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To those of you who use parabolic braces, what are some things you do to ensure you don't lose the guitar's low end? Specifically with the tone bars?

With the advice of Hank Mauel, I may use parabolic tone bars on a top that got a bit thin in the center. My Performax's drum went out of alignment I have a few of these redwood tops from Hank, and the tap is perfect for smaller guitars. I have another one joined and ready to go, but I'm not sure that I want to give up on this one just yet.

james,
i'm no expert (only 7 guitars done), and i've only used parabolic braces on one. my pre-carving dimensions for those braces were the same as my scalloped guitars, and in my experience i lost none of the low end.
my own hunch is that much of a guitars low end comes from how freely the entire top is allowed to respond to the tension/vibration of the heavier strings (the reason why some builders thin the outer edges of the lower bout). so as i worked those braces into their parabolic shape, i was conscious of removing mass and stiffness as i neared the perimeter. remember, just gluing the top to the rims is going to replace a whole lot of that stiffness and strength that you might be worried about while you're shaping your braces.
one huge structural advantage of shaping parabollically is that you keep the tone bars higher in the direct line of string tension.
also remember - i've only built 7. others with more experience might have more and better experience to share. i'd say forge ahead with the top you've got.
phil

James my friend as I read Phil's post I found myself agreeing with his observations completely.

My first parabolic braced guitar was grossly over braced and does not have the openness that I would have hoped for. One day I plan on getting in there with a plane and trying my hand at brace shaving.......

But the other 5 parabolic guitars that I have built to date have not lacked bass. I believe that some of the things that I do that encourage the lower frequencies to compete with the strong mids that parabolic guitars are known to have is exactly what Phil indicated - recognizing that the perimeter of the guitar, near the linings, will be well supported by the rim itself. So I taper my lower X legs to nothing just before the rim and I also thin the top as needed, this is a feel thing......, around the edges too.

In addition this may be helpful. I am not so very mindful of having a parabolic shape to my braces as I am concerned that I can tap directly under a brace, any brace, and not hear the dreaded thud...... instead hearing the top ring.

So.....once all of my braces are profiled to a sharp triangular cross section taking care to remove as much mass as I can to get to a sharp triangular cross section if the top does not ring near or under a brace I reduce the height of the brace in that area. Once that is done I re-profile the brace in that spot again achieving a triangular cross section and check the ring, or not, again. This reduction in height and re-profiling is repeated until the top rings as I want it to.

In the end my brace heights are completely dictated by the finding the minimal height that a triangular profiled brace will permit the top to ring in each and every location. This is of course not done for the upper transverse brace.

Lastly, if I am concerned that for one reason or another a guitar will inherently lack bass, an example is a smaller body style, I have further reduced the height of the lower X-brace on the bass side of the top. I will then note that taping on that side produces a slightly lower pitched fundamental note then taping on the treble side. If this works or not is a matter for speculation but I can tell you that my own psycho-acoustics thought that it helped.

Remember James that if you simply use the same brace dimensions and don't scallop you are going to build something much much stiffer. So keep that in mind and reduce the bracing accordingly.

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David White, Toronto

"All my favourite singers can't sing."

Thanks, guys. All three responses have been very helpful indeed. I'm in and out of my room right now, prepping the other redwood top, so I won't say much more right now.

Hesh, I remember that you like 1/2" height at the joint of your X brace. Do you like that initially for the tone bars, too?

Yep James buddy I now start at 1/2" at the X intersection for say an OM sized guitar and this is the tallest that any of my braces will be. As for the tone bars they end up considerably less at the highest point, usually at the center of the top, and it would not surprise me if they are just over 1/4" at the tallest point. The same holds true for my finger braces.

Mind you this is for a guitar that is built for light gage strings AND I use very stiff Adi bracing.

Thanks, Hesh. I really appreciate the tips here, and also the notes about what you're looking for when you tap. I was afraid I would need to scrap this top. Hank suggested going parabolic with it, and I feel comfortable going forward with it now. I'll be bracing with Lutz, so I'll go a bit taller with the braces. If nothing else, for security's sake.

One of the fringe benefits I've discovered about teaching is spring break. It's going to be a good week

James this is what I do, which is a lot like what others have said. Notice how long the taper is. You might not want quite as much if your tops are thin. How thin? That could be a problem with redwood with its tendency to split, though I know the stuff Hank has from Craig Carter is tougher than the usual redwood.

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Ken Franklin
clumsy yet persistent
https://www.kenfranklinukulele.com

Ken my friend that is a VERY nice looking top!

An interesting side note here is that some refer to parabolic bracing meaning different things. To some it means the long axis shape of the brace. To some it means the cross section profile of the brace and to others it is a combination of the two. I have seen straight sloped long axis with parabolic cross section called parabolic bracing. I have seen a bit more triangular shaped cross section with a parabolic long axis called parabolic bracing. And I have seen and built with a parabolic cross section, parabolic long axis and with scalloping.

So to simply say parabolic bracing can at time only tell half the story

I think most here refer to parabolic bracing mostly in terms of the cross section profile and do various things in the long axis

I put a gentle scallop in the long axis because the guitars that I didn't do that with seemed to lack a little warmth and complexity.

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Ken Franklin
clumsy yet persistent
https://www.kenfranklinukulele.com

Right you are Ken, I consider the scallop as an EQ adjustment and most all of mine seem require an adjustment to warm up the responce. It is amazing the change in tap responce you get as you start doing this

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Tickle your guitar daily, and it'll tickle you back.

Think of the top and the stress how they are applied . You will have compression , tension and rotational. With this in mind , you want to keep the strength where you need it. The brace will is naturally stiffer the taller it is. Wider isn't always better.
Also loads can be more efficiently handled with under tension. I personally keep the brace a bit higher from the bridge area to help control the rotational force. Also a bit more up at the fretboard to help with the compressive forces. Behind the bridge is where most of the sound will be generated. I will not final voice a top till it is under load for a while.
I like to scallop braces also and once the top is under the actual stress you can tune the top without guessing.
john hall
blues creek guitars

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

So what I'm hearing is that the way to make a guitar with "parabolic" bracing sound good is to make it not have parabolic bracing.

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Howard Klepper
http://www.klepperguitars.com

When all else fails, clean the shop.

See that is where the definition comes in. So is a brace system with a parabolic cross section but non parabolic long axis shape a parabolic brace system? This is the parable.

I have been thinking for a while now that I probably need to stop calling my bracing parabolic and instead call it non-scalloped.

Like some of the others noted too my profiles are no longer really parabolic and my cross sections are triangular. And like the others tapering the height of the braces, but not really radically scalloping them, seems to open up the top greatly here at Heshtone golbal headquarters.....

My brace shapes are now entirely dictated by the tap response that I hear and as such the final shapes are not parabolic.

Here is an example of what I am trying to convey here:

Hesh my friend if your were in this for the money you could say you build withTriparbolic bracing and charge 10% more

I think Michael is right about the definition problem. After all a parabola has a formula and I don't think anyone is building these with a formula. Mine are somewhat parabolic (in appearance) in cross section and gently scalloped and tapered over the long axis. Like John says, you have to put the structure where it is needed most to resist the forces working against the top. To me that doesn't result in a symmetrical pattern.

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Ken Franklin
clumsy yet persistent
https://www.kenfranklinukulele.com

Michael and Ken I completely agree except that in terms of what Michael said I like a 25% mark-up better......

Tri-parabolic has a nice ring to it.......

Ken, why wouldn't that result in a symmetrical pattern? I would think that the strings apply a symmetrical force

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David White, Toronto

"All my favourite singers can't sing."

I use more of a true parabolic brace and also keep my intersection heights around 1/2" also. Part of what I do is because of the stress concerns. I have been known to use sub 100 tops also, so all of the strength I can get - within reason - is useful. I usually taper the tone bars to nothing and like the effect of keeping them slightly shorter in length AND in height than the norm also.

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Reguards,

Ken H

Well, whatever we decide to call them, all the advice here is really helpful. Particularly the pictures. I'm really looking forward to voicing this top.

Comparing the tone bars to typical scalloped tone bars is interesting. They have less mass and only seem to be as tall as the shallowest area of a scallop. Do they really give the area more support?

Dave, the greatest stress on the top is between the strings, so the x-braces need to be taller there than behind the saddle. You also want the lower bout to vibrate with more freedom if you're looking for an open and responsive sound.

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Ken Franklin
clumsy yet persistent
https://www.kenfranklinukulele.com

James buddy here is an interesting question that you might want to think about: Regarding your very good observation that the parabolic tone bars have maximum height not that much different from the low points of scalloped braces - What is the primary purpose of a tone bar? Is it to provide structural support only or perhaps is there also a component to them where they function as meridians of vibration from one area of the top to another? Or, of course - both?

Also consider the lattice like lower bout bracing that we have seen here before on Somogyi guitars. No height to those braces what so ever either. They just spread the load when addressing the structural aspects of the question but what happens in terms of the concept of transmitting vibration?

Agreed, when you said symmetrical I thought you were talking about the other direction - "treble" side versus "bass" side.

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David White, Toronto

"All my favourite singers can't sing."