Phase cancellation - dual AT 4033'a

[GZsound]

I've been recording Bluegrass bands recently and one group wants to use two AT 4033 large condenser mics stacked one over the other about a foot and a half apart. They think the top mic will get the vocals and the bottom mic will get the instruments. I think there will be phase cancellation because the mics are so close together. Anybody got any ideas?

[Ethan Winer]

GZ,

 

> They think the top mic will get the vocals and the bottom mic will get the instruments. I think there will be phase cancellation because the mics are so close together.<

 

There's a "three to one" rule that says if you have two mikes that can pick up one source, the second mike should be at least three times farther away than the first to avoid phase cancellation. So for a single performer that is singing and playing guitar, you need to put the mikes fairly close to each source.

 

For example, if the distance between the guitar mike and the players mouth is a foot and a half, then he or she should sing within six inches of the vocal mike. And vice versa, with the guitar mike staying within six inches of the guitar.

 

--Ethan

[miroslav]

There's a "three to one" rule that says if you have two mikes that can pick up one source, the second mike should be at least three times farther away than the first to avoid phase cancellation. So for a single performer that is singing and playing guitar, you need to put the mikes fairly close to each source.

 

For example, if the distance between the guitar mike and the players mouth is a foot and a half, then he or she should sing within six inches of the vocal mike. And vice versa, with the guitar mike staying within six inches of the guitar.

 

--Ethan

 

Ethan, you have two microphones and two sources. What about two microphones and one source?

 

OK, so why use two microphones on a single source?

Well, what if you wanted to generate some space or create natural delay?

Or, maybe even though it is one source, set up the two mikes for "stereo" recording.

 

Just some ideas.

 

So how would you apply mathematics to some of these other situations in order to best avoid phase problems and comb filtering?

 

(This thread is also running on GM's forum. Maybe we can stay in one place to keep all the posts relevant? GM's thread has gone a little further than here, so let's continue there? OK? I'll copy my post over to GM's forum.)

[Ethan Winer]

Miroslav,

 

> What about two microphones and one source? ... how would you ... avoid phase problems and comb filtering?<

 

The solution is to keep the microphones either very close together, or fairly far apart. When I record stereo using two mikes, I generally use the "X/Y" method. Two cardioid mikes are placed with the capsules as close as possible (an inch or less apart), but one is pointing to the left of the sound source and the other is pointing to the right. The angle is about 90-100 degrees:

 

*

/\

 

I hope my ASCII art figure shows up okay. http://www.musicplayer.com/ubb/smile.gif

 

This way all sound is guaranteed to arrive at both mikes at the same time, avoiding any phase problems. Even with a wide source like a group of singers in a semi-circle, the sound arrives at both mikes together. Since each mike picks up a different part of the sound source, you get a nice stereo image.

 

I'll also point out that the only reason we care about phase is when a stereo recording might be reduced to mono for, say, radio airplay. If you are making a CD to send to mom and your friends, mono compatibility is not really an issue. In that case, all that really matters is if it sounds good.

 

--Ethan