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Not So Mysterious: Using Polarity As A Tool For Optimizing Drum Sound
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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.

Simple, right?

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.

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