Official Luthiers Forum!

Hi there!

Please go easy on me, I'm not just new here, but to guitar building in general. I'm so glad to find there is a whole community of people building guitars who will share ideas, what a relief! I was starting to feel a bit isolated building by myself in my living room.

Some Brief Background
I'm on my 3rd build, it's an OM with adirondack top and cherry back and sides. Some time remind me to tell you the story of the adirondack log I got for 10c bd/ft.

To The Question...
I have the top braced and glued to the sides. Everything is going pretty well. But when I hold it by the head block and tap over the bridge plate, I get a sweet, ringing, beautiful C# (right above middle C on the piano). So pretty. You don't even have to hold it to your ear to hear it. Now I know things change a lot as I keep going, adding the back, the neck, the bridge, etc. But should I be concerned at this point about balance of response in the final instrument? Should I shave braces until that note is balanced by others? Or should I just have another cup of coffee and glue the back on? Perhaps I should take up butterfly collecting instead?

Gratitude
Thank you for reading and any experience you might have to share!

I would listen for others input as I have never checked a top with the back off. With the back on without a bridge I look for a top resonance close to 185-190 Hz. Yours is around 270 Hz. Having the back on does make a difference to the tops tap tone due to coupling. Your 270 Hz may show that the top is a bit stiff.

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http://www.Harvestmoonguitars.com

Tap it all over and see what you get. Some people look for a wide range of differences (Bourgeois?) and others a nice nondescript thunk! If you are building to a plan and it is your 3rd guitar, the coffee pot might the go to tool.
As to the other option you should lean more toward a lepidopterarium. Why do things the easy way when you can make them complicated.

Whole screeds have been written on this subject. It gets very complicated very quickly! Voicing, or building to aim at particular resonant frequencies is a real tool however, it just takes a lot of reading/learning to get to the bottom of it.

From my limited knowledge there is probably not a lot of information from the frequency at your current stage except perhaps that you have a nice resonant piece of spruce which bodes well.

Dave

You have two under your belt. How did those turn out? What, if anything, are you doing different this time? Are you using a book or method to guide you or doing your own thing? Would you be interested in suggestions for a book to guide you?

Thank you all for your help!

It's good to know that you check after the back is on John, I should probably stick to that stage for voicing.

There is a lepidopterarium here in Denver that we were just talking about Clay. Might be time for a visit.

Thanks for the encouragement Dave, and I'm going to move forward with the back, then revisit any potential balance problems after that. I guess I was just nervous about finding problems later and having to figure out how to shave braces through the sound hole. I guess I'm going to have to learn some time.

Onward!

Oh Barry, those are good questions.

I tend to build guitars like I cook. I value recipes for the ideas they give me, but then I have some other ideas I want to mix in, and pretty soon I'm not really cooking that dish anymore.

The basic body shape is similar to a Martin OM. The bracing pattern is also OM-style. Unlike a Martin, the lengthwise brace profile is dolphin-backed, and instead of a popsicle brace I used an a-frame. Those last 2 bits I lifted from my amateur understanding of Lowden's designs.

There are 3 differences between this build and my 2nd, which did not have this pure single note at this stage.

1. That was Honduran mahogany, this is cherry back and sides. But only the sides would figure in at this stage.
2. I only let the a-frame braces into the head block 1/4 inch that time, this time I ran the a-frame all the way through to exit so it will meet the neck. So there is a stiffer connection with the head block.
3. The adi in the 2nd build was from the fat side of the log that had some heavy red-horse, thick hard winter grain. This adi is from the other side of the tree, all soft white, no runout, lacing throughout, tight grain. Both were very stiff, this one was lighter while being just as stiff. I do expect different sound from the 2 tops, I just have no experience to tell me what that difference will be.

So you can probably all tell I'm just a babe in the woods here. Hopefully with a few more builds I'll begin to understand what's going on and slowly make some better decisions. Also, hopefully I'll have a bigger place to store all these guitars I'm planning, or find some players who would give them homes.

I forgot to mention I got all the basics from Cumpiano and Natelson, which has been a great foundation.

Of course, being me, I diverge whenever I take a notion. Dovetail neck joint, gluing the sides to the blocks first, then gluing the top and back on after, etc. I'll probably fiddle with lots of things over time so that I can get a feel for how all the different ways affect things. I owe a huge debt to those guys for giving such a thorough foundation to a beginner.

My third guitar had a top with a similar tap tone. I was unhappy with the results. The guitar sounded stuffy and overbuilt. Of course, It could easily have been a result of other building mistakes on my part, so *shrug*?

https://youtu.be/Ei5-DkVTrEE

This should link you to Dana Bourgois's talk at Northern Lights

Thank you Paul and bftobin!

Paul, you're agreeing with that nagging feeling I've been having, this top is too stiff.

bftobin, that video had a couple of key points that really helped me understand braces better. I loved the marimba bar comparison and the visual of how they tune harmonics.

I've started shaving material off the braces. That single loud note is slowly beginning to be joined by others. The top still feels plenty strong. I'm beginning to learn.

Thank you all!

As has been said, there are volumes written on this: some pro, some anti.

I took a workshop on 'free' plate tap tuning with Dana last year, to learn something of the limits for scalloped bracing, and to compare his methods with mine. So far as I know, he tunes the 'free' top, that is, checking out and refining the tap tones before the top is glued on. Then he tap tunes the whole box when the top and back have been assembled to the rim, but before the binding goes on. I tracked him through that latter step back in '98 or so at a GAL convention. I don't know of anybody who looks at the tap tones/resonances of the top when it's on the rim without the back.

The problem there is that the rim itself is somewhat flexible, and has some mass, so it changes the way the top vibrates. Of course, you can say the same about gluing the top and back onto the rim: they're not going to work the same afterward either. However, when you look at the open back case the changes will depend to a larger extent on the specific details of the rim used. If you used the same top and glued it to a mahogany rim and a rosewood one you'd get different results. I'd expect, for example, that all the resonant frequencies would be lower on the rosewood rim, since it's heavier but not all that much stiffer.

I've been using Chladni pattern testing of 'free' top and back plates for a long time. This is basically a 'tech' method of tap tuning: it gets much the same sort of information in the form of frequencies, but also shows the shapes of the resonances. It also takes more time than Dana's method. IMO, it's the shapes that are more important on the 'free' plates. You can also do tap tones or Chladni tests on the assembled box, and it that case the frequencies of the resonant modes are more important, but what really seems to matter is how those frequencies relate to each other on the particular guitar. Again, there has been a lot written on all of this, both pro and con, and I'm sure not going to do a full tutorial via ASCII text!

Thank you so much Alan for your thoughtful response.

To be a bit more specific, I had made a beginner mistake with the braces. I had tried to emulate the Lowden dolphin-back shape, but had the peak of the brace at 3/4 inch (and this is an OM size). I had carve parabolic cross-sections. In retrospect this was just a bad design, way overbuilt.

The fact that the top resonated loudly and clearly with a pure, single note just seemed wrong, so I got started down this path.

So I have converted the braces to a scalloped shape like Martin scalloped braces. During this process I noted that the primary note dropped from a C sharp to an A flat, which is mildly interesting, but the important part is that I started to hear other notes at other points around the top, including some more in the bass registers. I was just looking for an overall sense of flexibility and a range of resonances rather than just one.

So I’m not nearly at the level of tuning the top for some specific target, more just trying to avoid completely overbuilding the thing.

Some day when I have earned my stripes with multiple nominally successful builds I will feel ready to learn the kind of tuning you’re talking about, but today I’m content to just not build a complete dud.