Tone woods- how are they determined. Measurement, opinion

[Bob Howell]

I've seen list but very subjective.
Walnut is listed as damping one place and resonating another.(Siminoff)
All maples are good.
Sycamore is a sleeper, one of best domestics.

Cherry and walnut are common here, and used by many.

I have a collection of beautiful figured cherry, walnut, and hard and soft maple.

Even saw somewhere butternut is resonating.
I'm thinking that's as good as we can do. No way to measure, just opinion on sound.

My ears are shot so I must rely on others if that's the only way.

So is that how tone woods are determined?

[Alan Carruth]

Quite often the popular tone woods are those that are used on furniture. It helps if some wood has been used by a famous maker in the past; Strad used maple, so it must be good. And so on. Anything you try to say based on properties will be subject to a load of exceptions, so take the following with a large dose of any seasoning you choose.

There's a distinction between top woods and back woods. For the most part we want tops to be as light as possible and still stiff enough to hold up under string tension over time. In general, this means using a low density piece of softwood for the top. The other main constraint is that the top have enough surface hardness to resist denting in reasonable use, which leaves out stuff like balsa for the most part.

For backs the most desirable woods tend to be ones with high density and hardness, and, if possible, low damping. The exemplar of back woods is generally taken to be Brazilian rosewood, which fits that spec better than, say, Indian rosewood or Mahogany for the most part.

Appearance has an effect. If you test the properties on black walnut and soft maple you'll find that they are generally quite similar. Nonetheless, walnut is often cited as sounding 'dark', while maple is 'bright'. 'Blind' listening and playing tests generally show that most folks can't reliably tell the difference between fairly dissimilar woods when they can't see them.

Any species of wood will vary quite a lot from one example to the next. It is, after all, a natural material. I'm working on a couple of mahogany guitars now, but the B&S wood actually has more in common with rosewood than the softer mahogany we often see. I'd be willing to bet that, when they're done, folks will say they 'sound like mahogany' rather than rosewood.

Those of us who do measure the properties of the wood we use find it very useful, but in general it has very little to do with why one wood is used instead of another. Basically, anything that it reasonably suitable can be made into a very nice sounding guitar if you know what you're doing. If you don't, the 'best' wood in the world won't help: it's distressingly easy to make a bad guitar from good wood.

[David King]

"Tone wood" is defined as the one selling for 5-10x what the same wood would cost you at the lumber yard.
All woods are tone woods in that they all excite or dampen at some frequency or other. Some are livelier than others or they have a more coherent tap tone but that can be just as troublesome in an acoustic instrument as the wood equivalent of a wet sponge (a heavily spalted burl say).

[Karl Hoyt]

I have a totally different view of 'tone' woods... Well.. Maybe not so different. My attitude about luthiery is that every single instrument we build develops its own personality. From a cigar box tele to an Adirondack/Brazilian beauty, they all have their own personality: which has as much to do with the builder's skills as the materials. The absolutely best sounding guitar I ever built had a lumber yard eastern spruce top. Second best was a small bodied guitar made with a piece of spruce that was found on a beach in Truro , Mass. In 1952.... Likely a piece of a shipwreck, married to some 40+ year old Birdseye maple. Every instrument has it's own personality and someone out there is going to fall in love with that personality. You give some lumber yard wood to a master builder and you'll get an outstanding instrument, despite the materials. Happy Sunday, all.

[Eric Knapp]

I'm a total newbie and can't call myself a maker yet. I got the Benedetto book on archtops and in it there's this one picture of a guitar he made that made me smile. The top is made from a 2x10 piece of pine from a lumberyard. He says it sounds great. One of the things I'm learning from you guys is building a great instrument is more about how many you make, not the materials. This is quite liberating. Just start building and when you're done do another. Start with nice wood but don't worry about the quality. Just do your best and learn. Repeat.

-Eric

[Alan Carruth]

David Kimg wrote:
" Some are livelier than others or they have a more coherent tap tone but that can be just as troublesome in an acoustic instrument as the wood equivalent of a wet sponge (a heavily spalted burl say)."

Ah yes. I got a very fancy mesquite burl back a few years ago, in a weak moment. If you can believe the numbers, it's a little bit worse as a back material than Masonite. I'm hoping to make a matched pair to check that out. Yes, at 2mm thick you can bend Masonite. The real issue is figuring out how to thickness and smooth that burl with hand tools...

[Brian Evans]

My own feeling is that for traditionally braced flat top guitars the back and side wood choice has a minimized effect on tone. I think the recent trend towards laminated sides and solidly brace backs emphasizes that. As does Martin's use of plastic laminate for bodies. Archtop guitars with traditionally carved, graduated and tuned backs are a different story - the back is almost as alive as the front and plays a big part in sound production, and I can only assume tone production. I am building an archtop guitar now that will have a spanish cedar back with figured maple sides and spruce top. The idea is quite solid sides that don't do anything acoustically, but a highly active back from light(ish) and resonant wood. It's going to be optimized for a light top and back. The goal is to maximize response even at the expense of volume with lighter strings for a fingerstyle player.

[Alan Carruth]

So far, the only guitars I have run into with 'pure' reflector backs are Ovations. Everything else has a back that moves, at least some, at some frequency.

From what I can see, the only back resonant mode that actually helps produce more sound from the guitar is the lowest pitched one; all the other tend to steal energy from the top without increasing the output of the guitar. This doesn't mean they're bad and should be gotten rid of; they do help produce 'tone color', which is something we all want. The trick seems to be to limit the loss from them. IMO, that suggests the use of a wood that is dense, and has low damping. A very low damping factor limits the band width of the back resonances, so they don't suck up energy from too many notes. Mass allows the back to act like a flywheel, storing energy, and also limits losses, since damping only comes into play when the back is actually moving, and a heavy back will move less. Hence a preference for rosewoods, which tend to be dense and have low damping, for a certain kind of sound.

[Bob Howell]

Is there a measure for damping factor? Or subjective.

[Barry Daniels]

Yes, but the water there is quite deep.

[Alan Carruth]

In theory its actually pretty simple to come up with a number for the damping factor. In practice it can be hard to be certain that what you've got is 'the' number, and it's not at all certain that there is a single number that adequately characterizes wood.

It's fairly easy to find damping by bandwidth. Any resonance will be active over a range of frequencies. If you have some way to drive it, and record the amplitude spread over frequency you can use that to determine the damping. Materials with high damping, such as foam plastic, will tend to have a broad resonant peak, while something like glass, with low damping, will have a sharply defined narrow one. That's why low damping materials tend to sound more 'musical'.

These days it's pretty common to use a spectrum analyzer to find bandwidth. You hold the thing up to the mic on your computer and tap it, recording the sound using a program such as 'Audacity' or 'Wave Surfer'. Run the Fourier Transform routine, and you'll get a graph that shows the power at every frequency. You find the peak in the spectrum that corresponds with the resonant mode you're looking at, and get the peak frequency and the two points on either side where the level is 3dB down from that peak. At those points the energy in the system is half of what it is at the peak, so the band width between them is called the 'half power bandwidth' . If you divide the peak frequency by that bandwidth, you get a number called the 'quality factor' or 'Q-value'. The higher the Q value, the narrower the band width, the less loss there is is the system.

There are other ways of doing the same thing. I usually use a signal generator to drive things, and record the frequencies of the amplitude peak and 3dB down points at a constant power input. I can also observe the way the thing is bending by the shapes of the node lines, as revealed by Chladni patters, which helps rule out extraneous factors that can mess up the reading. Wood has different damping along and across the grain. If the node lines of the Chladni patterns curve, it's sign that you're seeing bending in both directions, and the properties you're measuring are thus a mixture of the long-grain and cross-grain ones. There are ways to sort this out, but first you have to know you've got a problem.

Aside from that, there are other issues in making the measurement. It's easy, for example, to shift the pitch or amplitude of a mode in a test piece by simply putting your hand close to it, or shifting a support a bit. Often, for whatever reason, vibrating a piece of wood for a while will change the resonant pitch somewhat. We all know that the properties of wood are not very uniform, so there's a quandary involved. If you take a reading on a top half, say, then what your seeing is some sort of average, but the stiffness and damping can be much different on the 'bark' side and the 'heart' side. You could cut thetop into strips and test those to get an idea of the variance, but then you couldn't make it into a guitar.

Daniel Haines, who did a lot of measurements of instrument wood (with a focus on violin wood), found that there was a systematic difference in damping with frequency in most of the soft woods he tested. Basically, they tended to have low damping at low frequencies, and the damping rose as the frequency went up. The rate of increase was more or less steady until he reached about 2000 Hz, and went up much faster above that. Why he got the readings he did is anybody's guess. He was not at all sure it was not some sort of artifact of his test method.

For the moment those of us who do such measurements will probably keep it up. It's not a perfect system, but seems to be of some use. It would take a huge amount of work it would to actually settle some of the outstanding questions, and, as usual, since nobody gets paid to do this stuff....

[Joel Nowland]

The picture below is of the Benedetto guitar Mr. Knapp mentioned.

I have sets of insanely super quilted Sapele, which is flat-sawn, which for years I hesitated to use because it's tap-tone is like cardboard. When I finally used a set the guitar's tone was wonderful. it sold very quickly.

Putting a lot of concentration and experimentation into the guitar's soundboard has served me well.