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In my experience for top woods cedar seems to bring out overtones better than spruce. For back and sides rosewood tends to allow the development of overtones while maple tends to stop them.

How much of overtones is wood related and how much is builder related?

What can we do as builders to reduce or enhance overtones? (I would think heavier bridges would enhance overtones, and light ones reduce them, but I really don't know.)
SteveS39051.5610416667

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"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

Well, to be vague, I think overtones are everything related. Wood plays an important role because many of its characteristics are either mysterious, or not replicable. Stiffness, density, thickness all play crucial roles for each instrument and each wood specie being used.

One thing I hadn't really considered before is shape of the soundbox. I'll be teaching a class next semester and came across this cool website: http://www.exploratorium.edu/exhibits/vocal_vowels/vocal_vow els.html among others. Takes about 4 minutes to look at everything.

The idea is that we hear different vowels based on their harmonic overtones. Aah is different from eeh, at the same pitch, merely by what overtones are present or not. ANd, the crazy thing is that these things are strictly conrolled by the shape of our voice box. They have reproduced these shapes with a duck call to get some creepy, although fascinating, human-esque sounds from it.

So, do we change the shape of a dreadnought or OOO? That might be taking it too far.
But... in reference to your question, that is a deep well and one that I hope people can share their experiences with.

Great question.

Geoff's response was pretty cool. I'd love to hear more from the other guys like Al Carruth and Kevin Gallagher. It sounds like this is indeed a deep well.

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One thing that probably influences the overtone content of the sound is the damping factor of the wood. Damping can be measured by the number of cycles of vibration it takes for the amplitude to drop to a set proportion of the original amplitude, once the thing has been struck, plucked, or whatever. That is; how long does it take for the sound to die out once you've hit it? Materials with high damping, like plywood, die out fast, while lower damping ones, such as aluminum, can go on ringing for a long time. Obviously, at high frequencies you have more cycles per second, and the sound dies out faster, so high damping tends to cut out high frequencies.

Woods like Brazilian rosewood and Redwood tend to have low damping factors, while others like Englemann spruce and maple have higher damping. When I built a 'matched pair' of classical gutiars in oak and rosewood some years ago, I noted that the oak one had less 'ring' in the high treble, and associated that with the higher damping of the back and side wood.

The structure of the instrument can also effect the damping of the parts. One example of this is that arching the plates tends to lower the effective damping factor. The maple back on an archtop guitar acts, in some ways, more like a rosewood one because of the arching. I'm not sure if the low 'arch' we use on most flat tops is enough to make a difference, but it might.

I also think that bracing can effect the damping. When I made the 'Twins', the two guitars with different top bracing that I took to the ASIA meeting a few yearas back for people to try out, it seemed to me that the asymmetric bracing acted like it yielded a somewhat higher damping factor.

I will say that, in his computer model study, Wright found that changing the damping had no discernable effect on the tone of the virtual guitar he 'built'. however, his model was pretty limited in some ways, and that particular result does contradict a lot of experience. For example, Meyer found that a narrow, low-damped 'main top' resonant peak in the spectrum of a guitar tended to give a 'cutting' sound that people didn't like. There's a lot more work to be done on all of this stuff.

Finally, you often hear damping refered to in terms of 'Q value'. Q is simply the 'quality factor' of the resonance, and is measured by band width. You vibrate a sample and find the resonant frequency of some mode that only involves bending in one direction. That is called 'Fp' for that mode. Then you find the two freqencies above and below that where the amplitude is 70.7% of the maximum for the same input power. These are 'Fhi' and 'Flo'. Q= Fp/(Fhi-Flo). High Q = low damping. It's pretty easy to do this measurement on top or back halves when you get them, and you can find the stiffness at the same time. This is useful stuff to know about your wood, I think.

Alan, how about the chamber size of the instrument itself. It seems that lower overtones will be reduced in a smaller instrument due to the smaller volume of the chamber and smaller area of the vibrating parts of the instrument. This may be minor compared to other factors, but perhaps a generality is known about this rather simple aspect of an instrument.

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Alan, Once again, you struck a chord with me on this fascinating subject when you mentioned about maple arched plates as having LESS a damping factor.
Is it proper thinking then to fallow this path:
Say you have a back material that is not vary responsive and seems thud-like by its self, so you would say it has a high damping factor. SO, to help make this back a bit more responsive or have less damping factor, would you arch the braces a little more or maybe use an ex bracing system for this back as apposed to the conventional bracing?
Thanks,
Great question Steve!

Wade

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Wade
Nashua, NH
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That is interesting what Alan said with regards to Brazilian and redwood vs. englemann and maple. Would sitka come after redwood and englemann for damping? I have noticed with a few of my mandolins that the sitka spruce is heavy on the fundamental and fairly dry (that is my word for it) in the higher partials.

Also, I have heard that the stiffness of rosewood and the higher voicing of mandolins is a bad combination that makes it fairly harsh to the ear (maybe too many overtones?). Perhaps that is why maple is used, to filter some of those out.

It would be interesting to see a list of the amount of damping for various common species of top and back/side wood, and to get more info on if you couple opposites, or go for similar damping between top and back.

I love it when conversations on wood sound like adjusting tone knobs on an amplifier.... essentially it is the same thing.

Sorta makes me re contemplate carved back flattops.

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Great idea on arching maple plates but what about sanding the interior of the box to finer grits, 600-800 for instance, would this help the box be more responsive and have less damping?

[QUOTE=gburghardt] (maybe too many overtones?).[/QUOTE]
Geoff...You may understand this, but I thought I'd mention it for those who don't--every pitch played on any instrument contains the complete overtone series, and the complete overtone series contains every pitch possible above the fundamental of the pitch played. The nature of the "tone" generator and anything affecting the way it behaves determines which parts of the overtone series we hear, and thus the character of the tone. For instance, true "white" noise contains all pitches at an equal volume. In "pink" noise, some pitches are louder than others.

An interesting tidbit I've read says that more than a few pros use rosewood guitars on stage, and mahogany guitars for recording. The sound potential of a guitar may rest mostly in the hands of the builder, but it seems that we can't entirely discount the materials used.

That is interesting about the white noise. I never knew that. Same with pink.

Recently i also learned about geometric sound waves, where different overtone combinations make differently shaped waves (graphically i think). Need to learn more.

One thing, that should also be taken into consideration is the length of string. Carlton is right about all the overtones being present, it is just a matter of which ones are damped or amplified as a function of the structure of the instrument (I might be getting my terms mixed up here). Seems to me, and i could be wrong, that a longer string has the potential to amplify higher overtones, increasing the range of sounds coming from a single string and perhaps why ultra-professional pianos are so much longer than the upright piano at your local saloon. Maybe, for piano, the more overtones the better?

Steve Courtwright asked:
"Alan, how about the chamber size of the instrument itself. "

Generally speaking a smaller box will tend to make a more 'treble balanced' guitar. There are a lot of reasons for this. For one thing, the lowest resonance that can actually produce much sound is the 'Helmholtz' air mode, and that will be higher on a smaller box unless the soundhole is really small. Making the soundhole smaller, of course, gives less power in that resonance, so either way you tend to lose it with a small box. Small plates are stiffer and higher pitched, all else equal. There's a lot that gets into it.

Wade S. asked:
"Say you have a back material that is not vary responsive and seems thud-like by its self, so you would say it has a high damping factor. SO, to help make this back a bit more responsive or have less damping factor, would you arch the braces a little more or maybe use an ex bracing system for this back as apposed to the conventional bracing? "

That's an interesting question, and I'm not sure of the answer. The difference in arch height between the usual sprung braced plate and a carved one is considerable. I don't think small differences in radius on low-arched plates will have much effect, but then, I've been wrong before.

gburghardt asked:
"Would sitka come after redwood and englemann for damping? "
Sitka seems to have a 'funny' damping characteristic. The folks who have done the most measurement, particularly Haines, say that most spruces have low damping at low frequencies, and that it rises as you go up. There is normally a 'dogleg' in the graph at about 2000 Hz, and it rises much faster at higher frequencies. Sitka seems to have _high_ damping at very low frequencies, and it actually falls off until you get to that 2 kHz point, where it's like other spruces, and rises after that in the same way. This is odd, and I've never seen a convincing explanation, so take this with a grain or two of salt.


K.O.
"Sorta makes me me contemplate carved back flattops. "

Yup. One of my students made one with a Sitka top and maple back that sounded great. It was based on an old Epi 'Recording' model 3, iirc, but bigger than the original.

Serge Poirier:
"Great idea on arching maple plates but what about sanding the interior of the box to finer grits, 600-800 for instance, would this help the box be more responsive and have less damping?"

Well, it might lower the damping of the air modes, but I've never seen real data on that. Then you have to think about how the air and wood work together. It gets complicated.

gburghardt again:
"One thing, that should also be taken into consideration is the length of string."

Lots more there than you might think. For one thing, longer strings with higher tension have higher impedance. They also tend to have more energy in the higher partials, and those partials tend to be closer to being harmonic. All of this tends to give them a 'brighter' or 'richer' sound.

It's enough to give you a headache, isn't it? ;)

Thanks Mr caruth, my thought was that it would help on getting the overtones by having a very even surface to reflect the sound but i must agree with you in the complexity of it all, for instance, some will put shellac inside their soundboxes claiming that it helps bounce the soun!d better while others maintain that it would dampen it!

I guess someone will have to do a test some day...

Serge