Official Luthiers Forum!

I appreciate all the advice.

The pattern router bit made making my first parlor and om molds and bending forms a 1-2 hour breeze. And the flat sawn mahoganey, well I took the advice, ripped it turned 90 degrees laminated and glued.

Now that the whole neck building and carving is going well, I am approaching my first 2 boxes.

SO:

What is the difference between scalloped and parabolic bracing? Whice did ya'll do for your first builds?

Thats a wide open question. I scalloped, but the question about the differences could get interesting. It was pointed out recently that one is parabolic-like but probably not a true parabola.    Others can voice their opinions on the diffent effects on the sound, which I assume is what you are really asking about.

Good question! Opinions vary on this subject and some have very strong opinions.
For your first guitar, it really won't matter whether your braces are scalloped or parabolic. Both are good and are quite capable of producing a good sounding guitar.

I should stop here but......

For me, scalloping makes no sense, so I won't scallop anymore. If someone can explain why braces are scalloped, I'd love to hear the reason. I suspect it is because CF Martin did it and now most people copy it not knowing why.
Does anyone know why Martin did it?

No offense intended, I've made guitars with scallops and I suspect many that go parabolic aren't really sure why they do that. There are exceptions.

YMMV   

(I'm putting on my flame proof suit now.    )

_________________
"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

let me reword the question,

What is the difference in how you shape them. Do you use a chisel for both, is one type only sanded? and what is the difference in the overall shape.

 

Hesh, you do awesome work!

---

With regard to the original question about the differences in bracing types, here is the best explanation I have heard; scalloping and other similar forms of carving out the bracing are done in order to lighten the top and to make it more flexible. This increases the response of the fingerboard to the playing of the strings. This is a double edged sword however since a more flexible guitar will not usually last as long as one made more rigidly. I've also been taught that it's not what the guitar sounds like 1 month after you are done..but what it sounds like years afterward that matters most in a quality instrument.

This explanation always made sense to me....I can't take credit for it...just pass it along!

I would build it in whatever way seems to be the best way to you...it's the variability in this hobby that makes it fun...don't be afraid to experiment..and most importantly, let us know how it turns out!

Trev

_________________
http://www.PeakeGuitars.com

I am just not asking the question right,

Do the terms scalloped or parabolic refer to different types of brace wood? What is the ultimate difference between the two? Type of wood, shape, thickness?

Is the shape of a scalloped brace different than that of a parabola?

I know what a parabola is but what is the final shape of a scalloped brace?

To through out some science lingo...

Scalloped braces change the moment of inertia and therefore the modulus of gyration along the length of the brace. This allows the brace to react to or resist movement along its length. By removing height (and to some small extent, thickness) from the brace, the pitch and timber will change when energy (a tap or pluck of a string) is added to the top.

If you have a Peterson Light-speed strobe tuner, you could "see" the changes in the top, as you "tap-tuned" the top.

The parabolic brace(s) shown in the second photo that Hesh supplied add stiffness (strength) and remove flexability.

Gee I knew those strength of materials classes would come in handy...

OK, OK I read it in a book and asked my Statics professor, too.

Robert

Parabolic braces are thought to avoid "dead" areas on the sound board. Scott van Linge did a presentaion at 2006 GAL on this. I'm not sure I'm convinced, but the gist of it was to "tune" the SB. He had elaborate data and drawings on tuning patterns at various note frequencies. He was able to show that the spots right at the peaks of the scolloped brace were "dull" and shaving the brace to parabolic made this resonate better. Also at GAL, Harry Fleishman did a session on revoicing existing guitars by accessing the braces via soundhole or a designated access port and shaving various parts of the bracing . I think the Ervin Somogyi class on voicing might have info on this. There is a thread on this course here on OLF. My take on this is that there are many ways to "skin a cat" or rather brace a box each havimg merits. We'll probably be discussing these for years to come. I know from my limited experience that I am "over bracing". The experts who post here will have more experience and feel for their bracing.

_________________
Greg
http://garibaldiinstruments.com/

[QUOTE=SteveS]Good question! Opinions vary on this subject and some have very strong opinions.
For your first guitar, it really won't matter whether your braces are scalloped or parabolic. Both are good and are quite capable of producing a good sounding guitar.

I should stop here but......

For me, scalloping makes no sense, so I won't scallop anymore. If someone can explain why braces are scalloped, I'd love to hear the reason. I suspect it is because CF Martin did it and now most people copy it not knowing why.
Does anyone know why Martin did it?

No offense intended, I've made guitars with scallops and I suspect many that go parabolic aren't really sure why they do that. There are exceptions.

YMMV   

(I'm putting on my flame proof suit now.    )
[/QUOTE]

Steve,

A most interesting question. And here I was hoping that you might have an answer to such a question, rather than pose it.

Are you familiar with Left Brain Lutherie by David Hurd? There is a very active yahoo group dedicated to extending the understanding of the physics of stringed instruments. I've been following the discussions for a couple of months and find it fascinating. These folks really know their science. Perhaps this group could help shed some light on the situation. With your engineering background, this might be a good place to look ... the discussions are pretty erudite, more akin to a scientific journal, with an atmosphere that is open and inquisitive.

My naive luthier mind would like to think that scalloped bracing actually has a basis in physics, even if it was arrived at empirically. While certainly the shaping of the bracing changes the local stiffness of the plates, such a scalloping pattern does not make much sense if you were merely hoping to maximize stiffness of the edge support plate from a local load applied at the bridge. Parabolic bracing would do that wil lower mass. I-beams would be better still. Instead, could this scalloping pattern be used to control the vibration modes of the top plate?

leftbrainluthiers-subscribe@yahoogroups.com

 

Still much to learn.

-Peter O.

 

[QUOTE=RobertJeffery] To through out some science lingo...

Scalloped braces change the moment of inertia and therefore the modulus of gyration along the length of the brace. This allows the brace to react to or resist movement along its length. By removing height (and to some small extent, thickness) from the brace, the pitch and timber will change when energy (a tap or pluck of a string) is added to the top.

If you have a Peterson Light-speed strobe tuner, you could "see" the changes in the top, as you "tap-tuned" the top.

The parabolic brace(s) shown in the second photo that Hesh supplied add stiffness (strength) and remove flexability.

Gee I knew those strength of materials classes would come in handy...

OK, OK I read it in a book and asked my Statics professor, too.

Robert[/QUOTE]

I'm an engineer also...so I've got to chime in on this

I have my doubts about how tap tuning translates to the completed guitar due to the fact that when the guitar is in use, there are entirely different end conditions in place. I.e, there is a large difference from the standpoint of vibration response whether the edges of the soundboard are constrained or not. It would seem that just because you tune the soundboard to a particular resonant pitch when it is hanging from your fingertips does not mean it will resonate once the edges are constrained/glued to the sides.

If we had anything better than the lowest level of cosmos FEA at work you can bet I'd run some real #'s on this! Even so, I did some simple load/displacement analysis and it was really cool to see the deformed shape of the acoustic top. It basically forms a sine wave with the bridge at the peake and the soundhole near the trough. I'll post some screenshots tomorrow if I remember..

_________________
http://www.PeakeGuitars.com

Parser,

I think tap tuning simply gives you a plate that is predisposed to vibration in a certain frequency range, a lot like tuning a drum. To refine the top to vibrate effectively in the end state (edges of the top constrained/static) there's modal testing like Al Carruth (who taught me) uses.

_________________
Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

hesh, that was a good explanation with the pictures and all, thanks alot

 

jon

[QUOTE=PeterO]

Steve,

A most interesting question. And here I was hoping that you might have an answer to such a question, rather than pose it.

Are you familiar with Left Brain Lutherie by David Hurd? There is a very active yahoo group dedicated to extending the understanding of the physics of stringed instruments. I've been following the discussions for a couple of months and find it fascinating. These folks really know their science. Perhaps this group could help shed some light on the situation. With your engineering background, this might be a good place to look ... the discussions are pretty erudite, more akin to a scientific journal, with an atmosphere that is open and inquisitive................[/QUOTE] Peter,
Thanks for that link. I'll check it out.

_________________
"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.