Here are some comments on the 3-to-1 rule and its relation to stereo miking. Hope this stuff is helpful...
Suppose you're recording a jazz trio in mono for TV broadcast.
For simplicity, we'll say there's one mic on the drum set, one on
the piano, and one on the bass.
The drums sound funny -- sort of filtered. What's happening?
The piano mic is picking up the drums at a distance. The
drum mic is picking up the drums up close. So there are two drum
signals in the mix. One is direct and one is delayed.
When you combine a signal with its delayed replica at equal
levels, certain frequencies cancel out, depending on the delay.
There appears a row of notches in the frequency response where
the sounds cancel. This is called a comb filter effect, because
the frequency response looks like the teeth of a comb.
In general, if two microphones pick up the same sound source at
different distances, and their signals are fed to the same
channel, this might cause phase cancellations. These are peaks
and dips in the frequency response caused by some frequencies
combining out of phase. The result is a colored, filtered tone
quality. It sounds like mild flanging.
If you are recording in stereo, and don't expect the recording to be heard in mono, then the 3-to-1 rule doesn't matter. The phase cancellations don't occur in stereo; just in mono. In fact, the phase differences create the stereo imaging in near-coincident and spaced-pair stereo mic techniques.
To reduce phase cancellations between two mics, follow the 3 to 1
rule: The distance between mics should be at least three times
the mic-to-source distance. For example, if two mics are each 1
foot from their sound sources, the mics should be at least 3 feet
apart to prevent phase cancellations.
How was the 3:1 rule determined? It started with the following
phenomenon:
When you add a signal to its delayed replica at equal levels, and mixed to the same channel, you
get severe comb filtering with deep notches. But when you mix
direct and delayed signals at different levels, you get less deep
notches.
Specifically, if the delayed signal is 9 dB less than the direct
signal, the comb-filter notches are only +/- 1 dB, so for all
practical purposes they are inaudible.
How do we make sure that the delayed signal, picked up by a
distant mic, is at least 9 dB below the direct signal picked up
by the closer mic? Put the distant mic at least 3 times farther
from the source than the close mic is. Due to the inverse square
law, the level drops about 9.5 dB when the distance to the source
is increased 3 times.
So the 3:1 rule ensures that the level at the distant mic will be
down at least 9 dB, so the mixed signals will have comb filtering
of +/- 1 dB or less.
A ratio of 4:1 or more is even better. The 3:1 ratio is the
minimum to avoid audible comb-filter effects.
Suppose you have a close mic and a distant mic. The distant mic
is either used for reverb pickup, or to pick up another
instrument. You just set its gain to get a normal recording
level for that mic's sound source.
Suppose the close mic is picking up a loud piano, and the distant
mic is picking up a quiet acoustic guitar. You've placed the
mics following the 3:1 rule. But you have to turn up the
guitar-mic gain a lot because the guitar is so quiet. If so, you
might negate the 9 dB separation. That is, the piano signal in
the guitar track might be less than 9 dB below the piano signal
in the piano track, because the guitar-mic's gain is so high.
So there's more to it than just the 3:1 placement. The idea is
to get at least 9 dB difference between track levels for the same
instrument. You want at least 9 dB of separation, not exactly 9
dB of separation.
Here are some ways to increase separation:
*Mike close.
*Spread instruments farther apart.
*Put a gobo between them.
*Use directional mics, and aim the null of each mic's polar
pattern at the other mic.
*Don't use two mics when one will do the job. For example, use
just one mic on a lectern. If you must use two mics mixed to the
same channel, place them so their grilles touch.
*Record in a deader room. This reduces reflections into the
front of each mic, which can degrade separation.
*Use a pickup on the guitar instead of a mic.
If the close and distant mics are two cardioids aiming in
opposite directions, the mics can be closer than 3:1 and still
get enough separation.
