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Thread: Hard maple back thickness vs. soft maple

  1. #1
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    Default Hard maple back thickness vs. soft maple

    Newbie to mandolin builds but a dozen or so guitars in the past few years. I recently purchased a pre carved back made of very hard Birdseye maple that had a taped-on tag indicating it was a 1939 Washburn. I also purchased a pre carved soft maple back from Smith Creek. After getting both down to approximate Loar specs the difference in tap tones is dramatic. The soft maple rings like a bell with good sustain. The hard maple just thuds. The soft maple is an F5 whereas the hard maple is an non symmetrical two point with approximate A5 dimension.
    So, my guestion is....keep on thinning the hard maple and hope for the best (it's a beautiful piece of wood) or stop now and concentrate on mating it with a very good tapping top?

  2. #2
    Registered User fscotte's Avatar
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    Default Re: Hard maple back thickness vs. soft maple

    Just make em .170 in the center and .100 in the recurve. Not much else you can do with them since they already have a pre-carved arch.

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    Default Re: Hard maple back thickness vs. soft maple

    This is not gonna help you very much, but it is the best I can do. If the tap tone of a piece of wood is "thuddy" or dull, that will not be changed very much by changing the thickness. The "thuddy" character of that piece of wood is an indication of high internal damping. A piece of wood that rings well, giving a tap tone that seems like a musical note by comparison, has lower internal damping (think rosewood, cocobolo,...). When you change the thickness of a piece of wood, you are not changing its' internal damping. A thuddy piece of wood will remain thuddy, while a ringy piece of wood will remain ringy. When you thin a piece of wood, you are doing two things; one is making it less stiff, and the other is making it lighter. The lowered stiffness will tend to lower the pitch, aka frequency, of the tap tone, while the lower mass will tend to raise rise the pitch of the tap tone. In practice, the stiffness wins, so when you thin a piece of wood, the pitch of the tap tone gets lower. But thuddy wood will still be thuddy, albeit at a lower pitch. Similarly, ringy wood will still be ringy, albeit at a lower pitch.

    You may have known some of that at some level, so how does it help your carving? Not much. You need a handle on the stiffness vs mass (or vs density) of the two pieces of wood. In general, the stiffness (actually Young's modulus or Elastic modulus) increases linearly with density. Denser pieces of wood will be 'stiffer' (actually higher modulus) than less dense pieces of wood. The problem with that generalization is that the trend is based on averages. There are outliers, i.e.., denser pieces of wood that are not so stiff, and less dense pieces of wood that are pretty stiff. That puts us all back at square one, so to speak. Some, including some frequenting this forum, advocate a technique called deflection testing, aka compliance mapping of their plates during the carving process. That technique is good for obviating knowing the actual Young's modulus, but doesn't deal with the mass. It does work, more or less, and some will probably weigh in with explanations of the process.

    I have left some very low-density/low modulus back plates as thick as 0.160" (~4.1 mm), or slightly more, in the centers of those plates. I did one back plate on an F mandolin in birdseye maple. That stuff had a density of around 0.78 g/cm3. I ended up thinning it to about 0.115" (~2.9 mm) in the center of the plate. It was still a relatively heavy plate. It worked, in that the mandolin is OK, but I didn't like that stuff very much.

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  5. #4
    Registered User fscotte's Avatar
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    Default Re: Hard maple back thickness vs. soft maple

    .78 g/cm3? Holy mackerel... I thought using 54 g/cm3 hard maple was hard.

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    Default Re: Hard maple back thickness vs. soft maple

    This is just my point of view, so it's probably wrong, but I think of birdseye maple as full of flaws. Each pretty little node is a flaw in the wood, the grain changes, the hardness changes around it, it's just a mess. You often get flame in birdseye as well, making it worse. I get the best ring from clear wood, and densely figured wood I get less. For a back in particular, it's a good choice for visual impact, and I think the back carving is probably best judged by stiffness/compliance/deflection under load and not ring, however you look at it. A combination of the wood, the thickness, the recurve, all of that. In an ideal world it marries the top, and they have beautiful tone babies...

  7. #6
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    Default Re: Hard maple back thickness vs. soft maple

    Well, 'Birdseye maple as full of flaws'"..........

    That will certainly stir the pot, but I would agree from a commercial grading, but not musical or appearance grade point of view. I feel the same about bearclaw spruce , all varieties, wouldn't want a ladder made from it. But you grade for purpose and there are many categories in the grade book, and used to be more.

    I'll run for cover now.

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    Default Re: Hard maple back thickness vs. soft maple

    I'm somewhat hesitant about using birdseye for necks, but you can always abandon it after shaping and testing it. Depends on the particular piece. But it can work very nicely for backs, depending on the particular hunk. This is a mandola and very stable. It sounds very nice. Click image for larger version. 

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