November 11, 2009, by Dave Rat
It’s pretty common knowledge that if you get the wires mixed up when hooking up two loudspeakers that something “not good” happens.
Loudspeaker phase (actually, polarity) seems at first glance a pretty simple concept. If both loudspeakers 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 loudspeakers while standing midway between them and then listen again after reversing the leads on one side. You should notice a very apparent decrease in the lows when they’re wired the wrong way.
To picture why this happens, imagine a very simple pulse or “positive pressure wave”
being reproduced by both loudspeakers simultaneously. The two positive pressure waves add together and that means addition or “louder.”
Reverse the leads to one of the loudspeakers and one loudspeaker moves outward (toward you, positive pressure) and the other moves inward (away from you, negative pressure).
The pressure wave from one loudspeaker is being “sucked out” by the other loudspeaker, also know as cancellation or “not as loud.”
If we reverse the other loudspeaker lead as well, so that both loudspeakers have reversed leads, both will now move away from you. The two negative pressure waves add together, and that is once again addition or “louder.”
In most situations, it doesn’t much matter whether both loudspeakers move toward or away from you, as long as both are doing the same thing at the same time.
If it was just loudspeakers, all this would be easy .But things are rarely ever simple in the real world of live audio. There are microphones, amplifiers, instruments, drums and plenty more that also make lots of noise. Add in monitors pointing in various directions, and some interesting things happen.
Everyone Does It (Just About)
Nearly every engineer that uses a snare bottom microphone naturally reverses its polarity. Seems simple enough, and you can hear the added lows and punch when pushing the button next to the word “phase” on the console when both the top and bottom snare mic channels are at similar levels.
Fair enough - drummer hits snare, it’s head moves down/away from the top mic while also moving down/toward 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 than our loudspeaker example - negative pressure wave plus positive pressure wave equals cancellation, and 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 output connector.
Assuming you have a properly wired pin 2 hot sound system, that positive voltage on pin 2 will eventually manifest itself as a positive (outward) motion on the loudspeakers in your system. (Yes, I know some JBL stuff will go inward, and there are other exceptions to take into account.)
But why does this matter beyond snare bottom?
Because if you mic the kick from inside the drum (non beater side of the head), then the kick drum beat will produce a positive pressure on it’s mic, and therefore, in most pin 2 hot systems, a positive outward motion of the drum monitor loudspeakers.
Imagine sitting at the drum kit: hit the kick, the head is moving away from you, the drum monitor loudspeaker 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 loudspeaker reversed.
And not only do you have cancellation, but there is another problem as well; the drum head moves away from the drum monitor, the drum monitor loudspeaker moves toward the drum head and pushes the head in a bit farther, and then the reverse happens - the drum head rebounds toward you, while the monitor loudspeaker moves away.
The drum head and monitor loudspeaker are augmenting each others’ motion, creating “resonance” and increasing feedback susceptibility. This is generally not a good thing.
But all you need to do is reverse the kick drum mic polarity, and now the kick drum and drum fill outputs not only add together, but the combination becomes overall less resonant.
Makes Sense… But Does It Really Matter?
If you try this approach, keep in mind that time delay induced by digital processors and drum fill placement can greatly affect audibility.
So to really hear the difference and set the proper polarity, bypass all drum fill and kick channel EQ, and then bring up the kick in the drum fill to the point where it is just on the verge of feedback.
Now, by pressing the phase reverse on the kick channel, you should be able to determine which way is noticeably more stable.
OK, Did That. Now What?
If you did this right and have found a clearly better sounding kick polarity, use it as a starting point to set polarity for the rest of the drum kit.
Working around the rest of the kit, for every mic for the under side of a drum, set the phase the same as you did for the kick. And for every mic for the top or “stick side” of a drum, set the polarity the opposite of how the kick is set.
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 approach dictates.
Polarity is another tool that you can use to achieve the sound you’re seeking. here is no “right” or “wrong” way but having a method for quickly finding a consistent starting point can help take some of the voodoo out of your system.
What About The Cymbals?
Polarity issues are going to have the most noticeable effect on low frequencies, as well as when two sources are reproducing similar signal are in very close proximity to each other or mixed together.
Higher frequencies and things panned hard left and right in the mains will often sound different, but there will not be a definitive “ better” sounding choice.
How far is too far? I personally take it pretty far, and further, follow this same pattern on my house console as well. Both my kick and snare bottom are reversed polarity (I top mic my toms).
But how much of this you employ, is, of course, up to you. What we’re doing here is establishing a clean stating point for you to make decisions to change polarity on specific channels to achieve desired results.
Dave Rat is the co-founder and owner of Rat Sound, a leading sound reinforcement company based in California. Courtesy ProsoundWeb
Be seen. Be heard.
Voice: 800/426-8664 x255 / Fax: 800/399-8273