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I'm sure this has been discussed many times, but this is my first time here. I've built 6 OM sized steel string guitars and every one has had varying degrees of the " thuddy G" on the low E string (sometimes F#) and sometimes the same note on other strings, but not as pronounced as on the low E. I've read some info in American Lutherie and all I have come up with is David Wren's advice to keep the main box resonance 50 cents off a semitone. Can something be done after the guitar is finished? Is there any other info that someone can give me so I can avoid this problem. The guitars otherwise sound pretty good.

That's usually caused by the 'main air' pitch being too close to the pitch of the played note. What we call the 'main air resonance' is actually the lower part of the 'bass reflex couple', where the Helmholtz air resonance and the top (and the back, and sides too!) work together to produce a pair of peaks in the output in the low range. We call the two peaks the 'air' and 'wood' resonances as a shorthand way of saying where most of the energy is: both parts are moving at booth frequencies. Thus, at that low G pitch the top moves a lot, extracting energy from the string, and turning it into sound in a hurry. You get a note that's twice as loud for half as long. Since your ears are not very good at hearing changes in volume you notice the lack of sustain more than the added power. Since several parts of the guitar are involved, there are a few ways to change this.

The most direct in some sense is to alter the pitch of the Helmholtz air resonance. Changing the soundhole size is the most direct way, but you can also add a 'sleeve' inside to make the 'neck' of the 'bottle' longer. It's easy to try this with a paper ring that you can stick inside the hole. You can also make removable plugs with holes in them to reduce the hole size until you find something that works. I've used paper faced foam board for this: cut a disk that's a tight fit in the hole, and glue a piece of card stock to it to keep it from falling into the guitar. Making the hole smaller will usually lower the 'air' mode pitch, so the guitar might sound more 'bassy'. It will also usually be quieter.

The flexibility of the top and back get into the act too. Often I've found that, if the back is stiff (the 'main back' tap tone is, say, more than a fifth higher than the 'main top' pitch) loosening up the back can make a useful difference. This is a lot less risky structurally than shaving top braces. You can do non-destructive tests of this by adding some mass to the back using something like poster putty. This drops the resonant pitches just as shaving braces would, but it's easier to undo if you don't like it. Loosening the back can also drop the 'air' pitch, but generally adds power.

There are other ways to get this result, but those are the ones I'm most familiar with.

Alan, Thank you so much. I will try this out. I've tried the chlandi pattern method of brace shaping and got a wonderful sounding cedar top guitar. Thanks for that too!