Soundboards

[Ron Nossaman]

>Now oh great sage and master of all things piano I have a few more questions.

Uh oh, I'm about to be flushed into the open and found out for an idiot!



>How does one go about measuring "internal friction?"  Would it be greater in 
>a laminate board because of the laminations?  Does the glue joint between the 
>ribs and board come into this equation or is this a cellular thing?

I don't know for sure, having never done it. I rely on published figures,
if available, for this kind of stuff. I would think that driving a
specifically sized and mounted sample of the material at different
oscillation frequencies until you found the highest supportable harmonic
for the size, weight, and modulus of elasticity of the sample would give
you a pretty good idea of the internal friction losses. I don't have
formulae, so I can't really say for sure. If anything, I'd think internal
friction in a laminated board would tend to be less, since the internal
structure is (or at least could be) more tightly bound together. It's the
composition of the material that makes the difference. An adequate glue
joint is very easy to perform, and is adequate in either case.



>The stiffness of the SB assembly is determined to a large extent by the cross 
>sectional ratio of the ribs. Width in proportion to height. Right?

Yep, but the stiffness increases in proportion to the cube of the rib
height, so height changes have the most effect. Panel thickness is
important too.


>How does one go about determining the best ratios?  In the "killer octave 
>area" what changes are most helpful?

Consider ribs as simple center loaded beams. Start with a string scale.
Compute the string downbearing loads on the bridges, assign each unison
bearing load to the rib that's helping hold it up, compute the resulting
rib deflection, and play with the rib configurations until you get what you
think you want. To find out what you think you want, try setting this
exercise up on a number of different existing pianos and see what
correlations you can find between what your figures show, and how the
pianos sound. If you set up a scaling spreadsheet, you can add all this
other stuff and semi-automate the killer math stuff to make it more
survivable.  The killer octave needs stiffer ribs. The assembly impedance
in that area is too low as is, probably, the fundamental resonant
frequency. Check the length and cross sectional dimensions of the ribs
through that area against the load they must carry and the frequencies they
must handle and you begin to see the patterns. Machine crown the ribs,
instead of letting panel expansion crown flat ribs, and you will have much
better control of the process. 



>I know most original research in the piano field is now conducted by 
>rebuilders such as those on this list.  I am not looking for any proprietary 
>information.  Just point me in the right direction to look.
>
>Andrew Remillard

Check Del's Journal articles for an excellent starting point, and wade the
List archives for as long as you can stand it. There's LOTS of reading on
the subject. Welcome to the snake pit.

Ron N