Hesh asked:
"Al by tuning the back do you mean to reduce the stiffness of the back braces specifically in the lower bout area?"
Like Humpty Dumpty, I only meant what I said. ;)
If you can get the 'main back' tap tone to be fairly close in pitch to the 'main top' tone the two will couple more strongly. The back then becomes an actor in the 'bass reflex couple' that is making most of the sound in the low range. Normally, this will drop the pitch of the 'main air' resonance slightly, but, more importantly, it will give you more power in the low end, putting a floor under the sound, so to speak.
How you tune the back will depend on how it's working in the first place. Most people seem to think of the back primarily as a reflector, and make it as stiff as they can within reason. There is a lot of merit in that image: the 'main back' resonant mode seems to be the only one that can contibute directly to the output of the guitar, and above that you probably do want the back to be as unresponsive as possible in some respects. So, as often happens, we're on the throes of a dilema here: we want a back that can move at a low frequency and not at high frequencies, and that's hard to do.
Normally it is the lower bout braces that are too stiff, and reducing the height of those will generally be the quickest way to drop the 'main back' pitch. The low, wide braces that Martin uses on many of their instruments do, indeed, add less stiffness and more mass than they could, and tend to drop that back pitch relative to what it would be if they were stood up. It may happen that on a guitar with that sort of bracing the back waist bar will be the one that most needs to be trimmed. My feeling at the moment (and this could change with new data at any time) is that scalloping the back braces will do the most good. That said, I often end up with 'tapered' back braces. Go figure.
As far as I can tell, the 'best' pitch for the 'main back' mode would be about a semitone higher than the 'main top' tap tone. You can find these (cloes enough) by blocking the soundhole and taping on the plates. Tap the top at the bridge, and try the back somewhere around the center of the lower bout, or maybe a bit higher. If the pitch seperation between the top and back modes is too much (say, a third or more) they won't work together as strongly. If they are too close (much less than a semitone) they can couple too strongly and cause a 'wolf' of sorts. Since the top tends to 'play down' in pitch, it's safer to have the back pitch higher rather than lower.
This is one of those things I picked up from Fred Dickens many moons ago. The first time I tried it out was when a local player brought by a pair of guitars that had been made by another builder. One had great bass and 'flat' trebles, the other had nice trebles and a 'thin' bass. The player wanted be to effect a transplant of sorts: either improve the trebles of one or the bass of the other, and the luthier had been unable to do so.
I checked out the top and back modes of both. It turned out that the one that had the good bass had about a semitone of seperation on the main modes, while the other had a stiffer back. The top on the one that had the better bass seemed a little 'loose' to me, and that seemed to account for the lack of treble crispness. I liked the look of the top bracing on the other much better, and could easily get into the soundhole to shave the back braces. I worked mostly on the brace just below the waist, and the one below that, above the wide point of the lower bout (it was a 4-brace back).
As I reduced the back braces I would check the tap tone pitches every once in a while, and also play it to see how it sounded. At first there was not much change in the timbre, but as the pitches got closer the bass started to improve, and when the pitch relationship got to be the same as that of the other instrument, the bass response also became quite similar. Since, as it seems, much of the treble response comes from the top, and I had not touched that, I had managed to do what the customer wanted: the transplant the better bass into the guitar that alredy had good trebles, without effecting the trebles noticably. Thanks Fred!
"Also I am confused - are you saying that moving the sound hole forward can enhance base or is it the other way around. "
Moving the soundhole upward, toward the upper edge of the top, tends to drop the 'main air' resonant pitch. For a lot more on this, see Allen's 'The Basics of Air Resonance' in American Lutherie #1, or the first 'Big Red Book' if you don't have the mags.
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