Sound in soundboards

[Ron Nossaman]

>OK.  Question 1: "Wave energy moves the bridge".  Under the bridge, 
>say the bass bridge, I arrange a massive trestle with thick metal 
>blocks pressing up against the underside of the bridge line, and on 
>top of the bridge I place a long lead weight as heavy as I can lift. 
>And if this isn't enough I borrow hydraulic equipment to make sure 
>that bridge really can't move.  I haven't tried it, but you suggest 
>the effect will be to kill the bass of the piano as though the 
>soundboard and bridge were not there, since the loudspeaker effect of 
>the soundboard depends on the solenoid effect of the bridge.

That should be the case. Just like clamping the driving coil of a speaker
cone to the magnet. The transducer can't transduce if it can't move.


>2.  "Wave energy moves the bridge".  What is wave energy and does it 
>travel primarily along the bridge surface as you say it moves 
>primarily along the soundboard surface?  If it moves "along the 
>surface" does it travel in the varnish or in the air touching the 
>varnish or in the wood just under the varnish but no deeper, and how 
>does it travel?  So far as I know sound travels always as a 
>compression wave through whatever medium, but you are suggesting 
>something different.

But the soundboard isn't transmitting sound. It's dispersing string energy
and producing sound. Transducing.


>3. "travels primarily along the soundboard surface.....reflected back 
>into the soundboard panel."  So the sound, I understand, begins its 
>journey moving _along_ the surface as far as the boundary and then 
>gets reflected _into_ the soundboard. How does it travel then?

Like a ripple, with the stiffness of the transmission medium (soundboard
assembly) determining the energy transfer rate from the string. It's the
same kind of ripple that propagates along a struck string, and reflects
from the terminations back into the string - only there are a whole lot
more driving frequencies and excitation sites with the soundboard, so the
ripple pattern is much more complex. It's not really a compression wave,
though I'm not sure what it would be technically called other than a ripple.


>What I suggest is that the sound does indeed travel _in_ and through 
>the beech and the spruce because otherwise it could not travel at all 
>except in air.  I suggest also that the speed at which these 
>materials are able to carry the compression wave is elemental to 
>their choice for this purpose.  As to exactly where in relation to 
>the surface(s) of the board it travels, I'd suggest it travels 
>wherever the medium exists to carry it and that it travels most 
>effectively in the hard and long-fibred winter growth.

Then why does changing the rib height and feathering have so much more
effect than thinning the panel? The long grain stiffness has more of a
constrictive, than a transmission enhancing effect in practice. That's why
floating the soundboard in the tail enhances the bass in a small piano so
well. It relieves the built in excessive constriction of long grain
stiffness, and shifts stiffness control to the ribs, where it is more
manageable. 


Ron N