Rough Bracing
 

Mark was delayed in being able to move ahead with bracing the top due to low humidity conditions in his shop. Now the the % RH is in an acceptable range again, his rough bracing pattern now installed using Red Spruce braces and Brazilian Rosewood bridge plate. The rough braces have a 30’ radius dish.
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Chladni patterns are simply a 'tech' version of 'tap tone' tuning, with the advantage that you get to see the patterns of the resonances. The shapes of the vibration modes gives information about the way mass and stiffness are distributed in the plate, while the pitch is related to the overall ratio of stiffness to weight. Looking at the pitch of a given mode, then, can enable you to get the top to the 'right' thickness; stiff enough to hold up under string tension, and light enough to work well with the limited horsepower.

Looking at the mode shapes can tell you where to remove wood from the plate or the braces to get everything into balance, so to speak, so that the whole top is working well. You can get much of this information from tap tones, but it takes a lot of practice, and it's hard to communicate with other makers. There are also cases where you might not hear something in the tap tone that you'd see in Chladni patterns: the patterns give you more information to work with.

OTOH, tap tone tuning goes very quickly when you know how to do it. I spend hours sometimes tuning the top for a guitar using Chladni patterns, while Dana Bourgeois can tap tune one in a few minutes. There's no one 'right' way to do it; there's just the way that works best for you. ;)

The top bracing looks like an interesting hybrid of "X" and ladder bracing. What's Mark's thinking behind that choice, any idea?

Thanks for a very readable and concise explanation.

To be honest, I do not know the full method behind the madness. I can ask him. Mark has not always used this bracing pattern. Below I am showing a guitar by Mark closer to 20 years ago (left) and a guitar that he made for me about 5 years ago.

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With Mark's top bracing pattern at a high level, I see some similarities to one of Jean Larivee's top bracing patterns. I also see influence from classical guitar building in his sound hole reinforcement approach and stiffening of his rim linings. I also see with many builders who utilize Chladni patterns to interpret their tops a trend to use of symmetrical bracing patterns.

What I can say is lay people we see a "pattern" and think that is "it", when in fact many subtle factors in the choice of bracing material, the shaping of the braces, the variable top thickness, the dish and rim radius all contribute to how the top as a "system" sounds.

I wondered if there was some way in which the bracing pattern was influenced by the Chladni patterns. It's interesting to see the evolution of Mark's placement of the braces: One of the things about these build threads that I really enjoy is to get glimpses of how luthiers (at least the best) are willing to rethinking the process, to challenge themselves and are open to discovery.

In a sense it did to the extent that tops braced with traditional asymmetric “Martinesque” bracing patterns (there, I created a word!) are more difficult to interpret using these techniques. As I mentioned in my earlier post that some builders who use Chladni methods gravitate towards symmetry for easier interpretation of the patterns.

Forgetting Chladni, I think that you will find it is common that across a luthier’s years at their bench they modify their approach. Sometimes they start using traditional approaches and break out later in their careers and other times it is just the opposite (a return to traditional approaches)!

I like "Martinesque", it’s a good creation!:up:

Carving the Top Braces
 

Mark spent a couple of hours removing wood from the braces today. It’s not finished, but he tells me that the Chladni modes are indicating that the top plate/bracing balance is “good ”.

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The ring and a half mode is well shaped with crisp node lines that form up with fairly low power input from his signal generator/frequency counter. He feels that the frequencies are still a little higher than he wants to see, so he will be removing a little more wood in key places to bring them down while hopefully, retaining the overall balance.

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Back Braces
 

Mark has taken a pause in the refinement of the top braces and has moved on to ladder brace the Brazilian Rosewood back while the humidity in his shop supports these operations. He first installs his mahogany back reinforcement strip on his go-bar deck.

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He just made 30 new 46” hickory bars for his go-bar deck. Here is a shot of the raw four Red Spruce ladder braces being glued in place on his back dish.

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It's not so much that the patterns are hard to 'interpret' with asymmetric bracing; it's just harder to get symmetric patterns. I started thinking about the double-X pattern I now use during a post-tuning evaluation session with a student. I'd found the more symmetric patterns seemed to work better in producing the sound I was after, and he questioned why we were trying to put a square peg in a round hole, as it were. Since he was also one of my acoustics gurus the opinion carried a lot of weight. As it turns out the 'Martinesque' asymmetric pattern does give a more 'traditional' sound, so it's an option to keep open.

Thanks Alan...:up:

Yes, thanks for your input, and the wise reminder that the aim is the sound. The Chladni patterns -- and bracing patterns -- are valuable means, but not the end in themselves.

Back Braces
 

Mark sent me a photo of the Red Spruce ladder braced Brazilian Rosewood back (below). Mark uses a 12’ radius dish.

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Mark also told me that he thinks that the top bracing refinement is now complete. After trimming some material from all the braces a little bit and brought what he calls the “ring+ mode” (shown in a prior post) down by about 15 Hz to where he wanted it. He will evaluate the top again tomorrow with a “rested ear” in the morning and run a full set of Chladni modes. If he thinks it is as good as it can be, he will prep it for being glued to the rims.

Top is completed
 

After listening to the top with some fresh morning ears and running some confirmatory Chladni modes, Mark has declared the refinement of the Italian Spruce top and its Red Spruce bracing complete. He was kind enough to forward me the pages from his notebook.

• The braced top ended up being 228 grams (8.0 oz.)
  
• The top is thickest between the sound hole and the bridge plate at 0.133” (3.4 mm) and is thinnest near the end block at 0.095” (2.4 mm). Interestingly, the finished top differs about 0.012” (0.3” mm) in thickness across the lower bout from 0.098” to 0.110” (2.5 to 2.8 mm). Mark made the top a little thinner on the low E string side of the lower bout. The idea is to allow a little more mobility on the bass end of the bridge. The height of the lower X-brace arm on the bass side is also slightly less than it is on the treble side. He tries to follow the general rule that the brace heights and top thickness should correlate. The brace heights should be the highest where the top is the thickest, and the lowest where the top is the thinnest.
  
• The braces appear to be stiffest near the bridge and taper thinner toward the perimeter/linings. The top is also thickest near the bridge and thinner near the perimeter/linings.

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None of measurements likely mean ANYTHING without years of: 1) building guitars, 2) taking measurements during the build process, 3) recording them in a notebook and 4) evaluation of the finished instrument’s sound. Mark told me it wasn’t until he had about 50 guitars using these methods before any correlations to what made one of his guitars “great” guitar and another just “average” guitar began to stand out to him.

• Mark told me that he knows when it is time to stop carving the top braces because the patterns and frequencies presented match up, more or less, with good examples of his previous work recorded in his notebook.
  
• The Chladni patterns can be seen as a surrogate “fingerprint” of the overall structure of a top. It is a reflection of the top’s intrinsic properties, thickness and the impact of the bracing structure.

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