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Does anyone use the deflection test for guitar backs to correlate with the deflection test for the tops? This is of course for those who believe in active backs.

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Dean

I don't think in an active back such as a rubbery 0.08 mahogany, lightly braced dreadnought, since I build small box classicals with rather thick backs and very strong braces, but I recorded the deflection for my last 3 guitars. I am hoping that in time I might see a trend and relate to how those guitars sounded or projected.

Here are some numbers:

Indian rosewood, app. 2.5mm all over, deflection: 5mm
Indian rosewood, app. 2.3-2.4 all over, deflection 4.5mm

The density of the second back was 830 Kg/m3 but I don't know the first. In any case on paper it looks sensibly worse, while in hand the wood was so much nicer The softer back guitar also had scalloped back braces and a thinner top, overall sounding more open and warm compared to the hard back guitar, which is more clear and with more sustain. Of course the top must have the lion's share in the sound difference.

The third back:
3-piece of Madagascar rosewood 870Kg/m3 with a center panel of African Blackwood (didn't bother to measure the density but it is something ridiculously high anyway), average 2.3mm, some spots 2.4, the wings down to 2.1mm, deflection: 4mm. The weight is serious you can use the thing as bulletproof armor or smth of this sort This one again fails miserably in the stiffness/weight ratio contest.

The third back also features my strongest braces to date, in narrow and tall euro spruce weighting some 480 Kg/m3 (unbelievable stuff and I think the sap is 500) so I am really curious if I could sense any projection increase compared to my previous Indian rw guitars with lighter back bracing. It's not finished yet, still have to glue up the FB and bridge...

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Build log

Alexandru,

How much weight do you use in your deflection testing? I've been interested in trying this. I also have ordered Al Carruth's DVD on Chladni testing that I am looking forward to trying as well. I'm not set on active or stiffened backs one way or the other. I think both have their pro's and cons just like every other aspect of design.

I like to do whatever it takes in design to meet the needs/desires of the specific player receiving the instrument (as does most every one in the forum I'm sure).

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Bill Hodge


One does not simply, own enough guitars!

I use the same 500 grams scales weight for both tops and backs, since the compliance of spruce and rosewood seems to be very similar at similar thickness - it is only the weight that is nearly double for rosewood.

I've tried with 100 grams, not sufficient, and 1Kg, too much. 500 seems to track changes made by thinning pretty good. I clamp the entire width somewhere not much below the soundhole, while the weight sits somewhere close to the bottom. In a way it is good that it is localized instead of spread across more width- an offquarter, floppy top will bend some more right under it, so the test takes some of the cross-grain stiffness into the equation too.

While I record the numbers at a couple thicknesing stages for both the top and back, and hope to see trends and have some reference for future builds, I think that flexing and judging the wood with the hand should be in the arsenal of every builder. The hand seems to tell some things numbers don't. It can apply pressure much more dynamically (in any way you can think about it) than a mere fixed weight and spot does. At some stages there are tops that while nice on paper, they might feel a bit "off", lacking better words for the feeling.

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Build log

Thanks Alexandru that gives me a good start on that. Do you ever try testing the top once it's attached to the rim before adding the back?

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Bill Hodge


One does not simply, own enough guitars!

That's a good ideea! I know a couple builders doing deflection tests on finished guitars. Didn't try it myself so far since I don't have a dial indicator. I measure plate deflection simply using the caliper's depth arm at the butt end edge. It's not terribly precise, and it's a tad slow, but I trust my eyes and I think I can claim 0.1mm accuracy. For the moment it is OK. The dial is on my priority tool list

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Build log

You can pick up a pretty decent dial indicator at Harbor Freight on line for mighty cheap and they're accurate to about +/- .001"

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Bill Hodge


One does not simply, own enough guitars!