Modal Analysis (was Negative bearing (long))

[Richard Brekne]

Yep... thats about how I heard it described... sounds like a kind of modal
analysis variant to me really. I suppose then that the rest of the story is true
too then... that the patterns in the sand form only when the input frequencies
from the shaker approach soundboard resonate frequencies ? If so this would seem
to be a relative easy way to get some idea or not as to whether Wogram was
correct or not.

Couldnt you just run the experiment on an unloaded board, noting the reasonate
frequencies, and then compare to what you could see of the results in the loaded
condition... I mean true enough the plate block quite a bit of what you can
see... some more then others... but still should be able to get an idea
anyways... or what ?

RicB




Delwin D Fandrich wrote:

> > On the side and off a bit into the mists of Avalon if I may..... I seem to
> remember hearing somebody throwing one of those Steinway stories at me years
> ago about throwing sand on an unloaded soundboard and putting a set of
> frequencies on the panel to see what pattern the sand went to... and then
> trimming, sanding, beveling...whathaveyou the panel appropriatly to achieve
> a certain pattern they had found led to what they considered the desired
> result... does anybody know anything about this... whether or not its just
> another one of those myths... It sounds essentially like a kind of modal
> analysis technique..
> >
>
> To a limited extent, perhaps. These are called Chladni (sp?) patterns and it
> is an experiment that is fairly easy to conduct. You would need a simple
> sweep-frequency waveform generator, a shaker of some sort and some really
> fine sand. A little glitter mixed in might will make it sparkle nicely. The
> sweep-frequency waveform generator need not be expensive unless you are
> after frequency readouts of high accuracy. The shaker can be home-made. I've
> used an old two-way automotive loudspeaker which lost its foam diaphragm
> surround. (The greater the power handling capacity of the speaker, the
> better. And the wider it's frequency response, the better, although you
> won't find these patterns mean much above a few hundred cycles per second.)
> This speaker had a 'tweeter' mounted on a stamped steel bracket spot welded
> to the main ring. I took the tweeter out and replaced it with a Teflon
> insert. I carefully drilled a hole in the center of this insert. I glued a
> small circular piece of laminated balsa (to keep the weight down) fitted to
> cover just the dome of speaker. (Drill a hole (the same size as the hole in
> the Teflon)in the center of the balsa piece before gluing it in place. Now
> you need a stinger. This can be most anything. A Fiberglas dowel will work.
> Insert this dowel through the Teflon tweeter insert and glue it into the
> hole in the balsa disc. Now you can cut away the paper (or whatever)
> cone--the stinger will hold the wire coil in place.
>
> It's not terribly robust, but it will work. And it's a lot cheaper that your
> typical commercial shaker.
>
> Mount the speaker on an overhead arm, hook the stinger up to an appropriate
> driving point on the tenor bridge, run the signal from the signal generator
> through an old audio amplifier, sprinkle a bit of sand evenly and smoothly
> over the surface of the soundboard and crank away. Start with some very low
> frequency and very slowly work your way up watching for patterns to form on
> the surface of the soundboard. It's fun and very low calorie.
>
> This is one I've never figured out how to do on a loaded board. The plate is
> in the way and you can't see what's happening underneath it. You can't turn
> the piano over and do it on the back of the board because the ribs get in
> the way. Ah, well...it's still fun and low cal!
>
> Del




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