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  #31  
Old 05-20-2022, 07:14 AM
Henning Henning is offline
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I´ve continued somewhat yesterday, shaving the brace of the back just under the bridge. This turned the tap tone of the back to be: G3 +45%

Then I tried the tap tone of the top and had it to: G3 +40%

So I figured, from what you wrote and and others opinions too, may may may, this ain´t good.

Deciding today to string it up. So far, having played it all through, at all positions of the fretboard, I can´t say to notice anything that comes close to any wolf note, as I know them.

Concerning Chladni patterns. I understand the way they come out (the physics behind). But how would you use them?
You can find a node, and lots of them. But then you´d need to adjust the top in a way that is the right way. Which would be the tricky part.

I don´t know if I should do any more adjustments here. It seems as the balance between the bass E and bass A is better. Don´t if that result comes from adjusting the back brace or the fact that I raised the E string at the nut. (Or perhaps both?).
Eventually the braces of the top could be adjusted. But as koolimy said, it is a sweaty way to go. In my case it was a ´bloody´ way even with the back brace too.
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  #32  
Old 05-20-2022, 07:55 AM
Alan Carruth Alan Carruth is offline
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The 'rule of half' is always a good one: make about half of whatever adjustment you think you're going to need, and try it out for a while. It can take some time for things to settle in, and changes in humidity, for example, can alter things too. Part of the definition of a 'wolf' note for me is that it's problem caused by the alignment of resonances in the structure and air: if it's not a problem it's not a wolf. I once built a small guitar for my daughter that ended up having all four of the strong lower range resonances (top, back, air, and neck) within the range of a whole tone. It sounded great for such a small box. Good thing: it proved impossible to move any of them.
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  #33  
Old 05-20-2022, 08:58 AM
Henning Henning is offline
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This is the way the top looks inside. Five photos, please.
Whenever I see someone has made a top and is gluing it to the back and sides I see a wonderful smooth surface with equally fine bracing. That isn´t the case here. The saw dust is my fault. Beg you pardon for it. But please tell me (again?!) it doesn´t make any difference with the sharp edges of this Sigma (or the bracing of any guitar top):

https://www.dropbox.com/s/h1fmpw7wxw...58320.jpg?dl=0

https://www.dropbox.com/s/pg60q38co9...34_MP.jpg?dl=0

https://www.dropbox.com/s/j09apn7n8e...14381.jpg?dl=0

https://www.dropbox.com/s/6xlo97r6sc...22935.jpg?dl=0

Last edited by Henning; 05-20-2022 at 09:20 AM.
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  #34  
Old 05-22-2022, 01:04 PM
John Arnold John Arnold is offline
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Quote:
But please tell me (again?!) it doesn´t make any difference with the sharp edges of this Sigma (or the bracing of any guitar top):
I have not noticed improvement from smoothing the rough edges of braces. That is in line with my experience with 1930's Gibsons, which can be great sounding guitars. While I often do fine sanding of braces after scalloping through the soundhole, I don't expect any improvement over the initial carving. It is the height of the braces that is critical, nothing more.
Smooth those braces if it makes you feel better, and it will be nicer if you spend more time with your arm inside the soundhole.

The first thing I notice with those Sigma braces is the location of the upper end of the scoops on the X braces. Prewar Martins had that scoop starting very close to the X junction, and the curve of the scoop was much more gentle. A deep short scoop is less ideal because it tends to restrict the most active part of the top to a smaller area. It also is less than ideal structurally.
Seek out some photos of prewar Martin bracing and study those.
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  #35  
Old 05-23-2022, 02:05 AM
Henning Henning is offline
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Quote:
Originally Posted by John Arnold View Post
I have not noticed improvement from smoothing the rough edges of braces. That is in line with my experience with 1930's Gibsons, which can be great sounding guitars. While I often do fine sanding of braces after scalloping through the soundhole, I don't expect any improvement over the initial carving. It is the height of the braces that is critical, nothing more.
Smooth those braces if it makes you feel better, and it will be nicer if you spend more time with your arm inside the soundhole.

The first thing I notice with those Sigma braces is the location of the upper end of the scoops on the X braces. Prewar Martins had that scoop starting very close to the X junction, and the curve of the scoop was much more gentle. A deep short scoop is less ideal because it tends to restrict the most active part of the top to a smaller area. It also is less than ideal structurally.
Seek out some photos of prewar Martin bracing and study those.
Thank you, thank you so much!
This tells me, I was right in the first place in the intention to adjust the bracing.
When comparing the bracing to the Martin pre wars, I find them to be thinner as well. The adjustments will have to be all compromises.

Sic. I measured the thickness of the (Sigma) braces in the X. They are 8 mm in width (). According to the UMGF, the Martin 000 pre wars had ~6 mm width of the braces in the X. https://umgf.com/viewtopic.php?p=2674685#p2674685

"There is a misconception in the luthier world that tap-tuning the top will account in better voicing and sound. Sanding away parts of the braces in special spots doesn’t have a large effect on the tone. The thickness of the top has more of an effect then small places here and there in the braces. The top holds most of the mass compared to the braces while the braces provide more stiffness." https://www.portlandguitar.com/blogs...racing-science

Last edited by Henning; 05-23-2022 at 06:18 AM.
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  #36  
Old 05-26-2022, 12:46 PM
John Arnold John Arnold is offline
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I disagree with your last quote. The author has the viewpoint that emphasizes mass over stiffness, and apparently has little experience with aftermarket scalloping.
Did you read Buck's post?
Prewar 000's had 5/16" (8mm) wide bracing. It is the reissues that are not correct.
Stop obsessing over brace width and concentrate on the height. Shape of the scallops, locations of the peaks. Minimum height of the scoops.

Last edited by John Arnold; 05-26-2022 at 12:51 PM.
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  #37  
Old 05-26-2022, 02:48 PM
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Bruce Sexauer Bruce Sexauer is offline
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When I started building Schoenberg guitars, around 1999, Eric specified the braces to be 1/4” wide. I quickly noticed that the braces, narrower than I had previously used, telegraphed their location, especially in light reflected off the top as in stage lighting. Increasing the width by only 1/32” diminished this effect to almost nothing.

I consider the location of the peaks and valleys in sculpted braces to be of crucial importance. If you actually think about this factor from a structural POV you may see, as I do, that more than a few builders do not yet seem to have understood how they work. My “aha” moment came from viewing a 1929 OM top removed from the guitar. Not counting buying nearly 70 of my guitars, handing me that top was the best thing Eric ever did for me.
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  #38  
Old 05-26-2022, 10:00 PM
Wade Hampton Wade Hampton is offline
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Henning, one thing that you can do in addition to all of this experimentation with the braces is to keep your forearm off of the top while you’re playing. An easy way to do that is put a John Pearse armrest on the guitar - it lifts your arm up off the top and allows the instrument to ring out to its fullest extent and increases the bass response.

You can simulate the effect by playing a vigorous first position E chord with your arm on the top, then, while the chord is still ringing, lifting your arm off the top. It’ll give you an immediate, dramatic tonal change.

If you like that sound, the armrest will give it to you.

Hope this helps.


Wade Hampton Miller
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  #39  
Old 05-29-2022, 12:49 PM
Henning Henning is offline
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Thank you for your postings. I now have a Top G3 196 Hz and a back A3 +48% (220 Hz) to a Bb3 -47% (233 Hz) somewhere around 226 Hz.
I can see the lower part of the top is bulging below the bridge. As well, when looking inside the old Martins, the way they were scalloped, well it was much more a difference between the peaks and the depths of the scallop then what I can hope to reach with this Sigma.
As a matter of fact, I don´t dare to remove more of the top bracing (having taken away about 1,5- 2 mm, at some parts of the "valleys").
Eventually I will try to remove some material of the mid back brace, perhaps some of the lower back brace too. Wondering if the latter will make any difference to the back resonance frequency(?). The lower back brace is still quite high and adds mass to the back.

I will try the experiment with the free vibrating top, thank you Wade Hampton Miller.

Kind regards

P.S. http://onemanz.com/guitar/reviews/ac...acing-in-2022/

Last edited by Henning; 06-07-2022 at 02:24 PM.
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  #40  
Old 05-30-2022, 10:55 AM
John Arnold John Arnold is offline
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Quote:
Eventually I will try to remove some material of the mid back brace, perhaps some of the lower back brace too. Wondering if the latter will make any difference to the back resonance frequency(?). The lower back brace is still quite high and adds mass to the back.
Adding mass (by itself) lowers the frequency.
But when it comes to bracing height, you should think in terms of stiffness instead of mass. Braces are largely responsible for supplying stiffness to a top or back. Lowering the brace height reduces stiffness more than mass. That results in a lowering of the resonant frequency.
Understanding why that is the case will help immensely.

Take two rectangular braces. One is 1/2" wide by 1/4" tall. The other is the opposite....1/4" wide by 1/2" tall. Both have the same mass, but the taller brace resonates at a higher frequency because of the cube rule. Stiffness is proportional to the cube of the height.

Last edited by John Arnold; 05-30-2022 at 11:06 AM.
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  #41  
Old 05-31-2022, 02:16 PM
Sadie-f Sadie-f is offline
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@Henning, this has been a good thread to follow. My only complaint with my OM-28 is that she rings a little dead on G2 (I haven't put a scope on it, however sustain sounds 2/3 or less of E2, and volume / overtones are pretty lacking), she also feels attenuated on G3, G4.

So on the one hand, I have wondered if I can't shift this dead spot to say midway between F2-F2#. On the other hand, I've read of guitars opening up as late as 10 years into their aging, and I'd rather live with her as she is, than think that in time this will fix itself, and that 'fixing' it now might result in the guitar sounding less good a dozen years out.

I tapped out the top and back a long time ago, didn't keep notes, however the fundamental notes of each seemed in line with what I've read. I have to do this again, and will aim to use a piezo transducer I've got in hand along with an oscilloscope for the fullest picture I can manage.

I guess I can start / test by adding weight to the brace by the bridge and if I can get an improvement that way, I might use cyanoacrylate to hold a bit of lead or copper in place. That would hold for as many years as one could want, yet still be easy enough to remove.
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  #42  
Old 06-01-2022, 09:08 AM
Alan Carruth Alan Carruth is offline
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It's possible that your 'main air' and 'main top' resonant modes are both very close to G, an octave apart. The guitar converts string energy into sound very effectively at those resonant pitches, so you can get notes that are twice a powerful for half as long. Twice as powerful is just 3 dB louder, a barely noticeable difference, but you pick up on the lack of sustain.

Blocking part of the sound hole, say with a piece of cardboard, will drop the pitch of the 'air' resonance. You can do much the same for the 'top' resonance by adding some mass to the bridge. Try temporary changes at first to see what the effects are, and how little you can get away with to make the issue livable. Poster adhesive ('Blu-Tac' or 'Fun-Tack'), cardboard, and tape are your friends for this. If you decide to go to less temporary changes you can add mass with heavier bridge pins, or use a sleeve in the hole to drop the 'air' pitch. That way, if the instrument changes over time you can always back off.
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  #43  
Old 06-01-2022, 10:15 AM
Sadie-f Sadie-f is offline
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Quote:
Originally Posted by Alan Carruth View Post
It's possible that your 'main air' and 'main top' resonant modes are both very close to G, an octave apart. The guitar converts string energy into sound very effectively at those resonant pitches, so you can get notes that are twice a powerful for half as long. Twice as powerful is just 3 dB louder, a barely noticeable difference, but you pick up on the lack of sustain.
Thanks! I'd got to the first half of your thinking, that last bit didn't occur to me! I have been thinking of it in terms of when work happens, energy is consumed and my picture had been that a cancellation of the note was at work here (g3 & g3 out of phase). Your model is probably better, either way this is a great way to think on it!

Quote:
Blocking part of the sound hole, say with a piece of cardboard, will drop the pitch of the 'air' resonance. You can do much the same for the 'top' resonance by adding some mass to the bridge. Try temporary changes at first to see what the effects are, and how little you can get away with to make the issue livable. Poster adhesive ('Blu-Tac' or 'Fun-Tack'), cardboard, and tape are your friends for this. If you decide to go to less temporary changes you can add mass with heavier bridge pins, or use a sleeve in the hole to drop the 'air' pitch. That way, if the instrument changes over time you can always back off.
I hadn't thought of covering the sound hole, also a great idea, and yes, I didn't say it, but trialing mass changes with a dense putty was the first planned step.
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  #44  
Old 06-03-2022, 12:32 PM
Henning Henning is offline
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It's hard for an amateur to Foresee the effect of an alteration in either the bracing or the top when it comes to relation mass/stiffnes.
If the thick hard lacquer is scraped off the top and a cellulose or french polish is used instead, what might the result be?

I could just Guess that a big risk is turning THE guitar into a more tinny and thin sound instead. But Still more vibrant though
Regards
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  #45  
Old 06-03-2022, 01:57 PM
Sadie-f Sadie-f is offline
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Quote:
Originally Posted by Henning View Post
It's hard for an amateur to Foresee the effect of an alteration in either the bracing or the top when it comes to relation mass/stiffnes.
If the thick hard lacquer is scraped off the top and a cellulose or french polish is used instead, what might the result be?

I could just Guess that a big risk is turning THE guitar into a more tinny and thin sound instead. But Still more vibrant though
Regards
I don't think I'd try to improve on a well-applied top finish. I don't think a finish can ever improve the tone, it's always going to be a more dense and less stiff addition to a spruce or cedar top.

Granted no guitar should have a thick finish on the soundboard, however I would only remove finish in an older / damaged instrument and I wouldn't do even that without consulting / research.

Anticipating the effect of a change isn't as hard, since adding mass runs approximately the same effect of reducing the stiffness of bracing, I think that's a good way to preview what could be accomplished? I think that was already discussed in this thread, however I don't have time just now to comb back over it in detail.
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