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I am building a mandocello, Gibson style but with f holes, currently working on the top. I got to a point where I carved it to about 5 mm in the center and about 3,3 around the edges. If has some flexibility but still feels quite stiff. I tap and listen but having experience only with flat top guitars I don't really know what I'm looking for in terms of tap tone and flexibility.
I know there is no correct answer to this, but anyone who has built similar archtop instruments... how do you know when to stop removing wood?
Yes, I normally use the Chladni patterns the same way, but after reading Peter Coombe's article I thought it might be interesting to give it a try.
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One more update. I have been generating Chladni patterns for the top. I normally do this with the closed box, as most luthiers seem to think that on free plates it's not very useful, but I found this publication from Peter Coombe: https://www.petercoombe.com/publications/jaamim7.html ... So I thought I'd give it a go, more with the hope of learning something in the long run than anything else.
I got the three patterns seen below, of which the ones at 126 and 258 Hz could correspond to what Peter Coombe calls modes 1 and 4, but it's hard to tell. The frequencies of course are lower than on the mandolins. Obviously he builds mandolins with oval soundholes and X bracing, so it's a different thing.
I think I will try the same with the back and see if I reach some conclusion.
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Hello Jorge,
The Chladni patterns are interesting, but on free plates they don't give much info because the free plate modes are not very well correlated with those of the plate attached to the sides (and back).
Usually I make them on the closed box or at least on the plate (top or back) attached to the sides, but only as a diagnosis tool to check if the mode i am considering for whatever reason is the one that I expect.
About Chladni patterns, if you want to have nicer figures, i suggest you to mix different particles with different sensitivities (i.e. tea leaves, oregano and poppy seeds) so that some will stop on the nodal lines.
Also, to have a powerful but small diameter speaker helps a lot!
Kind regards,
Martino
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Thanks again Martino! :)
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Thanks a lot Martino!
That is definitly useful. Just one question, how much weight is used to measure those deflections? Or is that under the string tension?
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Yes, sorry, I have forgot to say that i measure the deflection with 1 Kg.
Martino
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Hello Jorge,
On finished mandolins (with bridge and strings on), I would expect a deflexion of 0.1 mm for very mobile instruments. About 3/4 as much for more "ordinary" ones.
As an example, on my F-Jazz (falcate bracing) I usually measure 0.09-0.1 mm, while on more common F5 there is a wide range but usually between 0.055 (tone bars) - 0.075 (X-bracing).
On one of my latest mandolas (a Mandola PeRfetta) i have measured a deflection of 0.12 mm but that's also a falcate bracing instrument and the angle at the bridge is very shallow.
I would suggest you to mount the top on the sides and use a bridge or a bridge proxy to have a reasonable comparison.
Since the tension on the mandola strings is higher (on classical mandolins/mandolas it is approximately 1.3 times higher), I would aim for the same deflection with the increased load.
I hope this can help. Keep us updated with your results!
Cheers,
Martino
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Hi Jorge,
Cool!
Have you ever tried to measure the flexibility/stiffness of the top using a mechanical jig?
I usually measure it with the Rialto Jig that you can see in the attached pictures, typically on finished instruments, but also on plates during the building (attached to the sides).
Also, if you attach it to the sides, it would also make sense to make a tap tuning at that point by measuring the spectrum and carving the bracing until you get to a reasonable stiffness or frequency.
Cheers,
Martino
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Hi Martino.
Thanks! Yes, actually I have a similar device which I made myself, I will take a measurement once the top is attached to the sides, as you say.
What I am not sure is what amount of deflection I am supposed to expect. I usually build flat top guitars, but those have much more flexible tops. Can I ask how much deflection you aim for in mandolins? I know a mandocello is different, but I'm thinking I could make a calculation of what to expect based on the size difference and also the tension difference from a mandolin to a mandocello. I think that would help, at least to know which range I am supposed to move on.
For flat tops I usually get between 0,10 and 0,15 mm deflection with about 1 kg weight. I expect to have less deflection than this, but if you could provide some numbers you get for mandolins, it would be so kind of you. As I said, just to use as a reference. I release you from all responsibility if anything should go wrong on my side :P
Cheers,
Jorge
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Just in case anyone is interested, I finished the top for now. It's still very rigid in the center, not sure if I should remove some more wood from the tone bars... I think I will move on for now and maybe decide to carve them some more after gluing the top to the sides.
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Thanks a lot Martino!
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Hello Jorge,
On my archtop mandolins i don't use tap tuning. Instead i tend to thickness the top until reaching a certain target mass and then I brace it to achieve a certain target stiffness. I have no numbers for a mandocelo though...