Varying bridge height (was Re: Floating soundboard)

[Ron Nossaman]

>It's been some time since I saw this, so I may not have it quite right.  But,
>Conrad says he works on a similar piano, so I guess I haven't completely
>lost my mind.  I do remember the opening in the board and the fact that the
>bridge was quite a bit taller at the bass end.  It could be that the
soundboard
>was level and the plate was set in at an angle, but that's not the way I 
>remember
>it.

The one I saw torn down a couple of years ago had the "ramped" soundboard,
"drain", and bridges super tall at the bass end. My impression was that the
cutout looked like an afterthought. I see a piano in final [sic]
development that is just too thin sounding in the low tenor. The bridge is
too close to the rim, making the assembly too stiff there. All the King's
voicers couldn't fix it, nor could an experimental scale change or fifteen.
Still sounds lousy. What to do? Shall we get new plate patterns made, new
rim presses, a clean sheet of paper and start over? Can't afford it, and
who's to say the next one would be any better? Too late at this point to
copy brand S. We're doomed! Let's go get drunk and forget it for tonight,
we'll die tomorrow. The result looks to me like a pretty good quick and
dirty affordable post design patch incorporated into production as a
feature.   



>1.  To provide a 'path' for the vibrations imparted at any one spot to
'travel'
>to all parts of the board.  If this is the function then what are the optimum
>dimensions?  Is it dependent on frequency?  I don't know.

You already know my views here. The bridge is structural.



>2.  To provide something solid to drive the bridge pins into.  If this is 
>the main
>function then the bridge really only needs to be big enough to hold the 
>pins.  Any
>extra material would be superfluous.  How long do the pins need to be anyway?
>Probably not as long as they commonly are.  Anything much below the surface
>of the bridge is probably not doing much.  So practically speaking, the bridge
>could be very short if this were its only function.

I agree. Pins probably are longer than necessary, and bridges could be very
short if holding pins were it's only function. This is obviously not the case. 


>3.  To provide some stiffness smoothing (impedance smoothing?) so that
>adjacent notes don't 'see' markedly different local stiffness in the board.

It is, after all, a bridge. It's primary job is load distribution.
Stiffness affects assembly impedance, as is most obvious where a bridge is
notched to go under a strut, making that section of bridge below minimum
stiffness, and the tone quality of the unisons immediately on either side
of the notch are reminiscent of mini killer octaves. This lack of stiffness
is also evident at the low tenor, where the bridge (structural support)
ends, making the assembly more flexible than it is just a few notes up scale.


> If
>there were no bridge or a very short bridge, the notes that attach to the
>soundboard directly over a rib would see more local stiffness than a note that
>attaches to the soundboard between ribs.  This seems to me to assume that the
>forces applied to the board by the vibrating string (and the subsequent board
>movement) are important to the production of sound.  As discussions here have
>shown, this is a topic of debate.  But assuming that this is the function of 
>the
>bridge what is the optimum height?  Since the strings at the low end of the 
>scale
>have more excursion and usually higher tension then the forces they are 
>applying
>to the bridge as they vibrate are greater.  So it would seem that you would 
>need
>more bridge stiffness to provide this 'smoothing'.  This could mean a taller 
>bridge
>at the bass end.

Not so. Beyond a minimum stiffness, bridge stiffness requirements aren't
(to me, at least) obviously different from one end of the scale to the
other. Assembly stiffness requirements are vastly different, and can be
dealt with by bridge placement relative to the rim, rib dimensions, etc.


>4.  To provide stiffness to the soundboard, rib, bridge system in addition 
>to that
>provided by the board itself and by the addition of ribs to the board.  If 
>this is
>the primary function then it seems you would want more stiffness at the
top end
>and less stiffness at the low end.  So the bridge might be taller at the top 
>end.
>Perhaps items 3 and 4 balance each other out so that the bridge should be the
>same height all along.

Whatever the reason, same height all along works fine if it fits in the piano.



>5.  To change the vibrational response (mode shapes) of the board.  I don't 
>know
>what the optimum dimensions would be for this but it seems likely that to get
>whatever response the designer decided he wanted he might have to use a bridge
>that had different stiffness and mass along its length.

Of all the folks who have designed, built, and installed soundboard
assemblies, what do you suppose is the percentage who have ever seen the
modal patterns of the boards they've built? And of that percentage, who
among them can anticipate any given resulting modal pattern from the CAD
drawing or sketch? It's a lovely concept, and a potentially valuable
forensic tool, but it would sure surprise me to find there was any use at
all for it in the design process. Optimal, also, depends on what you're
after, doesn't it?



>6.  To give an observed result.  Perhaps if you built several pianos of 
>similar design
>with different bridge heights you might decide you liked the top end of the 
>piano with
>certain bridge dimensions and the bottom end of the piano with other bridge
>dimensions.  This might lead you to think that your optimum bridge was one
with
>varying dimensions from top end to bottom end.

Or that your hammer sets were slightly different, or your panels were of
slightly different densities and stiffness, or your ribs were slightly
stiffer in one than the other, etc. There are some (sometimes major) tonal
differences from piano to piano of identical design, and sequential serial
numbers. With a two piano sampling, we can lead ourselves to believe an
extraordinary number of things. 



>7.  To serve as a marketing tool.  As with soundboards and other things I
won't
>mention bigger must be better, right?  The sales people can say that your
piano
>is better than the competition because it has a bigger bridge.  Or they
can say
>it has a more high tech design than the competition because the bridge changes
>dimension and is 'optimized' throughout the scale.  It has Patented Acoustic
>Construction as one manufacturer says (or used to say).
>
>Phil F

As long as it's designed and built without compromise.

Ron N