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I would love to hear your thoughts on choosing an appropriate top for a given body size/shape. For this discussion, I'm considering how tone is affected as I think I have a decent understanding the structural considerations.

Do you prefer to use a less stiff top on smaller bodies and stiffer top on larger bodies or do you want the same stiffness irregardless of body size and then thin the top more for the smaller body?

If you like the tone of a particular species, do you stick with that species no matter the body shape or do you choose a particular spruce species (or density) for a given body shape? For example, if someone likes the sound of red spruce on a dreadnaught........would that individual probably like red spruce on a 00 sized body?

Do you prefer tops with less overtones and more fundamental for smaller bodies and more overtones and less fundamental for larger bodies? Do you consider overtones at all when selecting you top wood? Does the back wood used on the build influence your decision for top wood?

Do you want more cross grain stiffness for a larger body and less for a smaller body? Do you care about or even measure cross grain stiffness when choosing a top for a specific body size?

I look forward to hearing your thoughts.

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Formerly known as Adaboy.......

I hope you don't view this as a highjack, but your question got me thinking.... I think it's directly related.

I've heard others discuss the importance of stiffness to weight ratio as a determining factor - more important than species of topwood - in impact on sound of a guitar. So, following this approah, density of wood, (measured in g/cubic inch and the like) and deflection testing are the recommended approach for creating and replicating desired properties.

But has anyone done any testing on the sound qualities produced on different species with identical or very similar deflection and density? (Maybe density isn't the issue - cedar's less dense than spruce in general, so you'd need a thicker cedar top to match the deflection of an "average" spruce top in terms of stiffness. Thinking as I type here) - But what about testing various spruce tops (Sitka, Engleman, Lutz, Adi, etc.) versus Cedar, Redwood, Doug Fir...... assuming consistent deflection parameters between the samples? E.g. thinning the top to the desired deflection, then testing the sound on the finished guitar...

This might be interesting...

To me,stiffness with and across the grain is the most important element for the top plate no matter what the size of the guitar. That will give me the lightest plate possible. Same thing for the braces. The type of wood will give you different sounds which can be good or bad depending on the stiffness. Stiff being my favorite so far for all wood types.

Ditto.
I use the stiffest I can find in the species I have. Different species will give a different tone IMO...Red/fundamental, tons of headroom, bell-like trebles; Sitka/a bit warmer; Italian/warmer still with very crisp, solid trebles; Engelmann/warm, can get a little mushy; and German/very warm, most complex, lots of headroom. How much of these traits is evident depends on the stiffness too. Probably about 60-80% of the tonal qualities can be attributed to the top and the rest to the back (if I had to put a number on it).
I don't have a preference as to what combinations of top/back wood to use for any individual instrument, It's more about what kind of tone I am trying to achieve.

Longitudinal stiffness doesn't change with size, but for smaller guitars I tend to use tops that are floppier across the grain, it seems to help with bass response. Overtones content does not seem related to top species, at least not the way I build and brace. I find common sitka less defined, "fuzzier" than other species, but that can be what is desired for a given instrument.

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

Has anybody seen or heard much about the talk that Mark Blanchard gave concerning Chladni tuning at H'burg a few years back? He has an interesting method of picking his tops based on it.

Mark has done a lot of work with the modes of unbraced tops, and keeps really good records. He noticed some time ago that the guitars he liked the best had some things in common in the unbraced free plate modes. In particular, there were three modes that he found to be of interest. The first involves mostly bending across the grain, the next was the familair 'ring+' mode, which does a lot of bending along the grain, and the third is a 'double ring' mode, with a node down the center line and a closed (more or less) ring on either side in the lower bout.

What Mark found was that the best guitars had the ring-type modes 'closed' on the unbraced plate, and the crosswise bending mode came in within about 30 Hz of the 'ring+'.

The relative pitches, and mode shapes, depend a lot on the 'aspect ratio' of the top; how wide it is relative to it's length. Obviously, on a wider top of the same length, the crosswise bending mode will be lower in pitch, and the 'ring+' might be too floppy in the cross direction to 'close'.

What he does these days is to take all the joined tops to some extra-beefy thickness, like 4.5mm, and cut them all to the shape of his biggest guitar, which is more or less like a Jumbo. He then looks at the modes. Any top that has the 'right' modes can make a good Jumbo, and the ones that don't get cut down to the next smaller size, which is narrower in relation to it's length, and tested again.

As Laurent said, the practical outcome of this is that the 'floppy' tops go on the smaller guitars, and the ones with higher crosswise stiffness end up on Jumbos and such.

I've been testing tops this way since I heard about it, for a couple of years now. I already test the wood to determine the lengthwise and crosswise Young's moduli, so what I'm doing is building up a data base of the range of stiffness ratios that will work for a given shape. This sort of thing takes time.

The next step, for me, involves looking at the lengthwise Young's modulus value, and using that to determine how thick to make the top. My assumption is that, over time, cold creep will reduce the effectiveness of crosswise stiffness in resisting bridge torque, so ignoring it for the purposes of strength can possibly be justiified, and simplifies things at any rate.

I'll note that Young's modulus along the grain pretty well tracks density, with any two pieces of softwood having similar density also sharing thier lengthwise stiffness characteristics. In general, a lower density top will end up thicker, but lighter, for a given stiffness. I like to save those low density tops for Classicals, where weight is a major deal, and use the denser stuff on big steel strings, where the extra mass seems to help with 'headroom'.

You'll note I don't talk much about the species in this. I'm tending more and more to the opinion that spruce is spruce; that the properties of the individual piece are more important than species properties. Although you can generalize to some extent, the range of variation in any softwood species is so broad that there's a ton of overlap. Engelmann spruce, for example, is usually low in density, but I have some that is as dense, and stiff, as any Red (Adi) spruce in my stash. _Maybe_ there would be a clear difference between guitars made of Red and Englemann of the same density, but I'm not all that sure. I fear the actual experiment would be very difficult and time-consuming.

Interesting replies, thanks.



After looking at body length dimensions, I see your point. The length of the body changes little between a 14 fret 00 and a 14 fret dreadnaught (roughly 1 1/8"). So there isn't a lot of variation. Interesting to read your comments that harmonics don't seem to be related to top species.

Al, I didn't hear Mark Blanchard's presentation but it was sure nice to read your summary. I assume that by keeping the cross-grain vibration mode frequencies close to the ring and a half mode frequencies Mark is choosing an appropriate cross grain stiffness for a particular body width. That's a very good tip. In my way of looking at things, it seems the ratio of cross grain stiffness to stiffness along the grain would have a big influence on the shape of the modes. For instance, seems it would be difficult to close the ring in the lower bout if the cross grain stiffness is too high relative to stiffness along the grain. So maybe low cross grain stiffness would go well with nonsymmetrical, Martin X bracing.

On my next build I would like to play with a lot of ideas by building a box where I can swap tops and change bracing height etc. without taring up the sides when removing the top/back.

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Formerly known as Adaboy.......

Yes, body sizes usually do not increase or decrease proportionally. However my point is more related to scale length, thus string pull: I use the same two scales (25.6" and 25") on all my body sizes. Whatever the body size, the length between neck joint and bridge does not change.
Overtones content does not seem to be related to specific top species IMHO. I've built very rich sounding red spruce guitars and very fundamental German ones. Controlling overtones is more a matter of relationship between top and back, body depth, back thickness and back & sides wood choice.
Dynamic range and volume seem directly related to stiffness, especially longitudinal stiffness, which makes a lot of sense. Of two identical guitars (they never are…), one with a stiff thin(ner) top, the other with a weaker thick(er) top, the former will have more volume and be more responsive.

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/