How do I "reverse phase" on mics?

[Conrad Josepi]

Okay,

Often when people talk about recording a snare drum with a mic on top and on the bottom, they mention "reversing the phase" on one of the two mics.

Why do you want to do this?

Also, how do you achieve this? Can it be done using Digital Audio software such as Cakewalk or Soundforge?

I appreciate any responses!

Conrad Josepi

[Emeric]

Hey Conrad,


When using two mic's in close proximity to one another like the snare drum example, what happens is something called "phase cancelation" - The wave produced by the snare drum is entering both mic's at the same time and at the same volume or amplitude. Unfortunately, one wave signal is positive the other is negative. (sorta brings us back to the unbalanced vs balanced stuff ). So 1 plus -1 is 0 I suppose, but only at certain periods throughout the recording. Some mixers have a button that reverses the phase of the signal as it enters the mixer. This may reduce the cancelation effect. On a snare drum this is fine, it is a short duration signal, and the resulting waveform is not quite as complicated (or long) as say, a piano.

You want to avoid this phase cancelation because you will end up with a thin, phazy kind of sound when using two mic's in close proximity. There is something called the 3:1 rule, which states that if your micing instrument A at a distance of one foot, no other mic should be within 3 feet of A. This is a good rule to follow as it will reduce the chances of this inherent electrical phenomenom.

In most editing type of software there is usually a function for reversing the phase of the waveform (sorry, I don't have soundforge or wavelab installed at the moment, I don't know the precise instructions but it is in there).

Great question, hope I answered part of it.

Emeric

[This message has been edited by Emeric (edited 07-01-1999).]

[aussie]

Hi there,

When you double mic a snare drum (one at the top and one at the bottom) it is likely you'll get phase cancellation when you mix the two signals because the top wave will be close to 180 degrees out of phase with the bottom wave (e.g for a sine wave the top wave would have a peak when the bottom wave has a valley). Of course it's not quite this simple, it depends on things like how far the mics are from the snare, if they are at similar distaces, even whether you're using the same mic on both sides.

Most mixers have a phase reverse button in each channel. My advice is to listen to the sound you get out of

a) The top mic on it's own
b) The bottom mic on it's own
c) Both mics mixed non phase reversed
d) Both mics mixed with one phase reversed

If you don't have a phase reversal button, you may try altering the distance of one of the mics so it is a different distance from the drum than the other one. But still compare the mixed result with just one mic.

When it's all said and done, if you like the sound you get who cares whether there are phase issues.

Cheers
Aussie

[Conrad Josepi]

Emeric,

Thanx for the explanation. That all makes sense to me.

Now I'm thinking that I can probably just invert the wav file, and that will achieve the phase reversal that I asked about.

i probably wont mic my snare with two mics, since that's not really the sound I need. It's good to know, because I'll be recording a lot of drummers in the near future.

It seems to me that the only reason you would want two mics is for very subtle sounds produced with extremely dynamic snare work. ie jazz brushes, hot rods....

Thanx again,

Conrad Josepi

[stevedruck]

Don't give up on the 2 mics for the snare idea! I've heard examples of "with 2 vs with 1", and it's well worth it, for any style of drumming!
It's real simple, to reverse the phase on a mic, you simply reverse the #s 2 and 3 on one end of the 3-wire XLR mic cord. To do this, open up the end of the cord that goes to the mixer(just because it's easier to expose the wires there), and switch the red & black wires. Simple. With clear markings to identify it as your custom reversed-phase cord, you're good to go!
OR, if you really want to be slick, you could open up the mic and switch the wires in it instead of a cord. This way any cord can be used without having to get out the soldering gun again. Make sure to mark the mic!
Steve

[punkin]

Yup...or build a simple in-line adapter strictly for this purpose...a short cable will do as well.

[SouthSIDE Glen]

Well, if he hasn't figurect that out in the past 9 years since he asked it, it's either because he's dead or he's moved on to another hobby.

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G.

[mixsit]

Ok, I bit too.
Good catch.