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Based on the archives and some of my own testing, I gather that well-liked full-sized guitars (15.5"-16" lower bout) typically have these tap test frequencies:
Main Air: 92 - 104 Hz (around G)
Main Top (open soundhole): 170 - 205 Hz (around an octave above)
Main Top (closed soundhole): 165 - 195 Hz (...)
Main Back (open soundhole): 10 to 60 Hz higher than the Main-Top-Open-Hole. (between a half-step and a ~fifth above)
Main Back (closed soundhole): 10 to 60 Hz higher than the Main-Top-Closed-Hole. (...)

But I have little data on OOO/OM bodies; from some posts, it sounds like the frequencies are similar. I was thinking the 'main air' mode would need to be lower given the smaller volume, unless the sound hole were much smaller.

(I included both open and closed soundhole data because I run across it both ways. The relationship seems fairly predictable when I can test it both ways, and the open hole is easier, so I've been gravitating to that.)

Any guidance is appreciated.

Thanks, David

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David Malicky

Actually, if you think about it, you really want the main resonant pitches to relate to the tuning of the guitar, not the body size. Having the 'main air' mode down around F-G puts it a little higher than the lowest open string in standard tuning. The very lowest note might lack fundamental, but pretty quickly you start to pick that up. If you make the 'main air' mode much lower you'll have wasted some of the band width of the resonance, and making it too high means you lose 'fullness' on the lowest notes. So, yes, most flat top and classical guitars do fall within pretty much the same range of resonant frequencies.

The only caveat is that it's hard to keep the 'main air' pitch down on a small box without making the soundhole so small that you lose it entirely. An alternative is to make the hole bigger, shift the 'main air' up by a fair amount, and have it in the same range as the 'main top' and 'main back' pitches. The lowest notes can sound surprisingly good: the second partial will be strong, all of the other ones will be there, and your ear will supply the missing fundamental, a la 'English Bass'.

Thanks, Alan. Yes, that makes sense, to match pitches to the guitar's tuning and not it's size. From your prior posts, it sounded like the pitches stayed the same, but I appreciate the confirmation. That's fascinating about the 'English Bass' approach... I'd like to try that sometime to hear it myself!

I asked in part because I've been doing a fun experiment on a (cheap, overbuilt) import OOO. Stock, the 'main air' was 117 Hz and 'main top' was 217 Hz with open soundhole. The sound was thin and quiet, lacking in bass, fullness, and complexity. After scalloping/trimming quite a bit in the usual places, the main air is down to 109 Hz (main top at 198 open hole, 193 closed). The guitar is much better and has a pretty satisfying tone now, but the low E string notes are still a bit thin, and the treble is not as strong as the mid-bass. (The full story is here... http://www.acousticguitarforum.com/foru ... 598&page=4).

The soundhole is 4.0" (a blessing for shaving as you know!) which probably accounts for the high 'main air'. I just tried blocking ~1/4 of the hole, which brings the 'main air' down to 105 and helps the low E string a bit. The A string lost some punch, but that may be good as it tended to overshadow the treble. Wow, this is fun! I'm thinking of a little more shaving on top and back to bring down all 3 frequencies, and a tad smaller soundhole.

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David Malicky

Try shaving the back braces down a bit, if you have not already. I usually find that the easiest way to improve the low end.

Thanks again, Alan. Another shaving on the back braces did help -- in total, shaving all 4 from 11/16" to 5/16" brought the 'main air' down 7 hz. And the back tap sustain is appreciably longer now. I also did about 1/16 more on the top X scallops and I'm now at these numbers:
'main air' = 103 Hz
'main top' = 191 Hz open hole, 182 Hz closed
'main back' = 215 Hz open hole, 209 Hz closed

The notes on the low E string have improved, but unlike the dreads I've done, most notes on the A string (A# and higher) have gone from too stong to too weak. This OOO body is fairly thin, so I'm wondering if that causes a narrower peak for the 'main air', and not broad enough to reinforce the A string notes? And/or, I may have shaved too much on the top braces... the X valleys are 3/16" high now (which seems short to me, but then the valleys on my much larger Epiphone AJ500M are 1/4" and its top is 0.108" thick!). If it matters, this OOO top is a hefty 0.123" thick.

Any suggestions on what might help the A string? On balance for this guitar, I'd rather have punch on the A string than E (if it's one or the other). I'm thinking of some reinforcements for the X-valleys and/or back braces to bring the 'main air' frequency back up a bit. All suggestions appreciated.

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David Malicky

Yeah, you might have gone too far on the top braces. The effect of the back on the 'main air' mode depends on it's relationship with the 'main top' pitch: if you'd left the top alone the top and back tap tones would be closer; the 'main air' pitch might be lower, and the mode stronger. One thing at a time, and wait a while, is usually the best way to proceed. Did you number the shavings from the top braces, so you could get them back on the the correct order?

Thanks, Alan. I tap tested after every change, and played it between the back and top shavings, but didn't wait for it to settle in first. That's interesting about the back and top couplings and their effect on the air mode -- on my own guitars, I've not experimented much with them very closely coupled yet, but that will be interesting to try. On this trial, it sounds like my last shaved back and pre-last-shaved top frequencies (about 18 hz apart at that point) weren't quite close enough to couple the air mode down.

Oh certainly, I carefully numbered and cataloged each shave and am actually conditioning the shavings right now in a closed plastic bag including apple cores and banana peels. Until those are ready, I've installed some taller spruce reinforcements to the sides of the X-scallops -- sacrilege, I imagine, but that has helped restore the A string notes, and straightened out the rotating bridge. It's still sounds better than when I started, and now I know how far is too far and a better path for the next one.

Thanks again! David

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David Malicky

Then this was a very valuable experience for you! I'm thinking this is something learned by experience only and will pay dividends on your future builds.

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Formerly known as Adaboy.......