Sound waves(The behavior of soundboards)

[Richard Brekne]

Keith Roberts wrote:

>  I say for sure that
> > compression waves have nothing to do with it. If they don't have anything
> > to do with it, I want to know why not.
>
> Me too. Lets build a bridge out of a material that doesn't support internal
> or compression waves but does everything the "bridge movement" camp says a
> bridge ought to do. Then the resulting change in sound will show us how much
> they don't have to do with it.
> Keith R.

Sounds like fun...

But seriously folks. So much of this disscussion seems to be arguing about what
comes first, the vibration (whatever sort) or the movement.  Forget about what
kind of waves move what for a moment and consider the following division between
the two camps.

One camp says basically that strings get hit by the hammer and start to vibrate
and then the bridge is moved, and then this moves the sound board.

The other camp says the strings get hit by the hammer and start to vibrate which
imparts an energy wave into the bridge which propagates through the entire
system causing it to vibrate.


Now really... depending on how you choose to read both of these statements, they
can either be totally opposed to each other or saying really the same thing in
different ways. Actually both statements seem a bit akward if you ask me because
what is really happening is that vibration and movement go hand in hand and
follow each other step for step.  Let me reword the two statements above

1: The string hits the hammer causeing it to vibrate which vibrates the bridge
which vibrates teh sound board.

2: The string hits the hammer causeing it to vibrate which imparts vibration
into the bridge which propgates throughout the entire system.


I go back to the begginning of this whole discussion and find three significant
falsehoods that were stated.

1. That <<sound does not move or propagate through the soundboard. >>  This is
clearly false. what significance it has in producing the sound, if any is
another matter tho apparently it has minimal affect.

2. That <<the soundboard / bridge can not accept longitudinal waves that result
in sound radiation from the soundboard.>> This is also clearly false, tho again
its  significance is another matter and again apparently minimal.

3. That <<the bridge / soundboard do not physicaly move, or that such movement
is not responsible for the production of sound.>>  This is probably the most
clearly false of the three, and despite the several retreats and restructuring
of this basic statement, it remains clearly false.

Sound is produced because some physical object or force compresses the air which
naturally enough wants to return to its natural uncompressed state, so it pushes
the air adjacent, which does the same and it all spreads out until everyone is
relaxed again. I am reminded of the chow line in boot camp where the drill
sergent wanted to pack as many of us in the mess hall as tightly as possible....
"Make the guy in front of you SMILE" he would shout. Now THERE was an
interesting compression wave...grin.

We are not dealing with magnetic forces here, or photon torpedos... or any of
that. Nor are we dealing with seperate entities or quantities. Vibration  IS
motion.


Richard Brekne
RPT, N.P.T.F.
Bergen, Norway
mailto:rbrekne@broadpark.no
http://home.broadpark.no/~rbrekne/ricmain.html