mixsit said:
They are slightly offset in time which would have some effect on the top end (I don't know how high) but look at AB stereo and ORTF. There offset is intended to alow a little phase differences so it's not an either/or situation.
The main point though might be to know that 3:1 doesn't fix any phase frequiencies, it just attenuates cross bleed.
With two mics an inch apart you may notice stuff starting around 5k.
As far as attenuating crossbleed- the magic level is around 9db. If you listen to two out-of-phase signals, once one of them is around 9db less in volume than the other, you won't hear audible cancellation, according to the propellerheads.
3:1 says if you put a second mic three times as far from the first as the first is to the source, the attenuation will be around 9db. So when you listen to both mics AT THEIR PROPER RELATIVE VOLUME you won't hear the comb filtering. It's still there, though.
3:1 is really about relative amplitude, not distance. 3:1 assumes the same mics set at the same gain and played back at their proper relative levels, ie the far mic is going to be down about 9db, just like you would hear it if you listened from one foot away and then backed off to four feet.
The classic misuse of this is a near/far mic on a guitar amp. You see it all the time.
"I put one mic a foot away from my amp and another three feet away, and I'm hearing comb filtering. Why? I followed 3:1"
No, you didn't. And you shouldn't have anyway.
First of all, the second mic is supposed to be at least three times farther from the first mic as the first mic is to the source, not three times the distance as the first mic is. So if your near mic is one foot away, your far mic needs to be at least four feet away.
Second, most people don't understand the amplitude thing. So they bring up their far mic track enough so it crosses that 9db threshold, and hear cancellation. And are confused.
Because thirdly, they thought there was a magic distance ratio that eliminates comb filtering. There isn't.
People who claim 3:1 spacing eliminates comb filtering when using a near/far mic placement have either 1) chosen a placement that puts the filtering either out of the audible range or in a range where isn't offensive or 2) unknowingly have the relative levels of their tracks set so the one is at least 9db quieter than the other.
