Soundboard mass

[Richard Brekne]

    "I think the reason they didn't use taller ribs is because with 
compression
    crowing, the taller, stiffer ribs won't bend under panel compression 
and you
    won't get adequate crown.  Isn't it true that the narrower taller 
ribs that
    you see on some older pianos were generally radiused?"



I would think this is reasonable speculation. The link I posted to the 
five lectures was intended to point out that there are more perspectives 
concerning ribs, their height, width and numbers then just how much load 
they are able to sustain.  In addition to increasing the soundboards 
stiffness in general a second and very important consideration for rib 
dimensions is to compensate for the differences in bending stiffness 
parallel to and across the grain (anisotropism).  In addition there is 
evidence to support the idea that wood under compression which does not 
exceed the elasticity limits of the wood in the direction of the applied 
compression actually can increase the stiffness of the panel itself. 
There are procedures today that take direct intent on effecting exactly 
this result. One such procedure is called Viscoelastic Thermal 
Compression, http://www.research.vt.edu/resmag/sciencecol/pressure.html 
which seeks to basically take advantage of the fact that there is a 
direct relationship between the stiffness of wood and its density. It 
basically increases the density of the wood without negatively effecting 
the strength of the wood cells in response to compression.

It is plausible to wonder if it is possible to some degree over some 
period of time the CC board approach increases the stiffness of the wood 
perpendicular to the grain by virture of an increase in the panels 
density due to compression that is low enough not to exceed the elastic 
limits of the wood employed. If so, then the use of lower wider ribs 
need not take the same regard to this requirement as in a panel with 
little or no compression.  Another point worth pondering is the 
difference of the distribution of such increase in stiffness over the 
panel as a whole might achieve.

As for the quote below... indeed the empirical methods and means 
employed by our forefathers may have included stumbling upon an 
desirable acoustic effect from using taller ribs.  Such ribs would 
indeed be more difficult for a compression panel to stress (bend) and so 
machine crowning these to some preconcieved radius  may have been an 
attempt to get around that problem to some degree... ie allow for a 
compression panel with higher ribs that did not need as much stress for 
the assembly as a whole to achieve a given crown. Compression Crown had 
been seen by empirical means to on the short term increase in resistance 
to and increase in load until overloaded and failure occurs. No doubt 
the limits for this were emprically established and panels were kept 
within what were perceived as within tolerances.

Speculating on what these fellows so long ago were thinking is a lot of 
fun. Earlier on there was a tendancy to align the grain so that it ran 
in the direction we run ribs today. Instruments of this sort were 
typically low tension affairs and downbearing forces correspondingly 
low. They also typically displayed very low rib height and often quite 
wide. This would allow for a good deal of compensation for bending 
stiffness using compression as the medium while not stressing the wood 
perpendicular to the grain so much as with stiffer ribs.  At the same 
time the stiffness of the wood along the grain may have been seen as 
sufficient to handle what little downbearing force was applied on such 
panels.  This is pure speculation on my part based on my limited 
understanding of all these issues... but it seems to make sense.  And 
indeed the hybrid board that Terry and I contrived works very well 
indeed. I am convinced that if I had greater insights into more of the 
issues involved that this grain orientation along with a laminated crown 
and modest ribs would provide a very satisfactory alternative to the way 
things are done usually today.  So called tunnel crown could be achieved 
with great strength, ribs could be contrived so as to satisfy 
anisotropic bending stiffness requirements and take less concern to load 
bearing requirements.  Seems to me worth persuing.  As do very low 
tension scales IMHO. I'm convinced that a very fine marriage between 
modern sound and the sound of straight strungs of old can be achieved at 
much lower general volumes using low tension scales.

Cheers
RicB