sirslurpee said:
Okay... I am n00b so I need to know about phase and why mics can be out of phase. I've done some reading, but all I can get out of it is that say for example you're micing a drumset, and you have two mics on a drum. One of them on the top of say a tom, and the other on the bottom of the same tom. Now, because sound is essentially a sine wave, when the drummer hits the tom, the top mic picks up the top of the wave, while the bottom picks up the bottom of the wave?
Cool, man, I'm feakin' wired on coffee after a gig, I'll spew out a bunch of jabber.
That's basically right. Much of the sound will cancel out.
The sine wave picture is really a graph. Sound "waves" are really air pressure variations. The sine wave is a picture of the pressure rising and falling over time.
Mic diaphragms react to these pressure changes. Micing a snare drum from top and bottom means that when one mic has positive pressure hitting it, the other has negative pressure hitting it at the same time. This is because as the head moves towards one mic, it moves away from the other.
Note that these two signals are not really considered to be out of phase, since phase implies a time delay. Rather they have opposite polarity. For every positive value there is an equal negative value. Phase shift is the result of delay.
The common solution to the top/bottom mic issue is to reverse the polarity of one of the mics, so the signals will be of the same polarity when they hit the mixer or recorder.
WARNING-POTENTIALLY CONFUSING BUT FORTUNATELY NON-RELEVANT RAMBLE, READ AT YOUR OWN RISK
Phase shift means delay. To talk about X degrees of phase shift, you need a reference point. 180 degrees phase shift is meaningless. 180 degrees of phase shift relative to a 100hz sinewave gives you a reference point. From this info you can figure out how much delay time and distance this translates into, and what effects that given amount of phase shift is likely to have.
Opposite or reverse polarity means that all values have their signs reversed. In a sinewave, this means instead of going positive first, it goes negative first. In audio, there is relative polarity, as in the top/bottom snare micing, where the signals are equal but oppposite, and the polarity standard, which is that "positive pressure on the mic diaphram shall produce a positive voltage on pin 2, and produce forward motion of the speaker cone".
For a single sine wave, 180 degrees phase shift looks exactly like polarity reversal on paper. Each cycle is identical and symmetrical, you can't really tell if the phase has been shifted or the polarity reversed. Two sine waves played over speakers will cancel with either 180 degrees phase shift or polarity reversal.
For complex signals, like multiple sinewaves or music, 180 degrees phase shift and polarity reversal have different results.
Polarity reversal will have the same effect. Total cancellation. Phase shift will not. Phase shift is delaying one signal vs. another. No amount of delay (phase shift) will make a non-symmetrical complex waveform like a vocal track cancel completely with itself. They can't be 180 degrees out of phase for all frequencies. And since the wave is non-symmetric, they wouldn't cancel completely in any case.
I've heard people describe polarity reversal as "180 degrees phase shift at all frequencies". But what is actually going on is different than that, and the results are different, so I don't feel it's an accurate description.
Blah Blah Blah.
sirslurpee said:
It's something with a time/distance from sound source issue? I don't quite get it 100% yet.!
Yes again, pretty much.
Phase shift is delay, introduced physically or electronically. Phase shift causes wave cancellations and reinforcements. The amount of phase shift determines which frequencies will be affected most. The volume of the phase shifted signal relative to the original determines how much audible cancellation takes place.
Adding out of phase signals together causes peaks and dips across the whole frequency spectrum, though certain ones will be more affected. Looked at on a spectrum analyzer, the combined response looks like the teeth of a comb, hence the term "comb filtering".
One physical example of introducing phase shift is recording a source with two mics at different distances, say recording a guitar amp from 3 feet with one mic and 1 foot with another. The far mic gets the signal after the near mic.
The far mic is two feet from the near mic. Two feet is 1/2 the wavelength of 280hz. When you listen to both tracks at the same time at the same volume, a bunch of 280hz will cancel out. The mics are picking up 280hz at points of opposite polarity (180 degrees phase shift). Two feet is a full wavelength of 560hz. When you listen to both tracks, 560hz will add together. There will be effects across the whole spectrum but this should give you the basic idea of what's going on.
Sound travels at about 1000 feet per second, or 1 foot per millisecond. The far mic is getting sound about 2ms after the near mic. You could get the same result listening to two identical tracks, and delaying one by 2ms or so with a delay unit.
Since you are listening to complex sounds, not single sine waves, the combined response continually shifts and changes amplitudes, which gives you the characteristic swishing and swirling sound of comb filtering. Basically pretty annoying.
You can almost always find a combination of distances that gets you a good sound, while keeping audible comb filtering to a minimum, say by spacing the mics so the most obvious effects are out of the sonic range of the instruments.
In today's digital world, you could fix phase problems from mics at different distances by simply nudging the delayed track until it matches up. No delay, no combing, unless it comes from reflections picked up by the mics.
You could get the same problem with only one mic, say recording a person talking in a very reverberant small space, or on a hard podium. The reflections will hit the mic delayed with respect to the voice, and if they are loud enough, can cause audible comb filtering.
Mostly people just move things around till they sound ok. If you know what's going on, it can help you either reduce the problems, or take advantage of the phenomenon.
Anyway, gotta go trick-or-treating.
