Modal Analysis (was Negative bearing (long))

[Delwin D Fandrich]

----- Original Message -----
From: "Richard Brekne" <Richard.Brekne@grieg.uib.no>
To: <pianotech@ptg.org>
Sent: December 04, 2001 11:53 AM
Subject: Re: Modal Analysis (was Negative bearing (long))


> 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