Originally Posted by
peter.coombe
Cohen and Rossing did the research mostly on arch top mandolins, but they also measured others and found the same thing but at different modal frequencies. So, yes a graduated carved soundboard does vibrate similar to a flat soundboard fixed bridge guitar. However, from the measurements I have made, it looks like a flat top mandolin is more similar to a flat top guitar and a carved top mandolin is more similar to an archtop guitar. They vibrate in the same way, but the relationship of the top to the back is different. So, when you measure free plate modal frequencies, you need to tune the back higher in the flat top mandolins (and guitars) to get the same relationship as the arch tops in the completed instrument. This gets you closer to what Gore and Gilet call a "dead back". They claim that in their experience the optimum relationship is a 4 semitone difference. I get bang on that by tuning the free plate ring modes to the same frequency, but on the flat top mandolins the ring mode of the back needs to be about 8-9 semitones higher than the top. The back needs to be so stiff and heavy that it is difficult to implement, but they sound much like a carved and graduated mandolin but louder because the top is lighter. It is a massive improvement in sound (sound quality overlaps my carved and graduated mandolins), and it is something I would never have guessed if I had not read the Gore/Gilet books. Making the back heavier and stiffer on a flat top mandolin changes absolutely everything, including the sound in a profound way. So, mandolins really do work similar to guitars. Note that what I have done has been on oval hole mandolins, F hole mandolins are different, the main back mode is about 6 semitones above the top on the ones I have measured, but they still vibrate the same way as a guitar.
Absolutely correct, empirical trial and error works and will get you there in the end, eventually. However, I have done something completely different with my flat top mandolins, and that required some knowledge of how guitars work. I would never would have guessed it (what most luthiers call intuition, I call educated guessing). I started out with a challenge. Flat top guitars can sound wonderful, so you should be able to make a wonderful sounding flat top mandolin, but in general they don't have a very fine refined sound at all. They range from just plain awful to not too bad but somewhat rustic sounding. I started out making them like Gibson, and they measured similar to a vintage Gibson flat top and sounded better but not in the same ballpark as my carved and graduated mandolins. Eventually I came to the conclusion that Gibson got it wrong. Obviously Gibson did not get it wrong with the Loars.
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