Saturday, July 2, 2011

Phase Vs. Polarity

Phase has to deal with timing and waveforms developing over time.
Polarity on the other hand has nothing to do with timing. It is purely a matter of direction of flow of electrical current.
You've probably heard the term "Out of Phase" or "180ยบ Out of Phase" 99% of the time people use this term when they mean "Reversed Polarity" Almost every mic preamp or a console out there has a "Phase flip" switch, which is misleading. All that switch does, is reverse the polarity of the signal. Since it clearly has nothing to do with the timing of the signal it should not be associated with "Phase". 

Phase problems or issues occur every time we have two or more instances of one source of sound. For example…
A guitar miked with more than one microphone, or a direct signal from a bass and a split of that same bass going through a bass amp which we've miked, or a close-miked drum-set and a room mic picking up that same drim-set. In all of the above examples we have one source of sound and more than one physical location where we capture that sound. Considering the speed of sound in air (1125 ft/s) it makes sense that one of our instances of that source would run slightly behind the other - (timing differences equals phase differences). The effect of the wire and cable lengths in the recording studio are so insignificant to the delay of the signal that we can just ignore them. (it takes about 0.00089 seconds or almost 1ms (millisecond) for sound to travel 1 ft in air at sea level. In order to get your signal delayed by 1ms just by adding length to your cables it would take you about 186 MILES of cable)
So you have your two mics on your kick drum for example - (one inside and one outside of the drum) about 1 ft away from each other. The outside mic will pick up the sound of the beater hitting the head about 1ms later than the mic a foot closer to the beater. So considering the speed of sound again, frequencies around 1125 Hz may get a boost while frequencies around 562.5 Hz may get lost. If we reverse the polarity of one of the two mics in theory the frequencies around 562.5 should get a boost and the range around 1125 Hz should disappear. Yes and NO. If we work with pure sine waves that would be the case - Before we reverse the polarity of the mic 1025Hz sine wave would fall exactly 360 degrees shifted over the distance of 1 ft and double itself in amplitude, while 562.5Hz sine wave would shift exactly 180 degrees over 1ft, so it would cancele itself completely. But since we are dealing with a complex waves and not just a pure sine wave, plus the fact that the two mics are probably different models and they pick up the sound of the drum at different locations of the drum itself, the two signals would have different frequency content to begin with. So after we combine them together we get a third frequency content of the sound of the drum. By flipping the polarity of one of the mics we drastically change the combined frequency content but that doesn't mean we completely lost our 1kHz range. Depending on the drum the mics the positions and also the rest of the drum mics like over heads and rooms you may end up reversing the polarity of one or two mics so you can get the desired sound... There is no right or wrong way... Just experiment and trust your ears. Rule of thumb is If you have two mics that are close to each other - 6 inches or less and you would like to mic some kind of a low frequency source - bass amp kick drum, guitar amp etc. make sure the polarity on of both mics is the same. After summing or mixing two microphones that have timing/ phase differences in the sound they are picking up, we create what's called a "Comb Filter". By reversing the polarity of one of the microphones all we do is shift the frequencies that get boosted and the ones that get cancelled out. In either case we end up with a severe EQ curve. At this point It is just a matter of preference and overall desired sound.

Also see article by "Musician's Friend" on the same topic. 

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