Sound in soundboards

[John Delacour]

At 11:04 AM -0800 12/3/01, Delwin D Fandrich wrote:
>Sound does not travel in the soundboard assembly. Sound is created as the
>soundboard panel vibrates in response to the energy input from the strings
>and compresses and rarefies the air around it. Wave energy from the strings
>moves the bridge which in turn moves the soundboard. The resultant wave
>energy travels primarily along the soundboard surface until it reaches some
>boundary. There it is either absorbed or reflected back into the soundboard
>panel. The difference is, perhaps, a subtle one but it is important to
>understanding the function of the soundboard.

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.

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.

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?

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.

JD