Tone bar patterns

[Skip Kelley]

I have one placement of tone bars I use that has worked great for me(Macrostie's) and wondered if what other were using and what opinions they have for the one used. I made an A-model a few years back and used the placement Apititus uses and it turned out great. Just wondering if there was somthing better out there.

[Stephanie Reiser]

I dont know if it is any better, but I use the location right off of Macrosties plans. I try an use those as close as possible.
Stephanie

[Stanley Cox]

I too used Dons plans. I noticed some builders use longer tone bars but the shaoing seems to be close to the same as MacRosties. My Storm is not finished yet so I cant say how it sounds. One of these days though---------
Stanley

[sunburst]

Don Macrostie himself said he tried moving the tone bars side to side and didn't hear any particular difference in the mandolins. I breathed a sigh of relief and concentrated on trying to figure out the rest of what makes a mandolin work.

[Jim Hilburn]

There have been some who say they don't think of a mandolin top as having a bass side and a treble side. But wouldn't it seem as though Loar or whoever must have been thinking of the top this way? Or else, why not make the tonebars equal on each side, either closer to or farther from the F-holes, but symetric, which is Bennedetto's approach to the archtop when x-bracing isn't used.
If you look at the bracing of a dreadnaught guitar, it's clear that the 2 long "tonebars" that extend across the lower bout from the x brace is intended to give the bass side less support while tightening up the treble.
I'm not talking about how it actually works, and what more current research has revealed, I'm only saying the originators laid out the bracing to enhance certain frequencies based on which side the strings laid.

[Keith Newell]

I too use Don's placing and I can hear a differance in treble to bass side when tapping the top. Every single mando I have built has a 1/2 step or so in note from treble to base right off before finish carving the tone bars. I have had very consisatnt sound also from finished instrument so am not changing placement of tone bars.
Keith

[Chris Baird]

The difference in placement between the tone bars does make them resonate at different frequencies. I've got a fair amount of FFT data to back that observation. The question is whether it really matters which side each bar is on.

[Dave Cohen]

The age-old question; should it be symmetric, or should it be asymmetric? One of the J. Meyer classical guitar papers in 1983 found that the exact position of the braces didn't really make much of an observable difference. Rossing came to a similar conclusion in his simulations paper in the "Chicago papers" collection of the short lived Journal of Guitar Acoustics. His concluding statement was something to the effect of "It doesn't seem to matter how you make it stiff; what matters is how stiff you make it" (no off-color interpretations, please!). In other words, the overall stiffness of the plates is what the force on the bridge from the string motion "sees".

Some of the plate modes are symmetric about the center axis of the plates, and some are antisymmetric. One thing that is sure is that all of the modes are global, both from theoretical considerations and from holographic results. That is, the modes occupy the entire area of the plates, unless they are choked off by a massive wastebar in front of a soundhole, as in the classical guitar. Even then, when driven hard enough, the (0,0) mode, f'rinstance, will venture into the soundhole area.

On the luthier side, there are guys like Bob Benedetto, who assert with more certainty than experimental results can confirm that the structure of the plates should be completely symmetrical. Otoh, there are plenty who structure their plates asymmetrically and insist that they are on the one and only true path. The latter camp likewise seems to have more certainty than experiment can actually corroborate.

The asymmetrically structured top plate goes back a long way in lutherie - back into the 19th century in the case of the classical guitar, and virtually back to the beginnings of the steel string guitar in the case of the Martin family/organization. So what should it be? I don't have an answer. Gila Eban has done some simple string replacement exercises in support of her asymmetric radially braced classical guitars. The experiment that would really be helpful would be admittance or impedance measurements -or maybe accelerance- done at different locations on the bridge. Even then, I'm thinking that it would be difficult to extract an easy answer from those measurements. Those transfer functions (i.e., admittance, etc.) vary drastically with location, and additionally are affected by the locations of nodes in the various normal modes. Confused? You should be!

So where should you put your "tone bars"? I think, like Don MacRostie and John Hamlett, that within limits, it doesn't make a lot of difference. You have a lot of latitude. Imo, it is probably a waste of time to worry too much about exact positioning of the bracing.

I finally got around to doing my bracing experiments this spring, and just got back from presenting the results at the Acoustical Society meeting in Vancouver. Without going into much detail, different bracing patterns do have an influence, but the influence is small compared to differences in individual pieces of top plate wood. I am starting to think that strict adherence to a given bracing pattern can be a hindrance to progress. When we really understand the effects of bracing, we will use different bracing patterns to adjust to differences in the top plate wood. F'rinstance, we might use a bracing pattern with more cross-grain stiffness to complement a piece of top wood that is itself a little low in that department.

http://www.erols.com/judcohen

[Chris Baird]

"F'rinstance, we might use a bracing pattern with more cross-grain stiffness to complement a piece of top wood that is itself a little low in that department."

That is certainly an interesting idea and right up my alley in terms of viable experiments.

[Skip Kelley]

Thanks guy's for your help! I have used Don's placement and have always liked the results. I guess I won't worry too much about it. You all are the best!!!!