Quote:
Originally Posted by Arnold Schnitzer
... there are makers who believe that the back table should be as stiff as possible and not vibrate, therefore allowing the top to vibrate more. Then there are makers who believe the back should behave as a secondary soundboard and vibrate like crazy. Whether you rework a back table's thickness (and by how much) will depend on which camp you are in. I've seen back thicknesses (round-backs) of 6mm all the way to 14mm.
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Interesting comment. Put me in the back-needs-to-vibrate camp.
As far as back plate stiffness goes
my understanding and belief is that the top and back plate should match in stiffness somewhat. A double bass resonates similar to big violin when it has a carved back plate. Customarily the plates are usually tap-tuned so that they are about a tone or semi-tone apart or thereabout. This seems to be traditional and results in a normal sounding and responding instrument (violin, viola or cello and roundback basses).
If the instrument sounds stiff and non-resonant it might be a candidate for a regrad. After measuring the thickness of the top, if excessively thick, I might consider taking it off for some thinning. Any structural concern would trump any thinning for resonant purposes.
While the top is off, I would assess the top and back thickness and get the frequencies of their modes, Taking into consideration the type of wood but regardless, I might try to match the back to the top by thinning one or both while the top is off. One tone to a semitone is what I would shoot for depending on where they are to begin with. There would no advantage to removing the back for this. The tap tone of the back on the ribs will be close enough.
If it were a flat back bass, the individual braces might be too thick or thin and can be thinned or rebraced to achieve a normal stiffness. there is no tap tone of a flat back, it acts more like individual sections with their own resonances in each section. The platform that the soundpost sits on I think of as a separate resonating body. But it shouldn't be too floppy or stiff. I have no tuning scheme for the back braces but here is my belief. Flatter, thinner back braces act more like a carved back, smoothing out the resonances with higher damping to spread the peaks of the individual resonances of the braces tones therby making it more like a carved back which has a more damped tones. Flat backs are peaky with closely spaced resonances, while carved backs have less peaks but are much wider with higher damping. They vibrate in a different ways, if you have lazor and a camera you can see it clearly, or better yet, just read about it.