Sound Boards

[Bob Howell]

I have glued up 4 Sitka spruce sound boards, two with a brittle tone when tapped, like a pane of glass, the other two more restrained. I plan a classic, two L-00 and a parlor. Which would be the best fit for each. I have not selected the wood for back and sides sets.

How important is this.

[Alan Carruth]

It's possible to make a good guitar of any plan from any top if you use it right. Some tops do tend to work better for certain shapes and types of instrument, though. You're planning three fairly different boxes, and it would help to match up the tops appropriately. The information in your post doesn't go very far in doing that, IMO. What would are some measurements of stiffness along and across the grain, and the density of the wood.

What you hear when you tap a top is a complex mixture of all the resonances that can be driven at the tapping point that are not to active at the point where you're holding it. It takes some time to learn to sort all of that out, and even if you have that skill it's not something that can be easily communicated. What you're calling a 'brittle' tone could be a sign of low damping on those tops, or it could be something else, possibly having to do with the way the resonant pitches happen to line up. I don't think anybody, no matter how good at tap tone tuning, could say much about which circumstance we're looking at here without actually tapping on the tops themselves.

Tap tones by themselves don't tell you how stiff the wood will be at a given thickness. Usually in making guitars we want to get 'just enough' stiffness to hold up under the string load, with as little weight as possible. Strength is not usually the limiting factor, so we're not looking for a 'strength to weight ratio' here: if a wood top is stiff enough it will almost certainly be strong enough. You can, of course, flex the tops along and across the grain and try to come to some judgment about how stiff the wood is. It gets complicated, especially given that stiffness changes very quickly with thickness: we're working with thin pieces here, and small differences ion thickness that are hard to measure can significantly change the stiffness. Some people do get very good at this, but it can take a lot of practice. Tests that I've heard about suggest that most people are not nearly as good at it as they think they are. There are 'tech' ways to get more exact measurements that are actually not too difficult to do on a shop level.

The ratio of stiffness along and across the grain also makes a difference. Basically, the wider the guitar is relative to it's length the more it helps to use wood that has high cross grain stiffness. The L-00's will do better with a 'floppy' top than the Classical.

In general, I think it's harder to make a really good Classical than a good steel string (not that either is exactly easy). In particular, given the limitations of nylon strings it's helpful to keep the weight of the top on a Classical as low as you can while still keeping the stiffness high. As it happens, the Young's modulus along the grain, which determines how stiff the piece will be at a given thickness, tends to track the density pretty well most of the time. If you pick the top that has the lowest density for the Classical, and make it just thick enough to hold up, you'll probably end up with a better guitar than you would have with one of the other tops. Note that the low density top will actually be thicker at a given stiffness, but it will weigh less, and that's what counts.

If you're catching hints in this that things are more complicated than they seem, you're right. There are a lot of good ways to address these issues, and lots of help available in learning them. Not all of these methods involve technical measurements, but they all do call for a certain amount of education, and that takes a little time. Or a lot, depending...

Finally, to reiterate, if you work carefully to good designs you can make a pretty nice guitar from any random set of decent wood. The better factories do this all the time. If the objective is to make, or learn to make, something better (whatever 'better' means), then matching up the wood to the design is important. It has been said that 'the sound is in the top', and getting the tops 'right' is the place to start for sure.

[Bob Howell]

I see there is no quick answer. I found a very nice WRC board 10 years ago and made 2 dulcimers from it. perfectly QS and even had beautiful grain. Still have half of it around so I made a 3 piece top from it to see how it might look. Even started the sound hole ring. The joints stick out due to color variations so I held off. It has a brittle tap sound like a pain of glass.

I have read little about deflection testing. I have only a plane to thickness my tops and I can not get the accuracy I would appear to need to be effective. I am laying up tops for #5 -6 as I work on the action of #2. Batch building. As I worked on the tops I noticed how different they sounded and wondered how to use that to choose which style to use it for.

Thanks for the explanation.

[Alan Carruth]

You'd be surprised at how accurately you can thickness things with a hand plane. The trick is to know where you are. It's not too hard to get a cheap dial indicator and rig up some sort of frame to make a thickness gauge. 'Real' ones are expensive, but don't work any better. Once you have something like that it's just a matter of planning across the top on diagonals alternately, so that you hit all the high spots, and checking frequently. Use a sharp plane, work slowly, and enjoy the process.

One advantage of and planning is that you can vary the thickness from one place to another. I usually taper steel string tops so that they're thicker at the upper edge and thinner over the tail block. Once I've determined the target thickness from measuring the properties of the wood I add a little to the upper edge number, and subtract it from the lower edge. The top ends up at the target thickness between the bridge and sound hole, which, IMO, is the critical area. The added thickness at the upper edge makes it a bit stronger, and tapering it that way seems to bring up the bass a bit. On Classicals I'll leave them at the target thickness or a bit more all along the length and taper them out in the 'wings' of the lower bout, which helps bring up the treble, IMO.

The 'pane of glass' tap tone of cedar is an indication of low damping: the wood doesn't dissipate much of the energy of vibration in internal friction.

[Clay Schaeffer]

If you can find a nice piece of WRC it might be a good choice for the classical rather than Sitka.

[Alan Carruth]

Sitka can make a fine classical, if it is reasonably low in density. Granted, that's not common in Sitka, while it is normal for WRC, but such tops can be found. The issue with cedar is that is has very low surface hardness, so that it dents very easily. If you leave it thick so that you'll have some material to sand away in the finishing process it will be hard to tell just how thick it ended up. In my process I start by defining what I think is the 'proper' thickness for the top, and 'tune' it accordingly. After that I don't want to remove a lot of material; it's already about as thin as I think it can be and still hold up. With some practice and a lot of diligence you can get to the point where you can make a pretty clean cedar top, but if you have not made very many guitars it might be better to use something like spruce, which tends to have higher surface hardness.

[Clay Schaeffer]

I think that the less dense pieces of spruce that I would select for a classical would dent more easily (although not as easily as WRC).

[Alan Carruth]

Density and surface hardness do tend to go together to some extent, but low density spruce does tend to have higher surface hardness than WRC of similar density. It would be nice if you could have it all, but that's not the world I live in.

[Bob Howell]

Can you use the growth lines to select a good Sitka spruce top for Classical guitar. Fast growth - wide rings, but is it lighter? I seem to recall summer growth is denser, but I am recalling conflicting information.

I picked out a wide grain Sitka top now not sure. I had choice of 6 sets from a box locally. I Could switch.

Bob

[Alan Carruth]

The hard 'latewood' lines add stiffness, but also add weight, and the weight tends to go up faster than stiffness. What seems to matter is not so much the grain count, but the ratio between earlywood and latewood. Close grained wood with inconspicuous latewood lines can have low density, and wood with wide growth rings and heavy latewood may not be low in density at all.

[Bob Howell]

Alaska Specialty Woods Has an ultra-light sort of tops from a group of old logs. Sounds like it would be great for Classicals.