n00b phase questions

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Kronpox

New member
Admit it- You didn't know the answer to these questions once too.

I'm not new to recording, but I've never quite understood phase issues- when two mics are out of phase, and what have you. How do you tell if two mics are out of phase? I've heard that reversing the phase on one of the mics fixes the problem, but how do you do THAT?

Thanks in advance.
 
If you sum the mix to mono, sometimes different elements will disappear, or you can lose some high-end as well. The main way that people fix this is with a phase reverse switch on the mic preamp. You can also wire up a special mic cable that switches the hot and cold pins of the mic cables (if I remember correctly, you may want to double-check which pins to switch, just to be safe).

Here's a little demo on what phasing is:link

Scroll down to where it says "Two sine waves travelling in the same direction: Constructive and Destructive Interference" and that is it.

Also check out this thread: thread
 
Kron,

> How do you tell if two mics are out of phase? <

First, there's a big difference between phase and polarity. The first involves time, and the amount of phase shift can vary continuously from zero to infinity. When two microphones or other signals are referred to as being "out of phase" what's usually meant is one has reversed polarity.

> how do you do THAT? <

You can buy XLR adapters that reverse pins 2 and 3 at one end, or build such a device yourself. That reverses the polarity, but does not affect the phase. Many mixers also have a polarity reverse switch on each channel to accomplish the same thing. Often these switches are even labeled "phase" though it's really the polarity that's reversed.

--Ethan
 
the other day I recorded my band's practice with two ev 635a omnis and a decent preamp, into the pc. the sound should have been superb, given the room balance. but what I hear on playback is not so crisp and clear. Every now and then the volume fades in and out in some of the mid frequencies. Almost like bad compression. That is one example of how phasing can mess up a recording. the sounds just hit each mic at coincidentally bad timing. once its down, theres really nothing you can do, except to pan it wide and hope nobody ever listens in mono.
 
Here's an example:

Mic a tom top and bottom head with the same distance/same angle from the head

when you hit the top head, the top mic will a signal that is moving away from the mic because the head is initially moving away (Negative Pressure Signal)

the bottom mic will see a signal that is initially moving towards the mic (Positive Pressure Signal)

these signals are basically reversed in polarity... not totally due to harmonics, but the fundamental is. You can view this with the recorded signals.

Now invert the bottom head signal to sum them... otherwise the signal is very weak because the fundamental is canceled.

As tracked, these two signals are "Out-of-Phase" per se.

Now there are other cases where two mics not equi-distant from the same source causes comb filtering. This is also considered "Out-of-Phase" but is generally not a polarity issue. This can result in a ringing/swirling type sound.

kp-
 
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IronFlippy said:
How does one tell if there's any cancellation going on?
Click to expand...
One way to tell is to pan 2 channels to the center. Mute one of them, if it sounds louder or more powerful with only one channel instead of two, you have phase cancellation.
 
Farview said:
One way to tell is to pan 2 channels to the center. Mute one of them, if it sounds louder or more powerful with only one channel instead of two, you have phase cancellation.
Click to expand...
Also if you switch a stereo track to mono and the sounds cave-in, disappear, or the level drops significantly, you have phase cancellation.
 
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