Phase problems -- miking a guitar.

1) Phase problems due to different distances from mikes to source

If you have two mikes and they're different distances from the sound
source, many frequencies will be out of phase as they reach the two
mikes.  For a simple pinpoint sound source you can always correct
that by dragging one track to the right or left to compensate.

It's important to understand in this case that for different
frequencies, the difference in phase (the "phase shift") between the
two signals is different. The amount of phase shift depends on the
frequency and the difference in distances (between each mike and the source).

2) Two close mikes

Let's take a case where one mike is 6" closer to the source than
the other, and all other things are equal.

For any 1kHz tones in the sound, the wavelength is about one foot.
Since 6" is a half a foot and 180 degrees is half a circle, that
means 1kHz will be 180 degrees out of phase.  When you add the two
signals together, this frequency will disappear (because one signal
is going up just as the other is going down).

The wavelength of 2k is half of the wavelength for 1K, so it's 6".
That means the phase shift for 2k is 360 degrees, which is the same
as zero degrees, meaning no phase shift.  This frequency will double
in amplitude when you add the signals together.

Just as you might guess, frequencies inbetween 1K and 2k will be
somewhere inbetween doubling and cancelling.  The same thing happens
below 1k -- as the frequency gets lower, the cancellation goes away.

Above 2KHz, it's a little more complicated.  At 4k, 6k, etc., we get
the same situation as at 2K -- the signal doubles (as we'd want it to).
But at 3k, 5k, 7k, etc., the signal cancels.  (This is called "comb
filtering", by the way.)

The smaller the difference in distance between mikes and sound source,
the higher the frequency has to be before making this mess.  When the
difference is less than half an inch this frequency is above the
audible range and you can generally ignore it.

So, if you're using two mikes to get a stereo image, a good practice
is to put one mike head directly over the other, with the two pointed
in different directions, like this (viewing from above):

   O
  /  \
/      \

where the O is both mike heads and the lines are the mike shafts.  The
angle between the two can be 90 degrees, or more or less to compensate
for how close to the source you are (narrower for more distant sources).
Really, the angle is a "fiddle and listen" thing.

3) Miking different parts of the guitar

On the other hand, if you're using two mikes to get a blend of
tonalities, other factors come into play.  A common practice is for both
mikes to be rather close to the guitar (inches, or maybe a foot or so),
but one is near the soundhole by the neck, and the other is pointed at
the top in the lower bout.  (I.e., the part of the top (tonewood)
between the sound hole and where you attach your guitar strap.)

In this particular case, there's a competely different aspect of phase
to consider.  The way the guitar works is that the changing tension of
the strings causes varying tension on the bridge, which then rotates
about its vertical axis and causes this portion of the tonewood to
vibrate out and in.  When this tonewood is going out, it's sucking air
in the sound hole, and when it's going in, it's blowing air out the
sound hole (so to speak).  In other words, whatever this mike picks
up will be (rougly, esp for low frequencies) the opposite of what the
neck mike picks up.  So you'd want to invert (180 degree phase shift)
one of them before mixing to taste.

4) Close and Distant mike

If you're planning to use one close mike and one distant mike, then
you can compensate for the distance after recording.  (You'll do
yourself a favor here if at the very start of the recording you make
a nice sharp sound, like snapping a bass string.)  Locate that sound
in the timeline view and zoom way in on it, keeping it in sight in
both tracks.  As you get closer, you'll see a delay between the two.
Drag one track to make it match up with the other.  Zoom all the way
in and adjust as you go, until they're lined up as closely as you can.

Bingo, you've fixed the biggest phase problems between the two mikes.
There may still be some, but nothing you can do about it anyway other
than to listen and try different mike positions until you find something
that sounds good.  But don't make the sound comparisons without making
this correction first.

Later, you can always drag one track a little off the other to create
a more spacious sound (if they're panned apart somewhat).  As you do
that, you'll be introducing new phase problems, so be careful -- but
it's a common and useful trick.  Just be sure to double check in mono
(there's a little button on the mixer panel).  If it doesn't sound
decent in mono, adjust and try again.

5) Two close mikes, reprise

You can also try track-dragging when the difference in distance is
small, but in that case it's usually easier to try lots of positions
where the difference in distance is zero, so you can just try and
listen, without adjusting the track each time.

Physics: http://68.99.178.61:8080/studio/physics/phase.shtml