Tone bars on the back?

[Steve Sorensen]

Has anyone experimented with placing tone bars on the back of the mandolin?

Seems like you could make some changes in tonal response by changing how the back flexes in response to the top . . .

Steve

[woodwizard]

I'm no expert... but what would you do about bracing the top for strength? Isn't that part of the funtion for the tone bars as well? And isn't there more vibration at the top?

[MikeEdgerton]

The tone bars aren't there to brace for strength.

[Lefty Luthier]

Placing tone bars on the back would be counterproductive. The primary function of the back is to reflect the sound waves. The best way to tune a backboard is by carefully thinning the center of the arch. This can lower the resonant frequency a bit, which is usually the direction you want to go when tuning your enclosure.

[sunburst]

Try it and see, Steve. I don't know of anyone having tried it, but I bet it's been done...(everything has been done).

I'll wait and see if Dave (Cohen) shows up and clarifies some things, but tone bars do serve a structural function as well as modifying top modes, backs do not "reflect sound", but vibrate in their own mode shapes. "Back bars" could change the back modes just as "tone bars" change top modes.

[woodwizard]

The tone bars aren't there to brace for strength.

True ... but don't they somewhat add top support strenth? ... seems logical

[MikeEdgerton]

Logical but not the intention. The Loar 700 series doesn't even have tone bars. I'll see if I can find the last raucous discussion about tone bars and structural support. It's been a few years since the last one.

[woodwizard]

With no tone bars or bracing I'd be afraid of the dreaded sunkin' top desease that you see in a lot of old Gibson carve tops. Usually caused by a loose brace.

[MikeEdgerton]

My point was that the tonebars weren't put there to be braces. Hans mentions in one thread I found so far that thinning a top and using heavy tonebars to brace them results in a thin sound so I guess I sit corrected that the tonebars can obvioulsy add strength (logical assumption). I just don't think that was the original intention. I'm still looking for that thread.

[PT66]

I would like to know if the original intent of those sticks glued to the top are for transmission of sound wave or strength. If you consider that the design by Orville was to copy violin contruction then we have to go back to history of violin contruction. I feel that they might be for strength, I'm not sure anyone tought about it any deeper then that.

[Willie Poole]

To add more fuel to the fire, I once read in an article in a magazine from a very popular luthier that the back and sides of an instrument only added about 1% to the sound of a mandolin....That builder posts on here some times so maybe he will chime in and explain it....Willie

[MikeEdgerton]

Well this message posted by Rick Turner pretty much sums it up for me, I'll change my view to his view of the bars being 50/50 (structual/tone) unless somebody else can direct me to the post that I can't seem to find. I also ran into this tragic/humorous post from James Condino.

[PT66]

I hate to disagree with Rick, but I have seen a lot of guitars with loose or broken braces that the top has wraped under pressure. If you brace too heavy you lose tone, if you brace too light you lose strength. It's a balencing act.

[MikeEdgerton]

I don't see where you're disagreeing with him.

[woodwizard]

Usually good engineering is a design that does more than just one thing well.

[tbyron]

According to Roger Siminoff in his book Building the Ultimate Bluegrass Mandolin, the tone bars are not needed to support the top. He says you can shave them really thin if you have to to get it tap tuned right.

Tom

[peter.coombe]

Getting back to the original question, I have braced the back in a few of my mandolins. It was not tone bar bracing, but neverless was braced. The reason was to raise the modes in the back so as to get the relationship with the top I was after. It worked, the resultant mandolins were fine sounding instruments, and I would not hesitate to do it again. The back does not "reflect" the sound and bracing the top or back just stiffens the plate and raises the modes of vibration. There is no reason why not to brace the back, although tradition says you don't so nobody does. Usually, however it is not necessary.

[Rick Turner]

Based on my work repairing, restoring, and building guitars (mostly), I believe braces are both structural and that they can affect tone by stiffening selected areas. I also believe that a rigid back makes for an instrument that projects forward better whether by reflecting sound or simply by not absorbing it.

[Steve Sorensen]

"The reason was to raise the modes in the back so as to get the relationship with the top I was after. It worked, the resultant mandolins were fine sounding instruments, and I would not hesitate to do it again."

Peter,
That's exactly what I was wondering about. Obviously most backs would not need bracing for strength since there is no bridge/string load.

Please keep adding details to this chat string regarding how (internal) tone bars on the back have been applied . . . and how that application worked out.

Thanks,
Steve

[MikeEdgerton]

According to Roger Siminoff in his book Building the Ultimate Bluegrass Mandolin, the tone bars are not needed to support the top. He says you can shave them really thin if you have to to get it tap tuned right.

Tom

OK, that may be where I saw that. I did read most of that book. I still think it came up in a message thread.

[Potosimando]

I'm glad Peter Coombe chimed in on this. Two of his mandolins that I have owned (one f-hole model, one oval-hole) have back bracing (vee-braced backs with x-bracing on the tops), and both have terrific tone by any measure. I no longer have the oval hole, but the f-hole stands tall alongside three other mandolins I have that each cost several times that of the Coombe. The Coombe has a hard edge of clarity to its tone that must be, in part, a result of a very-stiff highly reflective back plate--I have have never owned another mandolin that had such a characteristic "bite" to it, and I love what the bite adds to the already rich tone.

Until I learn differently, I am going to assume that the Coombe's bite is a direct result of the back-stiffening bracing. At any rate, it would seem like a very worthwhile thing for an interested luthier to pursue.

[Dave Cohen]

So many misconceptions here that I am hesitant to wade in. I have posted on some of these before. Fortunately for those who want to dig further, the 2nd Cohen & Rossing paper is now a .pdf somewhere in cyberspace. You can go to it for free at

http://www.jstage.jst.go.jp/article/ast/24/1/1_pdf.

So can you put "tone bars" on a back plate? The answer is that it depends on the back plate. If you have a particularly soft back plate, you might want to try a transverse brace, or a very splayed-out X-brace. Arched back plates really seem to like the (1,1) mode or sideways rocking motion. If that one is intruding down into the (0,0) or main modes territory, some lateral stiffness might help you out there. On the other hand, it might also raise the frequencies of the main modes, and you might not be happy with that. All you can do is try. Theory guides, but experiment decides.

Can you leave the "tone bars" off a top plate? Again, it depends on the particular top plate. I wrote an answer in the "Questions" column in American Lutherie #95 that has some bearing on this. Softwood top plates are anywhere from 7X to almost 20X stiffer along the grain than they are across the grain (depending on the particular piece of wood). So adding braces which are primarily longitudinal in orientation won't raise modal frequencies much, but it will bear some of the static load from the down forces from string tension. Otoh, primarily transversely oriented braces, such as ladder bracing in lutes, can raise modal frequencies a lot without dealing with the destructrive effects of string tension as well as do, say, "tone bars". The one thing that you can come closest to exerting rational control over with bracing pattern(s) is the ratio of stiffness along the grain to stiffness perpendicular to the grain of the top plate.

And yes, John Hamlett is right about backs not being primarily for reflecting sound waves. At least, they don't simply reflect traveling waves off the back plate and out the sound hole(s). Air mostly enclosed in a cavity has its own normal modes of motion, just as the plates do. The interior of the cavity is "reflecting" all right, but it happens off of the entirety of the interior surface of the cavity, resulting in 3D standing wave patterns of the air motion. In mandolin famly instruments, the first normal mode of the air cavity is the Helmholtz resonance, kind of a pulsing motion. The second is a longitudinal sloshing of the air in the cavity, back and forth from heel block to tail block. The third, at ca 1.1 kHz in mandolins, is a sideways sloshing of the air back and forth across the center seam of the mandolin body.

http://www.Cohenmando.com

[PT66]

How would this coversation be going if Bill Monroe had played a Martin Style 20?

[Dave Cohen]

I get an error when I click on the link to the ast paper, but the url is the correct one. Try typing the url in yourself. That gets me there.

http://www.Cohenmando.com

[Wudwerkr]

On my first build , I placed 3 supports across the back , approx 1/4"x 5/8" and scalloped at each end . I am playing with Idea of using 2 on my next and making an X brace and scalloping them like tone bars , so this thread has me wondering as well . wish if its been done someone would chime in