Sound waves(The behavior of soundboards)

[Richard Brekne]

Ron Nossaman wrote:

> >> So how could a compression wave traveling through the bridge possibly move
> >> the soundboard, and what's the physics behind it?
> >
> >Actually, that bothered me a while as well, but it was this rock in pond
> >example
> >that your camp through out that got me on to it. And I think I understand
> where
> >that reasoning comes from now.  Lets take another look at the pond analogy.
>
> And once again, you're talking about what happens in the soundboard after
> something has physically moved it, not what that something was, and what
> that something was is the entire point.
>

No, I don't think that I am really.  When one drops a stone into the pond the whole
pond hasn't moved and THEN the waves start propagating though it. Same thing for
the sound board, or any such medium. The wave propagation IS the movement. That's
what wave mechanics describe, at least thats part of what wave mechanics do.

>
> My McFerrin has been lost for some time, so I can't look up the experiment
> just now. The experiments with tuning forks only proved to me that the same
> assumptions were being made as to why the handle of the fork vibrates as
> were being made as to what moves the soundboard. I don't buy the force
> disturbance and longitudinal wave thing in the forks any more than I do in
> the bridge. Sure, longitudinal waves exist, but they aren't what's moving
> the fork handle any ore than they are moving the bridge. Cyclically applied
> force from a reciprocating mass is what drives both the fork and the bridge
> with a simple action/reaction. Like pushing a cotton ball with your finger,
> or is that an internal compression wave phenomenon too, pulling the cotton
> ball along?
>

Ron, to deny that it has been shown by many, both layman and physicists alike that
longitudinal waves can drive a sound board in the fashion I and others have
described is ....well.. There is no point in discussing if that's your stance.

>
> >This, I think... is close to the rationale and it certainly makes perfect
> >sense and
> >fits well with the physics that deal with wave motion through elastic
> >medium.
>
> It does neither that I can see. How is it the compression wave hitting the
> back or bottom side pulling something moves it when pushing it at the front
> or top does not?

Who said that ? I said any force applied to the surface of such a medium, from any
angle, creates a compression wave.

> That's my question, and basic to John's and Robin's
> theory. So then a string in the top half of the cycle when it's going away
> from the bridge causes a rarification wave starting at the bridge top and
> sucking the bridge up? Or is it being pushed up from the bottom by the
> rarification wave hitting the soundboard? I think it's about time to
> include the full string cycle in the compression wave theory as it has been
> in the action/reaction theory all along. Don't you?
>

It hasn't  been explained at all in the action / reaction theory and is problematic
there at best, as I have repeatedly said. It is indeed completely in the
compression wave theory as I also described in my last, though this seems to vary a
deal from JD and Robins description as they seem to have trouble with the surface
ripple thing.  I think perhaps a closer look into wave mechanics in general is what
is needed here as it is evident that there is far to much speculation as to their
behavior and far too little real understanding floating around.  Personally having
consulted with several on the net and elsewhere I find no reason to describe wave
mechanic basics in a fashion that clearly contradicts what these resources clearly
are saying. So if we are stuck at this point, then we aren't going to get any
further pursuing that disagreement.

> >It
> >also seems to my mind of thinking easier to deal with when it comes to this
> >business of the strings partials, or segments.
>
> Compression waves - somewhat. Progressive or traveling transverse waves -
> most definitely. That, however, is another topic altogether.

er... no comment.

>
>
> >I still don't really see how any of this is really so totally incompatible
> >with the
> >diaphragm idea. As I have said all along I would suspect the real truth to
> >all this
> >lies in some combination of these two rationale and probably some other
> >things we
> >lay folk and for that matter real physicists as well haven't grasped or
> >thought of
> >yet.
>
> At this point it has nothing to do with the diaphragm idea or anything else
> besides the cause and effect between an applied force and a resultant
> movement. We have not gotten one iota away from the initial assumption and
> statement that an internal compression wave initially moves the soundboard,
> which then moves the bridge. I'd like to know how that is supposed to work.

I disagree entirely. though you are certainly free to remain stationary in your
position. I presented my thoughts and what I came up with hunting around for
information on the discussion and you seemingly didn't agree with any of it. Are we
ok with that ?

>
>
> Ron N




--
Richard Brekne
RPT, N.P.T.F.
Bergen, Norway
mailto:rbrekne@broadpark.no