DarwinStrings wrote:So we know from the physics that the long and cross dipole have two antinodes that are 180 degrees out of phase with each other (this should also apply to the monopole cause the antinode in the middle of the board is out of phase with the rim or any part on the other side of the monopole node). So how does this cause comb filtering? It seems there needs to be a delay involved and the only way I can see that occurring and relate it to Frank's quote above is that there is some delay between the pressure waves coming off the two antinodes, in that they are a different distance from your ear causing a delay which is interesting because it would seem to relate better the the monopole and the cross dipole rather(for the player not the listener) than the long dipole (which Franks design suppresses).
Just the presence of two out of phase zones creates comb filtering. It doesn't technically require "time". It is true, the time constant is responsible for some of the phase smearing in the convoluted sound that hits a microphone or our ears. But two sound sources can be 180 degrees out of phase
and occur in the same time constant. That's the hard definition of 180 degrees out.
Electrically, if two identical signals (one out of phase) were combined in mono the result is silence. Acoustically, whether a guitar top or a speaker cabinet, the result is comb filtering. This can change with location. Like if you have a left/right speaker set out of phase, when you physically move yourself closer to the Left speaker, it begins to take over, and the sound of the right speaker is distant. When you're sitting dead center, the sound is thin and the ear-brain doesn't know what to make of the location. It may sound like there are speakers behind you or directly to the side right?
So now, just imagine two out of phase locations on the spruce, with a null in the center between them. They are pushing sound into the air that is convoluted. In this case the comb filtering I'm referring to occurs in the air in front of the guitar. It also affects the reflections inside the box, but we all pretty much understand those are highly convoluted anyway. There
is a comb filtering that occurs within the spruce top of course. I mean, that it would occur in a vacuum and is not related to sound waves penetrating the air. Like how a 12th fret harmonic combs/cancels the fundamental in a string, the spruce top is now the most significant part of a feedback loop between the string and the guitar. This is what I refer to when I say you can play my guitar out of tune, dropped 1/4 step, with dissonant chords, dissonant chords AND out of tune...and all 6 strings ring out more uniformly and with less beating. The more the harmonic extensions get caught in these "closed loops" (on a torqued top) the more they argue with one another when a chord is sustaining. They return vibrations to the string that are in argument with it. This changes the decay/sustain duration and is part of the "warbling" or beating sound as well.