Advice for Sound board flexibility.

[Bj Taylor]

I’m thinking of going in a different direction as far as carving my soundboard this go around. Instead of using Gibson prints I want to go more off of tapping and flexing the wood.However, I was having trouble locating information on flexing. Flexibility of the top seems more important to me Since there are so many factors to consider in inconsistent grain from instrument to instrument.

[Bill McCall]

I’m sure you have lots of experience with wood for tops and your hands can provide great information as to the appropriate level of flex. And your knowledge of tap tones, combined with the flexing, should allow the creation of excellent instruments.

good luck.

[Bj Taylor]

Thank you. However I am always looking for some additional, experienced advice. As much information as there is out there for tap tuning one would think someone would have documented, in more detail as to how far one should take a top using a flex method.

[Marty Jacobson]

Thank you. However I am always looking for some additional, experienced advice. As much information as there is out there for tap tuning one would think someone would have documented, in more detail as to how far one should take a top using a flex method.

What it sounds like you're asking for is some kind of target or metric for wood flexibility, and how that correlates to a functional soundboard.
The closest thing to that is deflection tuning, which has been discussed often here, including a few rigs like Adrian's which let you carve the soundboard while measuring the deflection at the same time (which is genius).

One thing to consider is that wood flexes differently along the grain and across the grain. Very qualitatively (and I stress that very qualitative info from other builders has caused me to ruin 3 otherwise perfect instruments, to a total of around 200 wasted hours of my life) - I look for "a bit" of flex across the grain, and "some, almost no" flex along the length of the grain.
"a bit" -- maybe a total deflection of 1mm, maybe 2mm, I haven't measured it - but how much force am I applying to get that deflection? No idea.

So yeah, I do a thing similar to what you're talking about. Not sure if that will help you at all since there's no way to know (at this moment anyway) how much flex I look for, and how much force I'm applying (not much), versus how much you'll think is good and how much force you're applying.

This is like a recipe given by a chef - add salt to taste. Hmm, that doesn't help me much if I've never had the dish before, or don't know how concentrated the salt is, or how big the pot of soup is, etc.

[fscotte]

I'll just post my notes I've saved to my computer over the years. Probably the only measurements that would be valuable to you is near the bottom, Macrostie's measurements he did on my Gibson A5L and my Isabel #6.

These are just my personal notes as I track the deflection of some of my builds, using MY jig. Your jig would be different. I use an assortment of interchangeable bridges on my jig, and even have a one footer bridge so I can check deflection at the very center of the top.

Note, back plate measurements are more difficult to obtain, and even Macrostie has trouble getting consistent back plate numbers.

All in all, I like to see something around .19" treble and .24" bass, or in that area, using 25 lbs. Its fairly consistent with my Gibson A5L deflection, which is a fine sounding mando.


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DEFLECTION NUMBERS ON VARIOUS MANDOLINS:

-2 footer bridge:
-2 caliper system (a little closer to bridge since no 3rd caliper)
GIBSON A5 Top: .020", .026" - 25 lbs



-2 footer bridge:
-3 caliper system: (.001" less due to being farther away from bridge):

Gibson A5: TOP .019", .016", .024"
Gibson A5: BACK .017", .013", .014"


Mine #8: TOP: .020", .018", .021"
Mine #8: BACK: .0125", .014", .013"

Mine #11: TOP: .031", .020", .025"
Mine #11: BACK: .010", .011", .010"

Mine #12: TOP: .023", .017", .024"
Mine #12: BACK: Very stiff, 3 transverse bars.





-1 footer bridge: 1 caliper

#8: TOP: .025" (25 lbs)
#8: BACK: .020" (25 lbs - before scraping away center tonebar)

Gibson A5: TOP: .026" (25 lbs)
Gibson A5: BACK: .024" (20 lbs - less weight used here)



Don Macrostie's Jig on visit to his shop 3/31/2015:

Gibson A5:
Top: .022", .028"
Back: .025", .021"

Isabel #6:
Top: .034", .029"
Back: .035", .037"

Note: Macrostie's personal target is a total of .050" to .060" deflection with his 25 lbs. The Gibson A5L has a total of .050" on Macrostie's jig.

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[Bj Taylor]

Where do I find the discussion about the deflection, Adrian rigs? I just learning how to navigate this website. I'm sorry if its right in front of me.

[Martian]

I do deflection tuning as well . You may find some on youtube, what I do, Is I have a jig that puts weight( I would have to look that up), on where the bridge would be replicating the string load. I have dial indicators on both sides of the bridge, and when I apply the weight, the top deflects. You read the indicators on both sides, then you attack the stiff side first with sand paper in the minimal area then re- check usually , both sides move some. so, for instance , If I want it to deflect 30 thousands, and I get 25 on e side, and 12 on the other, I sand on the 12 side . both will move some but the side you are sanding goes faster. You want to find the string load ( weight at the point of the bridge. I made a jig, to hole the plate, and pur a digital bathroom scale in place of the plate, then added water to my weight until I got 30 lbs, Now, I will also say everybody here is smarter then me, so this could be very arguable, but my mandos have come put pretty consistent

[Richard500]

As an outsider, I would think that static deflection gives some baseline hints about dynamic response, and maybe it doesn’t matter, but loads from a tunable transducer might be more to the point. I do understand that all sorts of mandolin geometries can sound great, and you’re not necessarily trying to replicate one maker’s universally praised sound, as, say makers of violins may do. So keep on experimenting, and keep the art and craft going.

[HoGo]

There are no staright and true answers to such quastions. There are too many input parameters in instrument to boil it down to just few numbers. The shape and height of arch, shape of recurve, type of woods and style of graduations and bracing all have their impact on result combined. If you are carefull and reduce tha variables to minimum (i.e. use sa similar woods as possible, same arching, same graduation scheme) then the deflection will tell you how the exact thicknesses work for you.
You have to count that you can get the same reading of deflection with very different arching scheme and graduations and tonebars shape position and thickness of the tonebars...