View Full Version : bracing questions

Mar-14-2004, 4:50pm
I am trying to understand how tone bars work and have a few questions, mainly about symmetry. First, just a few statements to find out if my thinking is straight:

1.) Braces act more as stiffeners than braces. The top should hold up to string tension even without the tone bars.

2.) Several different approaches are successful.

3.) These "plate stiffeners" ought to be made of stiff material that is also light. (For example, a cedar or redwood top that isn't terribly stiff should have spruce tone bars or even something with carbon fiber.)

I understand that most makers are copyists with regard to the parallel type tone bars of Gibson Loar instruments, but if you look at these as plate stiffeners, then shouldn't the tone bar on the treble side then be larger and stiffer than the bass side? Shouldn't the bass side be left to vibrate more "freely"? This may show my ignorance of how tops vibrate, but is there really a difference between one side of the top and the other when considering how the whole plate vibrates? Vibrations travel through all of the bridge and I don't think there is a great division just because the treble strings are on one side of the bridge and the bass strings the other. If all this is so, then is it perhaps safe to conclude that those parallel tone bars are of different dimensions because it helps to have a top that is *assymterically* stiffened?

I am just a novice on acoustics, but I can't help but wonder about these kinds of things. Right now, I am getting ready to put tone bars in a top that I have carved and I will probably X brace it. This system seems like a really good stiffening system and I have had good luck with it so far. I also had good luck copying David Cohen's very different system on one mandolin. But this symmetry issue makes me wonder whether I should make the X bracing assymetrical. Do other builders do this? Do any of you try to adjust the tone of your mandolins by somehow modifying the tone bars in situ? Who out there really experiments with bracing?

Thanks for any advice you might add!


Mar-15-2004, 3:03pm
At the ASIA symposium last summer, Don McCrostie probably saved me a lot of time and experimenting by saying that he had moved the tone bars from side to side in various mandolins, and didn't hear much difference.

I think of tone bars acting as beams to distribute the force of the strings to more of the top.

Dave Cohen
Mar-15-2004, 8:18pm
OF course, I experiment with bracing. I just got through doing holography on eight different pre-1929 mandolins, including one 1924 F5. I have been writing the paper for a few weeks now, and just got news that the Acoustical Society of America has accepted our abstract. So I will be presenting the paper on May 27 at the ASA meeting in NYC. I looked at several Gibson oval hole mandolins, a Lyon & Healy A, a Vega model 205 "cylinderback", an H1 mandola, the '24 Loar, and two Neapolitans (Eugene Braig's 1908 Martin, and Mike Schroeder's 1921 Calace). As is known, the Vega and the Neapolitans are ladder-braced, the rest of the ovals have a single soundhole brace, and f-hole mandolins have the primarly longitudinal "tone bars". Thanks to that assortment of mandolins, I am starting to get a better handle on what bracing does, though I still have a long way to go. Technically, I am not supposed to divulge the info in the paper until it is presented, though it can be shared freely after that. If there are any mando enthusiasts in the NYC area, I want to encourage them to come to the ASA meeting.

John, the tone bars do distribute the static load as you suggested, but the dynamic properties are way more complicated than that. One thing that can be said is that at low frequencies, it doesn't matter how the braces are arranged. The forces from the strings "see" the overall stiffness of the top, and the braces are "seen" as part of the top. I remember that Boaz Elkayam once braced a classical guitar with a Star of David pattern. I have no idea what it sounded like. At higher frequencies, the bracing pattern exerts progressively more influence on both the mode shapes and the mode frequencies.

Michael Lewis
Mar-16-2004, 1:17am
Dave, I for one eagerly await your "sharing" of the information you have gathered. We all have preconceived concepts of what goes on inside these things, and I think it would be helpful to actually have a real understanding of how the structure functions to make tone and volume.

Mar-16-2004, 9:42am
I, too, eagerly await your publication Dave! For now, I will X brace this current project, but will keep it symmetrical (especially after Michael's comment above).It's tempting to make them tall and narrow and then try fine tuning some after it's strung up.