Sound in soundboards

[Delwin D Fandrich]

----- Original Message -----
From: "John Delacour" <JD@Pianomaker.co.uk>
To: <pianotech@ptg.org>
Sent: December 03, 2001 4:26 PM
Subject: Re: Sound in soundboards


>
> 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.

Is this a serious question? About the only analogy I can think of is the
piano in which some technician inserted a series of maple wedges between the
belly bracing and the soundboard along the bridge line. It was an attempt to
re-gain soundboard crown. There was still some sound there, but it was muted
and thin.


>
> 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.

As I said, it moves the bridge. Primarily in a vertical mode (assuming a
grand piano), but there is some fore-and-aft rocking as well. How much
depends on the design of the bridge and location on the scale. More in the
bass than in the treble.

The bridge movement moves the soundboard causing it to vibrate much like the
diaphragm vibrates in the loudspeaker.


>
> 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?

Have you ever tossed a rock in a still pond? The effect is similar.


>
> 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.

As I said, there are excellent references available. None of this is really
all that new.

Del