Phase. What exactly is it?

[ajcarson1]

Hey everyone, i'm pretty new here, and I have a question,

What exactly is the deal with phasing when using multiple microphones?
When should I expect it, what exactly is it, what's the problem with it, etc...

Thanks for all the help!

[Electriclight]

It's kinda hard to explain without pictures... You know a sine wave is the simplest wave. So imagine a sine wave coming from a speaker. It goes up and it goes down over time. Let's say a 1 hz sine way goes above the center and then below the centerline and then returns to the center line for one complete cycle in one second. So over the course of time it's going up and down, up and down. Now suppose one micrphone is a foot away and another is 18" away. It could happen that at the same moment the sine waveis hitting the first mic on the up stroke of the cycle while the sine wave is hitting the down sroke of the cycle at the second microphone. The two signals would be reversed and would cancel each other.

Some cars put a microphone at the tailpipe to monitor the sound. then they replicate the sound but with an opposite phase. The whole car beomes quieter.

Here's an article that at least has some pictures and definitions:

Phase: Definition from Answers.com

The easiset way to handle it in recording is to reverse one of the phases and see if the signal gets stronger or not.

[ajcarson1]

Okay, great! I understand what your saying now. So basically 2 sound waves cancel each out.
So does this automatically happen when I use more than one microphone? How do i know if the microphones are having phase problems?

I've been micing a drum-set with several mics, and wonder how I can detect phasing issues.

[Electriclight]

No it's not automatic. It's almost random, because the wavelength of each frequency is different. I know there is a rule of thumb about mic placement. I'm just not sure what it is! LOL Someone else should pick up this thread now. hint hint

[darrylm]

It's called the "3 to 1 Principle". From The Recording Engineer's Handbook by Bobby Owsinski (p. 77-78):

The 3 to 1 principle is pretty important when considering any multi-mic setups because, if you observe the rule, you can stop any phase problems before they start. Simply put, the 3 to 1 principle states that in order to maintain phase integrity between microphones, for every unit of distance between the mic and its source, the distance between any other mics should be at least three times that distance. For instance, if a pair of microphones is placed over the soundboard of a piano at a distance of one foot, the separation between the two mics should be at least 3 feet. If the distance from the source is 2 feet, the distance between mics should be at least 6 feet.

The principle is not a hard and fast rule, but it certainly is a good guideline for eliminating phase problems.

(This is a great book, btw - I highly recommend it! It also includes a good section of interviews with different professional recording engineers.)

There are other stereo recording techniques that can be used for recording of a source (say, a choir) in a stereo field. These would be methods like X/Y, M-S, or near coincident Pair. I imagine there would be a lot of information available about these that you could find with a google search. I've personally used the near-coincident pair method on a smallish choir and had great results. In that method, you put two cardoid-type mics pointing about 110 degrees apart, with the capsules about 7 inches apart horizontally. I didn't quite get the angle correct, but it still turned out.

[ajcarson1]

Alright, the 3 to 1 principle makes some sense, but say I'm micing a drumset. If the microphone is 1inch from the source, I would be okay with several mics, feet away from that microphone right?
I guess I might be missing out on something here. Sorry for the ignorance.

[darrylm]

I haven't had a huge amount of experience recording a drum set, but from what I understand, that should be fine as far as phase problems go. Phase issues with drums can develop if you have a second mic on the bottom of a drum (e.g. a snare). In those cases the bottom mic often has its phase reversed via a switch on the mic or the mixer/interface.

Hopefully someone who's had much more experience recording drums will answer for you though... :-)

[ajcarson1]

Thanks guys, this forum has proven to be a lot of help so far.
So phase problems only occur when your close micing most amps/speakers/etc?

[Barryw]

Phase problems occur when more than one microphone picks up a particular source.

The sound hits the 1st mic, then a moment later (due to the speed of sound in air) hits the 2nd mic. These signals being slightly out-of-time do not add together nicely, and you get comb filtering type effects where some frequencies are boosted and some are cut and you lose the natural sound. Because the distortion is wideband it can sometimes be a hard kind of distortion to easily pinpoint, sometimes cymbals sound kinda wooshy, and sometimes things just don't sound natural and right overall.

The 3:1 rule is a good guide. If you have a mic 1 ft from the snare, there should be no other mics within 3 ft.

[ajcarson1]

Thanks guys, that answers my question then.

So in short, I should just follow the 3-1 rule and i'll be good to go (for the most part).