3:1 Rule...?

[Piotr]

So we know about the three-to-one rule when working with stereo pairs.

I am looking for similar suggestions using two mics in the same line of sight at different distances from the sound source. EG. a kick drum mic and a room mic, or a guitar amp mic and a room mic.

Please add any simple shortcuts you have for avoiding phase cancellation.

[alanfc]

I was a Sunset Apollo, what were you?

I have no meaningful tips for you otherwise

[Randy]

I like to check for weird phase by taking the second mic and pan-ing back and forth pretty quick and listening for a "whoosh" sometimes the whoosh is cool, but most of the time it will make mixdown a chore.

I also sum the board (not the software) to mono and turn off one speaker so I can get an idea of what is happening in real mono out of one speaker. You might be surprised what you can miss when two speakers are making a mono signal.

[Electricide]

well, it depends on distance. When your two mics are close to each other, it comes down to certain frequencies. If they are 1 foot apart, frequency X cancels, while at 1 foot 8 inches frequency Y cancels. High frequencies are more sensitvie to small distance changes because the wavelengths are shorter, and thus more peaks and valleys (compressions) occur within a given distance.

The farther the mics are spaced, the increased the time difference between a kick attack hitting the close mic and the far mic. There is a threshold at which point two sounds become distinguishable form each other, the first (close mic) as the initial, and the farther mic as an echo, or reverb. The distance between the two mics determines the difference in time that the sound waves hits both mics. Small differences cause comb filtering, or phase cancellation. Larger time differences are perceived as two seperate sound waves, or "echo/reveb/roomsound"

Sorry to be longwinded. So, I guess, it depends on what frequencies you want to deal with. If you're looking to add oomph to the kick, then you could determine the frequency of the kick, convert that to how many feet one wave length is, and place the second mic, say, exactly ten wavelengths back, so that each mic is getting the kick sound at the same point in the sound's cycle.

The quicker way is to move the second mic back from the first until it sounds good. Or until it sounds the worst, then flip its polarity. This can be tricky if you're tracking in your control room.

If you're just looking for room noise, then pull the mic back far enough so that it's heard more as a delay, and not a part of the initial attack.

I think the threshold is 40 milliseconds. Or was it 70? Anybody remember the minimum amount of time necessary?

[jajjguy]

...move the second mic back from the first until it sounds good.

That's where the 3:1 comes in. The farther mic should be three times farther from the source than the closer mic (if i recall correctly). So if Mic A is a foot from a guitar amp, mic B should be three feet or more away from it.

This is just a rule of thumb. Or just listen for phase problems and adjust.

[Piotr]

I've always found this rule works well for setting distances between stereo pairs. Say my drum overheads are two feet off the cymbals. I might try and get them four feet apart...

Sometimes it's fun to get out the old tape measure for all of this as well!

[Electricide]

ya know, I'm think about that, Walnut, and imagine a triangle, with the snare at one peak, and each mic 2 feet away, and the mics four feet apart. The snare sound still reaches the mics at the same time. But if one of the mics was 2 feet, 3 inches away, and the other at 2', you could still have phase cancellation when summed. I guess I'm wondering why horizontal distance apart compensates for phase problems, because changing it doesn't necessarily change the timing confilcts that cause filtering.

Or is it because the sound from one side of the snare is different enough from the other side, including the different room reflections, to prevent filtering?

[Piotr]

I think that's it.

Sound waves traveling toward both mics are going to hit each mic at slightly different times because the snare is not a single point in space, but rather a collection of points at different distances from each mic. Sound from one side of the snare drum will take longer to reach one mic than another and vice versa.

Cancellation will occur at the equidistant point(s). But that's stereo isn't it? There will always be some cancellation...