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Once you have a design settled in to your likeing, how do you compensate for various "custom" wood selections from your clients?

Say you have a guitar with:

- Indian Rosewood Back and Sides
- Red Spruce Top

How do you handle a build when your client says "figured maple back and sides" or "Sitka Spruce Top" or ....

What changes do you make to keep the sound in range of your ideal? Or do you just do everything the same and let the wood speak for itself?

Bring it!

I use Gore's techniques to measure the structural attributes of the wood (Stiffness and Density) and use those attributes in his equations to determine the target thickness for the plates. If the desired back wood is on the denser side like Cocobolo I will change the back bracing from what I do for an active back to stiffer braces for a reflective back.

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I think it’s helpful to think about how wood colors the sound. When I work on the stereo in my car I set a mic up, mute all but one driver, play pink noise, and adjust the EQ until the driver matches a pre-determines EQ curve. Thinking about guitars this way has been helpful to me. I like to think of the way I build the guitar itself as that curve. Will the general tonal footprint of the guitar “match” the curve I want?

I don't think the tonewood will effect the tonal footprint of the guitar, but it will color that footprint in an ornamental way through overtone content, the weight of the fundamental, reverberation (dryness, wetness), etc.

So I won’t change my design, per se. I’ll just hope I hit my target curve and understand what the woods are going to do to it ornamentally. Both are big if’s.


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What about top and bottom deflection? Is it all about the G/G frequency response, or more about the behavior of the pieces you are buildig with?

Lets all be honest here... I've been writing computer software for 30 years and recently completed a college degree wtih algebra and various mathematics cources. When the G & G books arrived I was so expetant... and then I started reading "Design." In the first chapter, there were more pages full of complex equasions than english. I could not decipher the math in this book once I got a few pages into the formulas. To me, it reads more like a doctoral thesis on accoustics than a luthier's guide to designing guitars. If you are hoping to understand acoustical guitar theory from reading these books, you had better be awesome at math. The only savig grace was the bullet points at the end of the chapers that summarized the conclusions that the equations proved.

No doubt the genteleman that wrote these books knew what they were up to, but their attempt to translate it to the common woodworker was lost in translation... at least for this old guy.

I'll duck now in fear of the firely arrows that will insue.

I think Trevor would say the equations are looking at the behavior to an extent by looking at the physical properties.

I struggled with Design, too, but do think the section on measuring the physical properties for top and back thickness was pretty interesting. Thanks to a spreadsheet floating around out there, it's also doable. I wouldn't know how to do the calculations in without it. Trevor, however, is a gem, and is nothing but helpful. I think Build is the best book on the subject.

No one should flog you. As makers, some approaches work for us and make sense, and others don't.

I agree that Build was a good read, particularly if you want to get into alternative bracing approaches. But, yea, without a lot of help (and perhaps a magic spreadsheet) I really can't get much out of Design. I did manage to take some tap samples into a spectrum analyzer and was able to identify what I "think" are their primary resonances... but other than pointing at the chart and guessing, I can't really do much with the info.

Maybe I'll bring the book into one of my math professors at college and see if they can help.

Trevors book is more or less a 4 credit college course and the build book could be a 1 credit lab or something. That's the way I look at it. It really needs to be taught by a professor in the appropriate setting over the course of a semester. It reminds me very much of physics classes or something like that. Even in a physics class you could get by without fully understanding the math. So you can get away with just reading it, doing your best to understand the mathematics and just move on and still gain a lot from it.

I've only built 65 guitars now so I am certainly no expert on this topic but it is something I struggle with. From the very first guitar I've ever built I have always held the top of the guitar in the highest regard and relegated the back/sides/neck to just being necessary for the structure of the guitar, can't have a guitar without a back and sides!

But many of the most experienced luthiers claim that the back and sides are important to the tonal palette of the guitar. Still it seems to me that it's not so much the species of wood but rather it's properties such as density, hardness, damping and so on. So TBH when someone comes to me with an ideal guitar in their mind using such and such wood I simply let them talk it out and agree to the terms and then build the guitar focusing on the top by doing deflection testing and when applicable the Gore acoustic testing. Which I am happy to say that when I started doing the acoustic testing it matched up with my deflection testing almost perfectly. So I still like to do both.

But there is so much talk out there about how maple is this and rosewood is that and walnut is this and mahognay..... Without having 200 guitars under my belt I'll just reiterate what Taylor or Martin or Collings says about 'tonewood' and then focus on the top

For the backs I will either make them reflective or responsive regardless of the species and depending on the design.

Many, many great guitars have been built without the gobbledeegook of mathmatical formulas and computer design.

I think that there are many builders that with experience build up an "intuitive" sense of the same properties I am measuring and using gobbledeegook to normalize. Most do not have the experience or the tactile sense to achieve consistency build to build. Even some of the great Luthier's had hit or miss guitars.

I personally find it fun to play with modal tuning, trying different resonance targets and see what they sound like. Using Gore's models and equations I was able to build three classical guitars with top resonances at 180 Hz, 190 Hz and 200 Hz respectively. I use carbon fiber on my braces, removing the ability to tune post build. Without the ability to normalize for the different attributes of the wood I was using, I personally would not have the consistency to be able to target specific resonances without the gobbledeegook.

But that is my personal preference and a way for me to enjoy building guitars in my retirement. Many good factory guitars are built with consistent dimensions for plate thickness and brace size. These guitar fall in a range from sort of OK to outstanding with most falling in the OK to good range. Starting that way and getting a tactile feeling for the wood and with time performing a bit of additional tuning on a guitar being built seems to me an acceptable way to go about it as well.

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Here is my 2 cents. worth . I/ve used both traditional gtr making methods an gore and I have yet to see a significant difference. The gore method requires more work IMHO.. I found a youtube recording by andrew york of home played on a 1888 torres gtr. The sound was beautiful. an lovely ditto for a 1925 santos hernandez played by brandon acker . May I suggest listening to various different old. used guitars an several contemporary ones e.g. Michael thames etc so you have a starting point for what kind of sound you are after. ?

I wish I could take a class where someone would walk through and explain all the concepts in Design to the extent that I could leverage it. I think it could be a valuable tool for consistency if it was accessible.

As for the sound of different species on the back, I feel like there is a noticeable difference between some and less so in others. For example, I had two hummingbirds for a while... one Mohogany and one Indian Rosewood. The difference to me was dramatic. Of course, who knows all the factors that were at play, but I have to believe the back wood had a big part to play in the difference. Mostly the overtones and generally thicker texture. This is why I'm using rosewood back and sides on my guitars, but I'm anticipating the day when someone asks for figured maple, which is the traditional wood for an SJ-200.

Robbie O'Brien is hosting Trevor Gore again this year for a small class July 10th - 12th. So far they have always added a second class. I took it a couple of years ago and found it really valuable.

https://obrienguitars.com/teaching

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http://www.Harvestmoonguitars.com

About 20 years ago I built two identical guitars. The wood was sawn successively of the same log, same braces, same everything. This was before I did deflection testing though. But the idea was to build them as close as possible. In the end I two very different guitars. So my conclusion to that experiment was that it's impossible to build two identicle guitars and or to tell what factors make a different when comparing two guitars is impossible to verify. IOW if you had a walnut and a cherry guitar you might conclude that they sound different because one is walnut and the other is cherry. However if you build two identicle walnut guitars they too will sound different so now what is it?

I've done pretty much the same experiment as jfmckenna several times over the past twenty years or so. Each time I tighten up on controlling the variables I get guitars that are more similar, but players and listeners in 'blind' tests are always able to tell them apart easily. Making 'identical' guitars deliberately seems impossible, at least as long as we use wood.

The 'Leonardo' project has explored the use of different tone woods for the B&S by making a lot of similar guitars and using statistical means to analyze the results of 'blind' tests. What they have reported so far is that players/listeners who can't see the guitars have no particular preference as to B&S wood, but when they can see them they prefer Indian rosewood by a wide margin. Note this is not saying that there is no difference between, say, maple and rosewood, just that listeners don't have a preference for one or the other.

My experience suggests that one of the more important variables is the mass of the back, assuming all else equal (which, of course, it never is). It's difficult to make a back with a lower density wood, such as soft maple, that has the same mass and resonant pitches as a rosewood one, so it's hard to make comparisons. I have made guitars with backs of similar density, and thus mass, but with much different damping (say, BRW as against oak or persimmon), and noticed very little difference in the sound. Given the difficulty of making matched guitars using 'identical' materials I have to think that damping may may be as important as we often think.

I suspect that top wood has a lot more bearing on the tone than B&S wood, but again, that's a can of worms. My (unpopular) opinion is that 'spruce is spruce': if you match for density and stiffness, any variation in the sound is likely to be random, ad no greater between species than it would be within the same species. Western red cedar and redwood, which follow the same rule for the relationship between long grain stiffness and density as other soft woods, usually have lower damping, and that ought to have some impact on the sound, at least in theory. OTOH, the best computer study I know of this (which is the only way to really 'control' things, sadly) says that large changes in overall damping don't seem to make noticeable differences in tone. It's hard to know to what extent material damping effects overall characteristics of the assembled instrument, of course.

The bottom line for me is that I'm happy to use a wide variety of woods for B&S, and do what I can to adapt the structure to get the results I want. Naturally it's easier to get a 'mahogany' sound out of mahogany (or something like cherry that has similar hardness, stiffness, damping, and density) than, say, rosewood or hard maple, and it's nice when you can take the path of least resistance. I've seen great guitars built out of a lot of different woods.

We will not be adding a second class this year. The only class being offered is July 10-12.

Trevor is also scheduled to do at least one edition of the course in Port Stanley, ON, July 21-23. I’ve sent my first payment but would have to dig through email to confirm if the course is full or if there is also a second edition.

I’ve been using Trevor’s books from the first month they came out. I may have more math and stats than the average maker, but I really think there is value for anyone investing time with these books, regardless of their physics background. I made my own version of a “magic” spreadsheet for the plate design parameters that generates comparable top thicknesses for all of my tapped top sets, all on a single Excel sheet. Very useful indeed.


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