Polarity to the People
By Dave Rat
Not so mysterious.
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It’s pretty common knowledge that if you get the wires mixed up when you hook up two speakers that something “not good” happens. Speaker phase (actually, polarity) seems at first glance a pretty simple concept. If both speakers are moving outward at the same time the sound adds together and if one is moving out while the other moves in, the sound cancels out, especially the low frequencies. Hearing this effect is quite easily demonstrated, listen to your home stereo speakers while standing midway between them and then listen again after reversing the speaker leads on one side. You should notice a very apparent decrease in the lows when they are wired the wrong way. |
To picture why this happens, imagine a very simple pulse or “positive pressure wave” being reproduced by both speakers simultaneously. The two positive pressure waves add together and that means addition or “louder.”
If you reverse the leads to one of the speakers, then one speaker moves outward (toward you, positive pressure) and the other moves inward (away from you, negative pressure). The pressure wave from one speaker is being “sucked out” by the other speaker, also know as cancellation or “not as loud.” Now if we reverse the other speaker lead as well, both speakers will move away
from you, two negative pressure waves add together and that is once again addition or “louder.” It does not really matter much in most situations whether they move toward or away from you as long as both sound sources are doing the same thing.
Simple, right?
If it was just speakers, all this would be easy, just wire all your speakers properly and it’s all good. Things are not that simple in the real world of live audio. You have mics all over the place, amps and drums that make lots of noise and monitors pointing in various directions and there are some really interesting things that occur in the real world, that when dealt with properly make noticeable improvements.
Everyone does it (just about).
Nearly every engineer that uses a snare bottom mic naturally reverses its polarity. Seems simple enough and you can hear the added lows and punch when you push the button next to the word “phase.” Fair enough, drummer hits snare, snare head moves down and away from the top mic and down towards the bottom mic. The top mic sees a negative pressure wave and the bottom mic
sees a positive pressure wave. It’s not too much different from the speaker example above, negative pressure wave plus positive pressure wave equals cancellation, for most applications, cancellation equals bad. The simple solution is to reverse the polarity of either the top or bottom mic to create addition instead of cancellation.
The way things are supposed to be.
First let me state that a positive pressure on a pin 2 hot mic should produce a positive voltage on pin 2 of its XLR. Assuming you have a properly wired pin 2 hot system, that positive voltage will eventually manifest itself as a positive (outward) motion on the speakers in your system. (Yes, I know your JBL stuff will go inward and there are other exceptions that you can take into account if you wish).
Understanding the first step.
If you mic the kick from inside the drum (non-beater side of the head) then a kick beat will produce a positive pressure on the mic and therefore in most systems a positive outward motion of the drum monitor speakers. Imagine sitting there, hit the kick, the head is moving away from you, the drum monitor speaker is moving toward you (negative pressure wave from the kick and positive pressure wave from the drum fill). What do we have?
Cancellation, just like in the home stereo with one speaker reversed. Not only do you have cancellation but you also have another problem as well. The drumhead moves away from the drum monitor the drum monitor speakers move toward the drumhead and pushes it a bit farther, and then the reverse
happens. The drumhead rebounds toward you and the speaker moves away. The head and speaker are augmenting each others motion creating “resonance” or “feedback.” This is not a good thing. Reverse the kick polarity and now the kick drum and drum fill outputs not only add together but they become less resonant.
Makes sense, but does it really matter?
Try it, keeping in mind that EQ and time delay induced by digital processors can greatly alter the audibility. So to hear the difference, bypass all drum fill and kick channel EQ, and then bring up the kick in the drum fill until it is just on the verge of feedback. Now by pressing the phase rev on the kick channel you should be able to determine which way is noticeably more stable.
OK, did that, now what?
The rest is easy. You have determined and put the kick in proper polarity to the drum fill. Now go around the rest of the kit and for every mic that mics the underside of a drum, set the phase the same as you did the kick, and for every mic that mics the top or “stick side” of a drum, set the polarity the opposite of how the kick is set. Now everything is all lined up and this system just takes a few minutes and will give you a clean starting point. Keep in mind that this is a starting point, upward facing monitors near a floor tom tend to be more stable with the opposite polarity than this system dictates.
I follow this same pattern on my house console as well. My kick and snare bottom are reversed polarity (I top mic my drums).
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