Out of phase?

[dum6sh1t]

Hi ya'll, Newb to the forum here. Great place! I've been lurking for awhile and it has helped tremendously with most of my questions using the search function. However (here comes the newb question), I am failing to understand mics being out of phase on a drum.

Let's use a snare drum for example with 2, sm57s top and bottom for sake of discussion. Now is the "out of phase" referring to the gap in time from the initial attack of the top head to the register of the bottom head? I am assuming one would want both diaphragms of both mics to oscillate congruently. Meaning if mic A (top) picks up top head going down, you would want mic B (bottom) to pick up bottom head going down simultaneously. This is where I get confused. Does it even matter if each mic is running through a separate track on the mixer? Is it a simple matter of switching polarity on the mics(re solder an XLR), or is there a third party piece of equipment necessary to eliminate this?

Also, if the case may be that it is just a matter of switching polarity and I choose to use other condenser mics, will phantom power be an issue?

 

[gecko zzed]

Taking your example, and simplifying it a little, let's postulate a drum skin suspended in a hoop, with one mike facing the skin from above, and another mike facing the skin from below, the same distance away from the skin as the first.

You then strike the skin. As it is being depressed (i.e. going down), the compression of air under the skin is equal to the rarefaction of the air above it. The skin rebounds, and the compression and rarefaction is reversed. The mike below the skin is detecting a mirror image of the the sound the mike above detects.

If you record the signals from the two mikes, and mix them together, they will cancel each other out and you won't hear anything. This, in the real world, is not strictly true, because things are rarely equal, and there are other confounding factors. However, there will be significant decrease in volume as a result of phase cancellation (the notionally equal signals are out of phase with each other).

Reversing the phase of the signal of one of the mikes means that no longer are the signals mirror images; they match: both go up and down at the same time. They are in-phase and you get an overall increase in signal when you mix the two together.

Reversing the phase is possible on some mixing desks, and some mikes have phase-reversal switches, but is very easy to achieve in the recording program itself, and that's where I would deal with the problem.

 

[Washburn100]

Hey Gekko,
Excellent answer and great way to picture things. You got me thinking about this. We always talk about phase and waves cancelling each other.

In wave theory, 2 waves out of phase cancel each other, but 2 waves of same frequency in phase are actually additive so the amplitude becomes the sum of both (at least with water waves in a pond). So can this happen in audio, and you suddenly get high volume?

 

[AlanSwenson]

You could also put the teeniest bit of delay on one of the tracks (the one that sounds first). trial an error till they are equal.

 

[Chibi Nappa]

In wave theory, 2 waves out of phase cancel each other, but 2 waves of same frequency in phase are actually additive so the amplitude becomes the sum of both (at least with water waves in a pond). So can this happen in audio, and you suddenly get high volume?

Yes. Waves are waves, whether in water or air.

 

[Chibi Nappa]

You could also put the teeniest bit of delay on one of the tracks (the one that sounds first). trial an error till they are equal.

That can work very well when there are only two mics. It can be impossibly tricky with 6 mics on a drum kit since the delay nudge may cause 4 problems while solving 1.


Phase can hit you two ways:

First look at the case of equal distance, opposite direction:

mic> \/\/\/\ (source) \/\/\/\ <mic

In the case above, you can see the mic on the left sees a positive peak while the mic on the right sees a negative peak. This is simply because each mic thinks "forward" is a different direction while the sound source makes no such distinction. Reverse the phase and there is no problem.


A much trickier case can come from different distances:

/\/\/\/\/\/ <mic
/\/\/\/\/\ <mic

Here we can see the problem is that the closer mic picks up the sound at a low peak while the further mic picks up the sound at a high peak. If the distance lands it exactly 180 degrees out of phase as shown here then no problem. Flip phase and you're done. But if you're not 180 degrees out of phase, one mic is always going to be screwing with the other unless you shift the phase with delay.

But if you have more than 2 mics, you are also phase shifting with relation to all of them. That rarely has a happy ending.


The bottom line is you will never, ever, ever be able to use more than one microphone without phase issues. But a phase issue can be annoying, destructive, and warbly...or it can be unnoticeable or even pleasant sounding. Learn to place the 2nd microphone while listening to the first and second together. Then place the 3rd listening to all three together and so on. Learn the sound of bad phase and avoid it with careful placement.