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I'm venturing into voicing and tapping and could use some feedback on the process.

This is a new body style for me (GA or SJ) so I don't yet know how it will sound. From reading Somogyi, I'm trying to build a step lighter. I started w/ fairly standard bracing, then tapped while shaving a few braces and thinning the top. I don't have a mold for this body yet, so am lightly holding the top/sides at the neck area while tapping. I shaved first on the treble side, then the bass, to compare effects.

Some details that may be useful:
Body: 15.6" lower bout, 10.0" waist, 11.2" upper bout, 20.5" long.
Top: Sitka, thicknessed to 0.100", with deflection of 0.20" (5 lb on 18" span). Density 8.0 g/in3 (30 pcf).
Bracing widths: 5/16" wide Xs and LFBs. 1/4" wide fingers and SHBs. Rectangular cross-sections, except I tapered the 5/16" ones a little as they seemed chunky.
Brace heights: Xs start at 0.55", scalloped to 0.30". LFBs are 0.39". Fingers max of 0.25". SHBs started at 0.25", now 0.20".
X: 90 deg, capped.
(Big UFB, for a pocket neck joint).
Bridge plate: Maple; seemed too big and thick after install... now thinned to ~0.95 ave. Bridge will be cherry, probably ~23g.
Sides: Baltic Birch ply, 1 ply sanded away.

Somogyi says to look for a live sound when tapping left/right of the soundhole. Mine was only sortof live L/R (but good just below the soundhole). The upper X legs initially had a linear taper starting 5" from the tip; I extended the taper to start at 6.5" from the tip, and also shaved the SHBs (see pics). I heard a difference, but more in frequency than liveness, and am hesitant to shave more or thin the top here (?). Any suggestions?

Tapping around the lower bout face showed a very uniform sweet spot extending to ~3" inboard of the perimeter, and ~2" below the soundhole. I sanded the perimeter of the top, tapering from 0.100" thick at 3" inboard to ~0.075" thick at the edge. But, tapping near the perimeter became less live than before, and the sweet spot isn't bigger. *But*, it seemed like tapping in the bridge area had more sustain. I had a recording of taps on Audacity from before, and sure enough the sustain is longer now and smoother in decay (see pic). (Main top freq dropped from 165 Hz to 157 Hz, for this config, though a bridge and back will probably change that.) Has anyone else found effects like this? Would you suggest thinning the perimeter further, or is 0.075" already pretty thin (15.6" lower bout)? Is 0.5 sec of sustain for this config considered 'good', or if not, what would you suggest next?

Thanks for any feedback.

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David Malicky

From the looks of things, you still have a lot of mass on your braces. Try shaving the sides of the braces to look like a triangle /\ , you'll keep the height where your strength comes from and reduce your mass allowing the top to be more "free" to vibrate. Start slow and record you progress. I don't think you should touch the top perimeter any more....

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What Rod says...

David: I agree with Rod and John. You have gobs of material that you can get rid of on those braces,take it off the sides not the height.Also the perimeter is thin enough. Good luck.
Tom

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Thanks for the feedback! And a consensus, even! I've thinned down all the 5/16" wide braces. No change in sustain, but main top tap frequency dropped a bit. Do any particular braces look especially ripe for picking? Thanks esp for the guidance on not thinning the top perimeter anymore.

I tried attaching the back with tape (hard to do very securely)... the main air tap freq is at 80 Hz and sustain is horrible (both probably artificially low from the taped/rattling joint). Main top freq is 167 Hz / 179 Hz (closed / open soundhole), so if accurate, it appears it is about there, or I could do a tad more. I think I'll attach the bridge and check again, before shaving more.

Thanks again!

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David Malicky

Your top is already quite thin.

I don't get the part about listening for a lively sound between the sound hole and the waist, if I understand you correctly. That is going to be one of the least active parts of the top.

You have taken the braces down a lot in height already. With your thin top, what I would suggest is that you complete construction and if the guitar is not distorting much after a month or so, try lightly shaving the sides of the braces through the soundhole. The lower diagonal is hard to reach and structurally less critical, so if you have big arms, you may want to shape it now. I don't like triangular braces; they have very little wood where it would add the most stiffness. I aim for a (I hate to say it, since the word is so abused) parabolic cross section.

I would also suggest you try to let your right brain join the party.

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Howard Klepper
http://www.klepperguitars.com

When all else fails, clean the shop.

A couple things....

Cleats -- yours look pretty beefy. Trim them down and thin down their edges too.... especially the ones in the lower bout.

The only other thing is the contrast between the tone bars and the X-brace... It looks like the tone bars are quite a bit heavier than the X-brace.. and I would think it would typically end up the other way around -- The X-brace being heavier and stiffer than the Tone bars.

But.. In the end, it's all gotta work together as a balanced system.... (Which is also something Ervin preaches..) Stiff sections end up doing more "Work" supporting the structure, but the Thin sections do all the flexing.

Thanks

John

I would suggest that at this point you build the simple deflection jig I've described earlier in the Tutorial section. Then see if you can borrow a guitar similar to the one you're building which is both dimensionally stable and has an acceptable tone. Create the deflection map of the lower bout for this "successful" guitar and record tap tones for the same areas you're doing now on your own guitar. Now make an analogous map for your guitar and compare the two sets of deflection and tone measurements. I don't believe that you have a finishing deadline at this point, so taking the time to do these reference measurements is really a good investment IMHO.

At this point it should be fairly obvious where your guitar top is stiffer than your reference guitar. Now you can both continue tapping, recording, shaving braces and doing deflection measurements so that you can follow the tap tone changes along with actual top stiffness measurements. Temporarily gluing/taping the bridge on is also necessary at this point. Doing a spectrum analysis of the tap tones of both guitars will also be helpful for your reference information.

Removing brace material in the center portion of the lower bout will have a considerably greater effect on overall top stiffness than material from the edges (natch) so going slowly at this point and making frequent deflection measurements will keep you from removing too much material.

aloha,

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David C. Hurd
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Thanks for the feedback and suggestions! I thinned the tone bars some, and shaved the diamonds a lot. I went ahead and closed the box, as Howard's advice is very sensible and this is my first attempt at building lighter. And my vacation week ends tomorrow, so it's good to wrap this one up. The deflection measurements make a lot of sense and I can see those will give me better feedback, and that I can't measure with tap tones.

Howard, on looking for a 'live' sound when tapping between soundhole and waist, I'm confused about it, at least with just a braced top/sides. I can only relay what Somogyi writes: "a well made steel string guitar top will sound live when tapped in the upper bout next to the soundhole" and to look for "some liveness in response in that part of the upper bout" (p. 115). I never really got that, but I don't know that my guitar is well made. Now that the box is closed, tapping there does excite the 'main air' mode a fair bit (but not like the bridge, of course).

With the box closed (and with bridge and a quick finish), I found a few interesting changes compared to my prior guitars:
- The 'main air' frequency came in pretty low -- 87 Hz (F). I recall Alan C suggests a main air of at least F# (~93 Hz). I tried opening the soundhole as large as I could (a slight oval equivalent to 4.25" dia round, which I thought would give ~92 Hz) but it only moved to 89 Hz. On my next iteration I'll space the bracing for a larger hole. Does anyone else use larger holes for 'responsive'/lighter-built guitars? Or do your main air freqs tend to be low?
- Tapping on the bridge sounds good! Boong! It's no bell, but much better than the Thump of my prior guitars. This is fun!

Thanks again,
David

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David Malicky

87 Hz (F) is not a bad pitch for the 'main air' mode. G=98 is more normal, but F will give you a nice 'growl'. I just don't see the point in going much lower than that.

Thanks, Alan, for the correction. I wasn't remembering one of your prior posts correctly, that below F tends to be 'tubby', not F# -- I'm sorry for the misquote.

I rechecked a Somogyi 'main air' freq from a video -- 85 Hz w/o bridge or finish. Not sure how his bridges affect that... his look both stiff and massive. So perhaps my low freq is normal for this approach.

I'm looking forward to some 'growl'!

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David Malicky