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Focusing on the Fundamental
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Author:  Wes McMillian [ Sat Nov 10, 2007 7:01 am ]
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This question was presented in another post, but was somewhat overlooked. If you are designing a guitar (OM in this case) with the goal of a more fundamental tone (less complex in overtones), what drives your design philosophy? What wood choices would you start with? I tend to think of maple and mahogany as being more fundamental in tone, but I base that on just the handful of guitars I've played.

Next, let's say you're building with a given B&S wood (possibly not in the above choices), what moves could be made to "tone down" and harness these overtones? Maybe the direction to go is with a better understanding of what contributes to those overtones and eliminate those? Do you build with more the "passive" or reflective back philosophy to take that color our of the picture? What cosideration would be given to the top?
Author:  Hesh [ Sat Nov 10, 2007 9:39 am ]
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Hi Wes - I wanted to bump this for you buddy and let you know what little I know about your questions.

When I think of emphasizing the fundamental over the overtones the first thing that comes to my mind is the coice of the top wood.  Sitka spruce IMHO is rich in the fundamental and lacks overtones when say compared to WRC, Adi, and Redwood.  It's a more in your face sound.

Glassy woods like BRW, Honduran rosewood are rich with overtones too where in my experience Mahogany has overtones to a lesser degree and so does Koa,

Now maybe others will weigh in here and help you out.  Sorry I don't know more to help you with Wes.
Author:  Vivian [ Sat Nov 10, 2007 9:42 am ]
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Hi Wes,

What a great question! I am sure that many other much more talented
and experienced guitar builders on this site can offer more
knowledgeable anwers to this question but I'll give it a go - at least
maybe I'll provoke those in the know to offer rebuttals to my ideas.

I think that the back&sides wood probably is not the major factor in
producing a fundamental tone. Think of all those Martin guitars made
over all those years with more prominent fundamentals - they were made
with rosewood, Braz and then East Indian, or mahogany. Based on my
perspective and education-thus-far, I think that the soundboard wood,
bracing wood and pattern, and the geometry of the body influence the
fundamental-vs.-overtone balance.

I think I'll stop there and let my brain cool off. I'll be ready for more in a
little while, if you would like further elaboration. Another resource that
may be helpful is the umgf.com, especially posts by BHguitars.

Holler back!
Viv
Author:  Alan Carruth [ Sat Nov 10, 2007 11:17 am ]
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Woods with low damping, a long ring when tapped, will tend to empasize the overtones a bit more. Until recently I'd have said 'maghogany over rosewood', but I just finished a mahogany/cedar small Jumbo that has all sorts of complexity. This mahogany has damping more like a rosewood, though, so....

Try making the body a little deeper, since you've already settled on an OM size. I'd say a larger body in general will tend to empasize the fundamental, since it will tend to be more 'bass balanced'. Another way to get a similar result is to make the soundhole smaller.

Use scalloped rather than tapered bracing on the top, to bring up the 'main air' and 'main top' mode activity and bump the 'cross dipole' up in pitch.
Author:  Rick Turner [ Sat Nov 10, 2007 11:34 am ]
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I'd say a relatively heavy ebony bridge would help along with a relatively shallow and on the soft side bone saddle.   Or try a deep slot with some spruce or cedar laminated to the bottom of the saddle to act as a bit of a high frequency shock absorber.

Al's comments, to be sure.
Author:  Alan Carruth [ Sat Nov 10, 2007 11:47 am ]
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Thanks Rick, forgot the heavy bridge.

The interesting thing is that when I tried making measurements of the vertone content of plucked strings on same guitar with polyethelene and bone saddles I couldn't find any consistent differences. I sure could hear them, though! I must have been measuring the wrong thing.

Oh yeah: try to make the neck stiff and the headstock light, so that the 'neck mode' will be tuned to the same pitch range as the 'main air' resonance. This tends to give a very 'dark' tone to the bass.

Tuning the 'main back' and 'main top' resonant pitches (tap tones) to be within a semitone (back higher is best)will help increase the mid-range 'fullness'. For heaven's sake, don't tune them to be the same, unless you like wolves!
Author:  Howard Klepper [ Sat Nov 10, 2007 11:50 am ]
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I would ask for you to further define what you mean by fundamental. Sure, it's easy to define it as the bottom frequency in the string's series. But some of the comments seem to take you to mean a bass-heavy sound, typical of big-bodied Martins. Another way of looking at it would be strong treble fundamentals, which those same guitars often do not have. The guitar has a wide range, and there are tradeoffs to be made depending on which fundamentals you want to be strong.
Author:  Wes McMillian [ Sat Nov 10, 2007 12:00 pm ]
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I'm not fully settled on the OM size. In the other post it was suggested a larger body,too, and one suggsted a 0000 or a variant of it. Which poses another question. The 0000 is a shallower body in ratio to top surface area(and known as a good recording or stage instrument, or so I've heard, which would tend to agree with the "more fundamental tone" thing), but I'm hearing now a deeper body would have a tendency to show less complexity? Hmmmm....

I've never heard of using a wood strip laminated under the saddle as a "high frequency shock absorber". I'm curious, how does this work? Guess it sorta "bleeds off" the overtones? It would seem as if something else is lost in the process, though.

I undestand the ebony bridge, though, and was going to incorporate one this time.
Author:  Wes McMillian [ Sat Nov 10, 2007 12:07 pm ]
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Thanks, all, there is some good info coming out here.

Howard, this discussion is actually a spin-off of another in which I am building a guitar for a Travis-style picker with a fairly light touch. In that was the discussion that this style tended to lend itself to a guitar with less overtones, hence this discussion. As far as what I mean by that, well, that's about it!

Actually, thinking about it, I guess there's a reason Ol' Merle played them big ol' guitars after all!
Author:  Rick Turner [ Sat Nov 10, 2007 12:13 pm ]
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String it with flatwounds...
Author:  Rick Turner [ Sat Nov 10, 2007 12:14 pm ]
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Or Thomastik-Infeld Plectrum Bronze...

Author:  Wes McMillian [ Sat Nov 10, 2007 2:48 pm ]
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An OLFer in another post reminded me of a Frets.com article I hadn't seen in a while. This Martin DM/D-1 bracing caught my eye as an odd animal.

Here's what Frank had to say:

"Notice that there are fewer braces and the cross braces are tapered quite thin at the ends for good flexibility at the edges of the top. This lightweight bracing pattern gives the 1-series a terrific bass response, but because there are no diagonal braces, slightly less complexity of tone, especially in the treble range."

So, just in the interest of education, what about having "no diagonal braces" would lend itself to "less complexity of tone"?

Author:  K.O. [ Sat Nov 10, 2007 7:36 pm ]
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I have a theory about the top as a balanced dipole (more cancellation, reinforcement, collision over a broader range of freqs) Vs the top as an extremely unbalanced dipole or monopole(an archtop) Which would to me seem to have less cancellation, reinforcement, collision over a narrower set of freqs..

The above bracing to me looks like it might sacrifice longevity in order to gain bass response. I get the feeling of wanting to be a little more conservative with the scalloping in the lower bout. I also get the feel of wanting a slightly taller tighter x. To go along with my theory I would use parabolic stiff for the upper half of the x. Some bass response would be sacrificed of course.

I think simplifying bracing (as above) makes for a simpler impedance map and that might also help.

To me dough fir as a top wood has a crisp sound that is mostly fundamental. I may be a hack who is hearing what he wants to hear though. Downside to doug is I feel it can be a self destructive top as it ages.

As far as the boat I have always though maple to have the least complex tone.

Take this all with a grain of salt. I am not quite sure where I fall on the hack to gifted dedicated amature scale.
Author:  Rick Turner [ Sat Nov 10, 2007 7:50 pm ]
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Douglas fir is fabulous for delivering splinters into the hands of woodworkers!   At least the splinters aren't as toxic as they are with redwood. I built a couple of kitchens with VG Doug fir for stiles and rails using gray Formica panels in doors and on cabinet ends. They were just beautiful...

OH, this is about guitars!   Well, watch out for the runout...those splinters will send you to the ER!

Author:  Kent Chasson [ Sun Nov 11, 2007 4:47 am ]
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Tapered (as opposed to tucked) brace ends also helps get rid of some of the high overtones.


I suspect using something like basswood for the top linings (as opposed to something harder like Mahogany) would help too but I haven't tested that one.
Author:  Howard Klepper [ Sun Nov 11, 2007 5:00 am ]
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Well sheesh! I hadn't known that treble response was something to be eliminated! Probably would help achieve the ideal sound if you also eliminated sustain. As Rick suggested, use flatwounds; or maybe those nylon tape wounds, if you can get them in a guitar set. Then weave some cotton bias tape through the strings right in front of the bridge.   That ought to do the trick.
Author:  Kent Chasson [ Sun Nov 11, 2007 6:04 am ]
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Getting rid of high overtones on the lower notes does not necessarily mean eliminating treble.


Sure, if you combined all the elements above, you will be wanting for high end.  Maybe the preface should be "all things being equal".


But it's easier than saying... with a stiff, light Engelmann top, untucked braces, loose edges, a BRW bridge and bridge plate, no scallops, basswood lining, and a hard tightly fitted bone saddle, BRW back and sides, etc, etc, etc, I get a guitar with strong fundamental on the low end and trebles that ring.


Oh, one more thing, I find (all things being equal...which they never are) that flatter tops also reduce high overtones on low notes.
Author:  Blanchard [ Sun Nov 11, 2007 1:26 pm ]
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Wow, this is interesting !!!
Sooooo many ways to skin a cat.

I will add that running the top braces across the grain more will generally produce a more fundamental, less colorful, more even tone. Opening up the X brace angle and running the tone bars more straight across helps accomplish this.  The finger braces can also be run more horizontally rather than perpendicular to the X brace.

Mark
Author:  Alan Carruth [ Sun Nov 11, 2007 2:00 pm ]
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In the spectra I've got, the guitars that have a 'thick' sound or a strong fundamental all the way up have 'main air' and 'main wood' peaks that are tall and broad. These 'bass reflex' peaks seem to put a floor under the tone, in a manner of speaking.

Many researchers feel that the bass reflex couple acounts for most of the actual output power as high as 1000 Hz. That is; the 'main top' and 'main air' modes are so effective at producing sound that they are still pumping it out at a good level even a couple of octaves higher up than their center frequency. Most of the other modes have some built-in cancellation. That's why enhancing the low end can give a more 'fundamental' sound all the way up.

Making the body deeper spreads out the 'main air' peak, while making it a bit lower in amplitude. Tuning the 'neck' mode to the same pitch as the aidr spreads out the peak as well, giving a higher total output while dropping the peak levels a bit. Tuning the back drops the pitch of the 'main air' resonance and adds to its power. Making the soundhole smaller drops the 'main air' pitch and makes it a bit weaker.

You can spread out the 'main top' peak by scalloping the braces and leaving peaks in the upper and lower parts of the top 'wings' outside of the bridge line. This moves the cross dipole mode up in pitch, keeping it from acting as a phase-cancelling 'cuttoff' to the 'main top' peak. Making the wings of the top thinner, and the bracing out there lower drops the cross dipole pitch, cuts off the width of the 'main air' peak, and gives a 'thin', 'forward', or 'cutting' sound in extreme cases.

You can also tune the back to couple with the 'main top' peak, giving an output like a 'Butterworth filter' with two peaks and a dip in between. Tapering the top toward the tailblock drops the pitch of the 'long dipole' mode, and changes the way it couples with the 'A-1' air resonance, probably giving more output there (around 350-450 Hz).

The shape of the spectum above about 350 Hz seems to have more of an effect on tone 'color' and 'carrying power' than on 'warmth' or 'fundamental', although, of course, you can't really seperate things out that neatly. If the bass reflex part of the spectrum is strong it seems to give that 'solid' and 'fundamental' sort of tone no matter what else is going on. It seems to be quite possible to have a good 'fundamental' sound and still have lots of 'color', brightness', 'complexity', and 'carry'.
Author:  K.O. [ Mon Nov 12, 2007 8:29 am ]
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So I'm buying into my own theory..


Does the cross dipole effect become more prominent as the long dipole moves from balanced dipole to unbalanced dipole/mono pole?

Is the cross dipole a range of freqs/amplitudes that are shaped on the top by its weight/stiffness/impedance map?

Author:  Alan Carruth [ Wed Nov 14, 2007 7:22 am ]
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K.O. asked:
"Does the cross dipole effect become more prominent as the long dipole moves from balanced dipole to unbalanced dipole/mono pole? "

The cross- and long dipoles are seperate modes, and each has its own effect on the tone. It is intersting that the long dipole modes on most flat tops are near 350 Hz, which is the about the pitch of the 'A-1' air resonant mode, which is set by the length of the box. These two modes couple strongly quite often, and the resultant 'peak-dip' in the spectrum can fill in a hole where there is no other resonance putting out sound.

BTW, on some steel strings, instead of cross and long dipoles, you get dextral and sinistral diagonal dipoles. Sometimes one of those will couple with the air, and you get the two diagonals plus a 'normal' long dipole.

"Is the cross dipole a range of freqs/amplitudes that are shaped on the top by its weight/stiffness/impedance map?"

Yup. On Flamenco guitars this mode is often quite low in pitch, (as low as 190 Hz as opposed to 220-250 for 'normal classicals) owing in part to them sometimes leaving out the outer two fan braces. This cuts off the 'main top' radiation at a relatively low frequency, yeilding a tall, narrow spectral peak. This, in turn, contributes to the 'cut' that Flamenco guitars need.

Steel strings with scalloped bracing can have the cross dipole mode almost as high in pitch as the long dipole: say around 300-325 Hz. It helps them to sound more 'full' with the brightness of steel strings.   
Author:  K.O. [ Thu Nov 15, 2007 12:35 pm ]
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Thanks for the response Mr. C.

Man it is embarrassing, I read words I did not type into what I did type

Does the cross dipole effect become more prominent

Should have read

Does the cross dipole cutoff effect become more prominent



?
oversimplified,
dex and sin dips = unbalanced cross-dips(to treble and bass side)?

cross dipole cutoff effect = the cancellation/scatter effect caused by the collisions of crossing wave forms?

Is there a weak(ghost) cross dipole in the upper bout?

All else being equal, a stiffer bridge raises the frequency of the cross dipole and moves the point of most excursions outwards?

Sorry guys/gals I do a lot of visualizing which involve forming some assumptions and I worry that some of my assumptions may be inaccurate.

I feel I could benefit from a long discussion of the factors that affect the cross dipole.

I start my visualizations simple, a bridge within a circle of uniform stiffness then try to factor in bracing, body shape and then the tensioning of ares of the top by the strings.

Thank you for your patience.
Author:  Alan Carruth [ Fri Nov 16, 2007 5:54 am ]
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K.O. wrote:
"I feel I could benefit from a long discussion of the factors that affect the cross dipole."

The cross dipole is mostly effected by the crosswise stiffness of the top. That can come from the top wood, the bracing or the bridge (which is a brace, after all). Obviously a narrower pattern will tend to have higher cross dipole pitches, all else equal.

X-brace angle effects the ratio of lengthwise and crosswise stiffness. If you use normal Martin-style tone bars, that are more or less parallel to each other and to one arm of the X, the angle and stiffness of those can effect the cross dipole. Again, in some cases, the long and cross dipoles turn into diagonals because of the tone bar angle and strength.

"I start my visualizations simple, a bridge within a circle of uniform stiffness then try to factor in bracing, body shape and then the tensioning of ares of the top by the strings. "

As far as I can tell, string tension has no effect on the pitches of the lower top modes, such as the dipoles. It ought to effect the long dipole more than the cross one, if anything.

Unbalanced cross dipoles, with one side taking up more area than the other, are an interesting problem. In theory, if both sides reach the same amplitude, there should be some monopole radiation from such a mode. Some folks claim that this makes a more efficient top. My experience suggests that making the structure asymmetric enough to get significant monopole radiation at the cross dipole pitch decreases the efficiency of the top at the monopole frequency, and costs power overall. I have to say my research along these liners has not been exhaustive.

If the amplitude of the two sides is different, then they may or may not cancel out, depending on the magnitude of the difference.

Since any dipole such as this, where the centers of the two radiating areas are less than a wavelength apart, must have a null in the radiation, the effect of unbalanced areas will most likely be to shift that null line.

As a practical matter, the darn thing is going to do what it does, no matter what you want it to do. It's really easy to over think these things, and spend 'way too much time worrying about marginal issues. Ask me how I know.....;)
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