Sound waves(The behavior of soundboards)

[Ron Nossaman]

>When I walked away from this topic, having proposed such a test and 
>had it arbitrarily dismissed, in my own mind I had hardly travelled 
>beyond the point at which the string meets the bridge, though others 
>were anxious to generalize and get by any means to talk of the 
>rippling soundboard.  I can't myself make such a leap of faith, so I 
>would like to return to this meeting-point.

And that's the problem when a discussion is immediately dragged in fifteen
different directions at once, isn't it? It wasn't dismissed, as I recall,
it was suggested that you try it. So try it. Don't just weight the bridge
down, because it and the soundboard will still oscillate vertically even
carrying a considerable amount of weight, as is demonstrated in a strung
piano. Clamp it, glue it, screw it, top and bottom along it's entire length
to very deep and rigid beams solidly mounted to the rim and see what sort
of sound quality the soundboard produces. Just drive a few wedges between
the bracing and a few points under the bridge and see if there is any
noticeable difference in produced sound. If the bridge doesn't have to move
for sound to be produced then this should make no difference. I predict
that it will if you do a realistic job of it. Back scale length should make
no difference either for the same reason but anyone who has added length to
the segments between the bridge and aliquots or hitch will tell you that it
most certainly does. 



>At the same time, the whole bridge is subject to the downward 
>pressure of the other strings and is in equilibrium between this and 
>the upward force of the soundboard.  In the case notably of 
>Grotrian-Steinweg, the bridge itself is very broad and is stiffened 
>on the reverse of the board by a substantial "counter-bridge". 
>Experience shows that no reasonable increase in the height of bridges 
>(and consequent exponential stiffening) will lead to negative tonal 
>effects, but rather the reverse.  The effective bridge, therefore, of 
>our most illustrious predecessors, seems to be by design a pretty 
>rigid affair and might be made even more rigid to good effect.

Correct, and at this point we need to establish what a bridge does. To my
mind, a bridge is just that - a bridge. It isn't a rib, and is a major
factor in assembly stiffness control only when it is too flexible, as we
clearly hear at scale breaks when the bridge is deeply notched to clear
plate struts. The bridge's job from a stiffness standpoint is to
structurally connect the ribs, and as long as it is sufficiently stiff to
do so without unnecessarily robbing the system of energy by flexing overly
much, it is doing it's job. Given sufficient minimal stiffness, it has very
little to do with the overall stiffness of the assembly except at the ends,
where the stiffness continuity is broken by the bridge not extending any
further. In making a bridge super stiff, it can be made too massive, which
can negatively affect the system, but that needn't be addressed yet. The
diaphragm action is primarily cross grain to the panel, or along the ribs
in a reasonably conventional design. Let's keep it simple and not bring in
the aberrant examples yet.



>The vibrations cause by the transverse movements of the taut string 
>are passed into the bridge at a point equivalent to the point of the 
>tuning fork pressed against the bridge, and this point in both cases 
>is static and not mobile.  From this point the vibration, or 
>molecular disturbance, radiates into the elastic medium that is the 
>beech or box or maple + the steel of the pin and travels as 
>compression waves in all directions as fast as the medium, the grain 
>direction etc. allow.  Virtually every molecule of the wood or steel 
>will be displaced and oscillate in response to the kicks and shoves 
>from its neigbours.  It is the oscillation of the molecules next to 
>the glue line, excited by kicks and shoves from all directions within 
>the bridge, that will now raise a rumpus in the soundboard.  The 
>bridge so far remains unmoved, its internal tranquility severely 
>disturbed but outwardly unmoved, unrippled, unfurrowed.
>
>Serious comments only please.
>
>JD

I see a string with an (initially) vertically oriented transverse wave
(imparted by a vertically traveling hammer) physically moving the bridge,
which physically moves the soundboard, which physically moves the bridge,
which physically moves all of the strings in the system. How does the
energy from one moving string visibly move other strings elsewhere in the
system if the bridge and soundboard don't move? I haven't tried this, but
by my understanding of your thinking, a string stretched across the edge of
a soundboard panel mounted perpendicular to it so that the panel edge was
acting as a bridge, would sound just the same and work just as well as the
more conventional configuration? 

Ron N