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The details of the violin you raise:whole body modes, bass bar, sound post, f-holes, varnish... are aspects whose physics are still uncertain even after all these centuries. Making good violins is considered difficult and the fundamental reason is the vagaries of wood as a precision building material. If violins were made of aluminum or steel, you could make them all identically, to specified dimensions and they would all sound the same. You can't do this with wood, since every piece is different. The Hutchins' free plate tuning method addresses the manufacturing problem of how best to make the most important parts of the violin (the front and back plates). Hutchins' publications teach that builders should start with oversized parts which are scraped down until their free-plate resonances meet her design targets of specified mode shapes at specified frequencies. This is different from building to a blueprint. She demonstrated that this method produces good instruments. She never claimed that free plate modal analysis would explain everything. Her final two volume collection ("Research Papers in Violin Acoustics 1975-1993", published by the Acoustical Society of America, 1997)contains 121contributed papers; most are works in progress and show how much more there is to study. The most glaring omission is a full finite element model of a violin with a coupled treatment of acoustic radiation. As far as I know this has still not been done, but should be possible.

The foam rubber spacers are 25 mm high, and the speaker cone surface is another 25 mm below the top surface. The distance is not critical. The foam spacers prevent the plate from buzzing against the top of the speaker cabinet. You move the foam supports to align with the nodal lines of the mode you are trying to excite. Hutchins published articles show the plate is to be moved so the maximum sound pressure is applied at the location that gives the best response for each mode.

Not likely to damage the amplifier, which will have electrical limits built in. But it could damage your hearing, so wear ear protection. Other people in the household will complain, since it is a very annoying sound. The 20 Watt speaker I use is just barely powerful enough. I have to turn the power level up to its highest settings to work at the low frequencies needed for the viola.

The app is "Audio Signal Generator" Version 1.8, on the App Store from developer "Media Punk Studios".

@@chrispurcell7069 ah I really appreciate your swift response. Thank you for your help and keep up the making 🎻