Orthotropic Soundboard (was Laminated Soundboard Ribs)

[Phillip L Ford]

On Tue, 12 Feb 2002 22:49:25  
 John Delacour wrote:
>
>Spruce, or any wood, particularly softwoods will have very different 
>values of Young's Modulus according to the direction of the grain. 
>That's why ribs are needed in the first place!
>

>And all this presumes that the rib's sole purpose is to act 
>as a girder, which is not so.  The stiffness of any part of the 
>system will also affect the wave speed of the forced vibrations, not 
>to speak of the resonant (modal) frequencies of the board as a whole.
>
>JD
>
John,
I've seen this expressed before in other places; since the board is orthotropic
and the speed of sound is less across the grain than along and waves don't
propogate in the same way across the grain than along the grain, etc., etc. the
ribs need to provide a cross grain path for the sound or waves.  I wonder how
important this is to soundboard behavior.  I'm not convinced that if you turned
the grain of the soundboard 90 degrees to the current norm and left the ribs as
they normally are (in other words the grain of the board parallel to the line of the
ribs) that the board wouldn't function just as well, even though the ribs aren't
providing a cross grain path (although admittedly the bridge would be).  I think
it would be an interesting experiment.

I'm wondering if the main purpose of the ribs isn't structural (just acting as a
girder as you put it).  Structurally it seems they serve two purposes:

1.  To reinforce the board so that it can resist the downward force of the strings
and do so without adding the weight that would be necessary if the board was
thickened enough to sustain the load without ribs.

2.  To change the vibrational modes of the board.  The ribs 'break up' the board,
so to speak, and change its natural modes, which is probably essential in an
instrument with a large frequency range over which it has to work.

Perhaps these functions are the primary ones and carrying waves, damping, etc.
are secondary or insignificant to the soundboard's function.

Another aspect of this is that some people seem to feel that there is something
magical about the board having orthotropic properties and that wood is the
choice for musical instrument boards partly because it is orthotropic.  I'm
unconvinced about this.  I think a soundboard needs to have high stiffness to
density and some level of internal damping.  When pianos were developed the
only material available with these properties was wood (softwood).  Now it just
so happens that this material is orthotropic.  Some people have concluded that
this means that the soundboard needs to be orthotropic.  Perhaps they're right
but I'm unconvinced.  I've seen pianos with laminated boards (which I've taken
to be essentially isotropic) that seem to serve the same function (whether they
do it as well as an orthotropic board is still an open question).  Ron Overs's piano
which he displayed in Reno is an example.  I don't know enough about the way
the board is constructed to know if it is truly isotropic.  Perhaps Ron can comment
on that.

Phil F


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