Keep in mind that the top on a 12 may not have all that much higher a tap tone. It needs to be something like twice as stiff as a 6, but that only requires making the top 25% thicker, and the bracing 25% taller, so it's not twice as heavy. The bridge plate is larger, of course, as is the bridge, and that helps drop the 'top' pitch some.
Basically, it seems to be a good idea to have the pitch of the 'main back' tap tone higher than the 'main top'. How much higher is somewhat a matter of taste.
The closer the two are in pitch to each other, the more strongly they will couple. This enhances the power of the 'main air' mode, and drops it's pitch. In theory, the effect is strongest when the 'top' and 'back' pitches match. In practice, if they're too close you can get strong 'wolf' notes, some of which can be pretty odd. A semitone of separation seems to be 'enough' to avoid problems.
People seem to prefer guitars where the back pitch is higher. In some cases, at least, putting the string on seems to raise the back pitch a bit, and may drop the top pitch, although that's not as firmly established. I have also seen guitars where the top pitch has dropped about a half semitone in the first month or so of playing, although I don't always have them around long enough to be able to say that's the rule. Starting out with the back pitch a bit on the low side thus invites convergence in the tap tone pitches, which could put one into wolf territory, so it's probably best to avoid that if you can.
Not everybody likes the sound of a guitar with tight top-back coupling. Many 'vintage' instruments have the back higher in pitch by a musical third or more, and if that's the sound you're going for that may be the best way to get it.
|