Official Luthiers Forum!

I bought a sinker redwood top to build with. Compared to my other spruce tops it doesn't seem very stiff. I guess redwood just isn't as stiff as Sitka, but I'm wondering what size guitar I should use it for. ...probably something smaller, like an OM or L -00. I was thinking on using it for a grand auditorium, a la Taylor GA. Perhaps a smaller size would better. What do you think?

_________________
Steve Sollod (pronounced sorta like "Solid")
www.swiftcreekguitars.com

Yep, redwood is one of the most variable woods there is. I have some from Dave Maize that feels almost as stiff as spruce for a given thickness, but light as a feather. I don't do numerical deflection testing, but I'm pretty sure the stiffness to weight ratio is far above typical for any softwood. I've used other redwood that's more normal stiffness to weight, but still lighter than spruce, so just leave it thicker.

I've never understood the logic of using high density woods on large guitars and low density on small. Seems opposite. I guess the goal is to keep the numerical thickness constant while having higher stiffness on large guitars... but then the large guitar top will be much heavier. Using a thin high density top on small guitars, and a thick low density top on large should keep the weight closer to constant, due to "cube rule of stiffness". But I haven't built enough to verify the tonal effects of that theory.

Bottom line, don't worry about numerical plate thickness. Measure stiffness using your fingers or a deflection testing setup, and use that top on any size guitar you want.

I'm no expert but I have three sets of redwood, one of which is sinker. They are all as stiff or stiffer than the Sitka & lutz I have.

_________________
Joe Beaver
Maker of Sawdust

All the redwood I've tested, including several sets of 'LS' wood, have had high long-grain stiffness. They've also been fairly dense, and actually less stiff for their density than many woods. They tend to make a slightly heavier top than some of the other softwoods in the same density class, but the low damping factor helps make up for that.

I have handled redwood that was much less dense, but that was before I started testing, so it's hard to say how it compares.

As far as I can tell, all woods vary a lot, and I don't think redwood is any more variable than most. Then again, I haven't tested more than forty or so tops of any one wood, and far fewer than that of redwood., so I may not have a good handle on the range of variation.

"I've never understood the logic of using high density woods on large guitars and low density on small. Seems opposite. I guess the goal is to keep the numerical thickness constant while having higher stiffness on large guitars... but then the large guitar top will be much heavier. Using a thin high density top on small guitars, and a thick low density top on large should keep the weight closer to constant, due to "cube rule of stiffness". But I haven't built enough to verify the tonal effects of that theory."

I think one reason for that suggestion is that on small bodied guitars, especially those which are deeply waist-ed, the sides support and add some stiffness to the top, and you don't have large unsupported spans, so you can get away with using a less dense top. It may even be preferable in some cases, if you believe that a minimum thickness of wood is needed for good overtone production (avoiding the thin membrane - banjo sound)
Large bodied guitars are often played hard - and a stiff , somewhat heavy, less easily driven top may be desirable to give a bit more "headroom".
There are exceptions (and exceptional guitars) - some large bodied guitars are great fingerstyle instruments and some small bodied guitars may have plenty of headroom, but that is not how it generally goes.

It's because it's not a linear relationship, the load is proportional to the square of the span(similiar to the cube rule of stiffness), so larger guitars need to be stronger to support the same string load.

_________________
A man hears what he wants to hear, and disreguards the rest. Paul Simon

Roger Knox wrote:
"... larger guitars need to be stronger to support the same string load."

Well, stiffer anyway. Of course, you can get the stiffness you want by simply leaving the lower density top a bit thicker.

Back in '96 Howard Wright noted in his thesis on guitar structure and tone that the change people could hear the most was an alteration in the ratio of top area in the lower bout to mass. A higher A/m ratio produces more sound. His thesis advisor, Bernard Richardson, pointed out in a subsequent article in the CAS 'Journal' that it'is easier to get a high A/m ratio on a small guitar than a large one, due to the way stiffness varies with span. Basically as you make the lower bout larger you have to beef it up to retain the proper stiffness, and the weight goes up faster than the area. This would argue that there'a a bigger payoff for using low density wood on large guitars, since that would help you keep the weight down.

You do, of course, need to leave the top thicker. If my experience is any indication, you also want to use the stuff that has higher cross grain stiffness on the wider patterns. Jumbos tend to be wider in relation to their length than smaller guitars like OMs or 00s, because they're not that much longer than the small guitars, but a lot wider. Something else to keep in mind.

Since stiffness is usually proportional to density, couldn't you make the argument that the higher density top also has greater stiffness, so it could be left thinner to keep the weight down? I understand that gets back to the stiffness/weight ratio, and if lower density wood tends to have a higher stiffness/weight ratio, then the low density wood on large guitars makes sense.

Physics is so much easier with ideal materials...

_________________
A man hears what he wants to hear, and disreguards the rest. Paul Simon

The thing is that the Young's modulus varies more or less linearly with density in the range of values we're dealing with. Stiffness of the plate varies as the cube of the thickness times the Young's modulus. When you crank the numbers through the equations the less dense wood ends up making a lighter plate in general. Note, however, that there's a lot of variation in wood (it's not an engineered material, after all). In the testing I've done, about 2/3 of the samples fall within 10% of the predicted line when you plot Young's modulus along the grain against density. That's pretty good for a natural material, but it's still a pretty fair scatter, and some samples fall 'way off the line. The samples that are well 'above the line', with a notably higher Young's modulus than the density would predict, can end up making lighter tops than some samples with lower density and lower than expected Young's modulus values. Again: rules of thumb are nice, but you don't really know until you've made the measurement. And even then....

If I understand correctly, a lower density top would probably be better on either a small or large guitar, but the larger guitar would be more likely to show the benefit of a lighter top.

_________________
A man hears what he wants to hear, and disreguards the rest. Paul Simon

I'd say it would be easier to get away with using a dense top on a smaller box, so long as everything else worked out right. Smaller guitars tend to be narrower in proportion to their length, so a dense top with high cross grain stiffness might not work as well on a small guitar. Here's where you can get rid of all your dense and floppy tops...

For a centrally applied load on a beam, the central deflection is proportional to the cube of the span.

(deflection = wl^3/48 EI)


What Al said.

It also means that many guitars are too big for their own good (if you're after a loud guitar). The more guitars I build, the smaller they get (and the louder they get).

If you have high density wood that you want (or have) to use, it is better used on a small guitar. I was faced with this issue when I built "The Shed" guitar, which has a radiata pine top. The radiata pine I had was relatively low stiffness, relatively high density (compared to the spruces) so a smaller guitar was the way to go. It is a steel string guitar on a classical guitar body shape (i.e. 00 sized).

The book has the required formulae for sizing panels. Having measured the material properties of the top wood and knowing the size of guitar you want to build, the formulae tell you how thick to leave the wood for a consistent vibrational performance. That's how I figured out how thick to make the radiata top (and to demonstrate that the theory works in practice). Obviously, denser, less stiff tops end up heavier for the same vibrational performance in modal resonance terms, but because they are heavier they have a lower monopole mobility, which means they are less responsive and not as loud.

There's more stuff on the variation in wood properties in this "Wood for Guitars" paper, including data for redwood (compared to Engelmann, WRC, Sitka and Euro spruces).

_________________
Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au