Sound waves(The behavior of soundboards)

[John Delacour]

At 11:28 PM +0100 12/16/01, Richard Brekne wrote:

>John Delacour wrote:
>>....If the solenoid of a speaker is jammed, no sound at all will be 
>>emitted from the cone.  The two cases are completely different.
>
>Touche'. In otherwords.... the diaphram analogy is far from adequate to fully
>describe all of the sound producing elements of the soundboard ??.

As another illustration of what I've been saying about the bridge, we 
might hark back to the Kundt experiment that you, Richard, drew our 
attention to during the Great Compression Wave Debate.

You will recall that a thick metal rod was clamped firmly at its mid 
point and made to vibrate longitudinally by means of a resin-covered 
chamois leather pulled along its first half.  At the end of the 
second half was fixed a disk which fitted quite closely, like a 
piston, in a long air-filled glass tube.  The compression wave 
travelled along the rod and caused the disk to set up a compression 
wave in the air whose length could be measured using a scattering of 
cork granules, etc.

The rod is firmly clamped and rigid; it cannot move longitudinally 
and yet it can carry a compression wave thoughout its length and 
excite vibrations at the same frequency in the disk, to which it is 
attached at a right angle, just as the bridge is attached to the 
soundboard.  The disk creates a compression wave in the air in the 
tube by its _transverse_ vibrations, just as our soundboard does. 
Thus a compression wave in a static body is converted into more or 
less complex transverse vibrations of a flat body, whether the disk 
or the soundboard.

In the case of the bridge/soundboard pair, the bridge is "clamped" 
only in so far as it is held firmly in equilibrium between two equal 
and opposite forces.  The bridge is the locus of the points where 
these forces oppose each other and is roughly speaking at a location 
where the system is the most flexible, a good distance from the rigid 
fixings of the perimeter.  The tranverse movements induced in the 
board disturb this equilibrium and the whole structure of board, bars 
and bridges etc. is set in vibration of a complex nature.

I'm not handing this out as gospel, but merely as the very simplified 
view that I currently have of the way things work, which seems to be 
logical.  I'd very much appreciate any educated comments on this view 
from the more qualified.

JD