Sound waves(The behavior of soundboards)

[Ron Nossaman]

>> 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. 


>Then too its been shown beyond any shadow of a doubt that the sound board 
>assembly
>accepts longitudinal input which results in sound being created from the 
>assembly.
>McFerrin quotes an experiment, Robin and JD have cited similar things with 
>tuning
>forks and other examples, and I have this upright standing in my shop that 
>has no
>strings... I attached my brass rod clamped in the middle experiment to the 
>bridge
>and induced longitudinal waves in the rod and boy did the panel scream. All 
>this
>put together tells me that the string vibrating at the bridge termination does
>nothing more then exert a force on the surface of a 3 dimensional wave 
>conductive
>medium, which must react as all such medium 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? 


>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? 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
>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. 




>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.  

Ron N