Hummmm well you cant really tell with fft unless you have a reference. You "seem" to know what your talking about but abit of your theory is off. I suggest not placing the mics so close together as the closer the mics are to one another the more "likely" phase cancellation will occur.. I would suggest doing some more studying on this subject, I would invest in some mics that you dont have to "fix" to get them to sound good. I would hardly call wearing two sets of mics stealth taping... I would look at getting a pair of AT 853 mics I am not going to plug my mics because well.. I dont have too. But I think you should look around and listen to some of the other veterans here that are trying to say the same thing I am.. If you dont like the sound of your mics. There is one company that did this AKG used two capsules in there 3000 one for low end one for high end. They used a crossover to divide the mics so they did not overlap each others frequence response. You then end up with a time coherent signal with "less" phasing issues. Also you will not be able to "fix" the phase issues unless you use a 4 channel recorder and then sent the individual tracks to a computer so you can then change the time arrival. But even then you will still have comb filtering issues. Sounds to me like just buying a better set of mics might be simpler
I will probably be replacing these mics eventually, I'm just looking for something to temporarily use maybe for the next year or so. I don't have very many recordings with me at the moment, but if memory serves correctly I've enjoyed a lot of recordings made with DPA 4061's -- but at the moment I don't want to spend $700-ish (I think that's approximately what they cost)... I'm moving into my first apartment and there are lots of other ways that I could more responsibly be spending that much money.
Two sets of mics isn't extremely stealthy, but I haven't been caught yet. Whenever I'm worried about being able to pull it off with 2 mics, battery boxes, minidisc recorders, and two times the blank discs and batteries in the past I've just guessed at which mic I would prefer on its own for that particular show and leave half the gear at home.
My degree is in mathematics and I've only taken one class about the physics involved with sound (and that class was sort of geared towards the non-physics, non-math crowd so a lot of stuff was glossed right over). So I'm definitely no expert and might have some misconceptions. If you could help me understand where my theory is off that would be wonderful, as I've been thinking about this for a while and can't figure out where my thinking has gone sour.
You say that the closer two microphones are to each other the more likely phase cancellation will occur, and you have a lot of practical experience with this sort of thing so you're probably saying this from experience which is hard to argue with. But I just don't see how the physics and mathematics behind it explain this.
In a simple model where there's one sound source, and one mic placed 1 meter away, and a second mic placed right between them then there's no problems when there are simultaneously high pressures at both mics or low at both mics, which would be whenever the wavelength of the signal the source nicely divides the meter distance from the source to the 1st mic and the half meter distance from the source to the 2nd mic. When the wavelength is something that
almost evenly divides those distances they won't be perfectly in phase, but would be only a few degrees out of phase. This wouldn't be a huge problem as long as it's only a few degrees out of phase, would it? However, with this setup than sounds coming from the source with a wavelength of 1 meter, or a wavelength of 1/3 meter, would be 180 degrees out of phase at the two microphone positions. If the 2nd microphone is moved closer and closer to the 1 meter point where the 1st microphone is, then when it is very close to 1 meter from the source, then the sound with a 1/2 meter wavelength will be only a few degrees out of phase, as will the sound with the 1 meter wavelength and the sound with the 1/3 meter wavelength. To find anything that is severely out of phase you have to look at the shorter wavelength, higher frequency sounds. Keep moving the two mics closer together and everything within the audible spectrum should be within a few degrees of being perfectly in phase (there would just be tons of ultrasonic phasing that we can't hear) -- theoretically this could be continued infinitely by pushing the microphones closer and closer so that more and more low frequencies are even closer to being perfectly in phase and the only frequencies that are dreadfully out of phase are even higher and higher. Taking this to the limit you'll end up when every finite frequency is perfectly in phase and the distance between the microphones is zero so that they're essentially the same microphone (the better but likely more expensive solution that you've suggested).
In practice it is surely more complicated than this, but I don't see how, if two microphones are sufficiently close to each other, there would be be a great deal of audible deconstructive interference or phase cancellation within the audible hearing range of hearing -- all of the significant phase cancellation would be happening at ultrasonic frequencies. Everything would be slightly out of phase at all audible frequencies, but not by too many degrees if the mics are sufficiently close enough. If the mics are farther apart, then some audible frequencies would suffer absolutely no phase cancellation (such as the 1/2 meter wavelength in my example). And so for that particular wavelength a greater distance is better than when the two microphones are very close together and they are out of phase by a few degrees. But when they're farther apart like that there would also be audible frequencies that are severely out of phase such as the 1/3 meter and 1 meter wavelengths that are 180 degrees out of phase.
I'd really appreciate it if you'd help me understand where my theory is off at!