboard 
function unless it is utterly inadequate. If you have near vertical bridge 
pins, inadequate side bearing angle, and zero to negative front bearing, 
the strings will whang like a badly played bottle necked guitar. That's 
coupling.


>Second, what Del calls "a system of opposing springs" captures
>that part of the concept I've been missing the most. I knew
>that a board assembly is a downward-hardening spring, and
>therefore, in the absence of downbearing, would be an upward-softening 
>spring. If one were to remove the downbearing entirely,
>it would rise to its equilibrium level as an a complex of
>unloaded beams, very much as a wooden girder, floor joist, and
>floor system, supported by a perimeter foundation, would do at the
>top of a basement, if a heavy live load were removed from the
>floor above. And at that equilibrium level, the board would be
>too flexible at zero amplitude, if the string were mechanically
>fastened to it (with help from the hitch pin...) at its own zero amplitude 
>(condition of rest and silence).

This is precisely the situation found by the thousands in zero bearing zero 
crown killer octaves in compression crowned soundboards out there in piano 
service land.


>One way of visualizing the problem lies in considering the consequences of 
>the fact that the gaining of downbearing necessarily changes, from a right 
>angle, the angle of the string to the downward vector of board assembly 
>movement.

What problem? The string moves the bridge top both up and down, and back 
and forth as it goes through it's excursion. Bearing vector deviation from 
90° shouldn't be a measurable factor in the bearing ranges found in a 
piano, though I haven't tried it. Back scale lengths are, though. Short 
back scales restrict bridge movement.


>The provision and adjustment of the adjustment of downbearing, therefore, 
>is a means to control the rate of vertical transmission "beyond", and, so 
>to speak, horizontal reflection backward from,
>the coupling of the string to the bridge.

Not in my world. To me, bearing is a means of loading the soundboard and 
raising it's impedance and spring rate to accommodate what the string plane 
has to give and take.


>If I haven't made a new mistake here, my next question is:
>are *lateral* transverse vibrations of the string (or, large lateral 
>vectors of vibrations that are significantly "slanted" from the vertical) 
>thought to be transmitted in any way that the theory
>of the board assembly, and of the interaction of its components,
>has been able to explain, or (usefully) speculate about?
>
>Thanks for your help!
>
>Randy Jacob

Usefully? Not to my knowledge. I've had much better results ignoring most 
of that sort of stuff and, viewing the strings as a considerably more 
simple input and energy storage system for driving and being driven by the 
soundboard, concentrated on trying to understand how the soundboard worked 
instead.

Ron N