----- 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
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