I see that Dennis's nomenclature has influenced you:). I am glad that you appreciate seeing what goes into the construction of a guitar. They all aesthetically look a bit different from the outside but why do they all sound so different? I post these threads for players in the future considering commissioning a guitar getting an in depth understanding of how the luthier approaches their work and how it may differ from others.

Rosette
 

Mark worked on part of the rosette today.

In the first shot you can see that Mark cut 10-36 degree trapezoid pieces of Brazilian Rosewood cut from a piece similar to the headstock veneer shown in the same shot. The grain of the rosewood is radially arrayed when assembled and contrasting color is organized to create light against dark for visual interest. The piece are located on a backer.

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The glued Brazilian Rosewood pieces adhered onto backer are cut out into a 0.240” wide circular rosette ring.

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Here is the Brazilian Rosewood rosette ring glued up with surrounding 0.040” curly Koa purflings with 0.020” Black Fiber in a UHMW polyethylene mold.

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A bit more on the rosette...

Mark created thin .020” Black Fiber/.040” Curly Koa/.020” Black Fiber outer ring in the UHMW polyethylene mold. Curly Koa purflings will be used throughout the instrument.

Sometimes simple is beautiful...:)


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Backstrip
 

Mark was able to install a .040” Curly Koa/.085” Macassar Ebony/.040” Curly Koa backstrip to the joined Brazilian Rosewood back set. He is struggling a bit with humidity control in his shop this week and is bolstering his humidification system before continuing working on the top.

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Another gorgeous guitar coming together. Beautiful rosette! That BRW is some kind of incredible. Thanks for sharing Bob.

Thanks again Dennis...:).

My friend, luthier and Brazilian Rosewood importer Rodrigo Moreira told me that Brazilian Rosewood like this is typical of type grown in trees grown in cacao plantations to provide shade (cacao likes shade) in Bahia region on the Atlantic coast. The insect holes seen are from a well known parasite that proliferates among cacao that kills the Dalbergia nigra trees. This tree was likely felled due to the parasites.

% RH
 

Mark’s shop was having a low humidity crisis that was slowing down work on the top and back of the instrument until it could be resolved. Despite extensive humidification he was only only able to raise the humidity to 35%. The problem was that his pellet stove heater was drawing combustion air from inside the shop and exhausts it outside. It’s was sucking all the humidity out of the room. Today, Mark added a pipe to bring in some combustion air from outside. It resulted in bringing the shop % RH above 40% so once the wood equilibrates, he can continue to work on the guitar.

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In the interim, Mark inlayed a Green Abalone “B” into the Brazilian Rosewood headstock veneer today.

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The Tone is in the Top...
 

This photo shows the installed Brazilian Rosewood/curly Koa rosette installed in the Italian Spruce top.

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The second photo is from Mark’s notebook in which he records the data for each guitar that he makes. This page shows some of the Chladni modes as he works the Italian Spruce top of my guitar. What Mark calls Mode 6 relates to the long-grain stiffness of the top and Mode 5 relates to the cross-grain stiffness of the top. Mark considers the long-grain to cross-grain stiffness ratio of his tops important to the final sound of his guitars.

You can see how these two modes come closer together frequency wise as he adds the rosette (added mass) and cuts the sound hole (interrupts the long-grain fibers) with the final Mode 5 and 6 frequencies ending up only 6 Hz apart indicating to him a top with nicely balanced top in terms of the long-grain to cross-grain stiffness ratio.

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I am fascinated by the patterns of the Chladni modes on the top. I have to confess that I don't know very much about the thought behind the whole process. Does Mark also test the Chladni modes of the back of the guitar? I seem to remember reading somewhere that one of the ideas behind this particular method was to have the top and back exhibit the same patterns so that they were sonically working together -- but it may have been an article about violins rather than guitars.

Mark does use Chaldni modes to tune his backs as he adjusts and braces the plates. The same principals apply to both the top and the back, it is just somewhat easier to visualize what is going on with a ladder braced back. Mark uses target Chladni modes to determine the Brazilian Rosewood back thickness and to guide him through the carving of the Spruce braces.

The modes are determined by the stiffness of the back plate itself (sans-bracing). Given the cross stiffness nature of Spruce ladder braces, they don’t really affect long-grain stiffness much at all. Mark believes that it important to get the target long-grain back stiffness by manipulating the Brazilian Rosewood plate thickness. The cross-grain stiffness Chladni modes can then be adjusted by adjusting the stiffness of the ladder braces.

Graduating the Top
 

Mark graduated the Italian Spruce top today. The process starts on his ‘graduation board’. Layers of tape build up areas on the board where he wants the top to be thinner.

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He places the top face down on the graduation board, and runs it through his sander.

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After each pass through the sander, Mark checks the Chladni modes. The process is repeated until he gets numbers that he likes.

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These are the final modes for the unbraced top plate.

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This is the graduation map of the top after the initial graduations are sanded and scraped in. The top is thickest between the bridge location and the sound hole and thinnest near the end block.

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It's always interesting to see the eminently practical solutions that luthiers come up with for something that conceptually seems complicated: Layers of tape that raise the edges of the wood slightly so that in sanding it "flat", the top is actually tapered in profile.

That is a simple and clever solution. However, to me, the rest of the process seems very complex. I've never even heard of Chladni modes until now.

It is a wonderful enlightening thread though, and I look forward to learning more.

That is exactly why I post threads like this...:)

Luthier's work in very different ways to achieve a common goal (a great sounding, easy to play, beautiful to look at, structurally sound guitar). Regarding Chladni, Mark learned the basics of this from luthier Al Carruth's "Free Plate Tuning" course >24 years ago. Since then, he has developed his own approach to using these to achieve consistency in his guitars.

Top Bracing 1
 

Mark has begun to brace the top of my Bristlecone. Here is a photo of some rough cut Red Spruce bracing stock, a Sitka Spruce popsicle brace and a Brazilian Rosewood bridge plate ready for use.

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Here is the top in Mark’s go-bar deck. A faint pencil layout of the bracing pattern can be seen. A laminated (lower laminate is cross-grain to the top and upper laminate is aligned with the top grain) sound hole reinforcement plate is installed and an angled upper transverse brace to accomodate the cutaway lining.

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