AV Police Squad

Back to Basics: Polarity

As you may know, I am a huge fan of Good Mythical Morning. One of their new segments on the Wheel of Mythicality is letting their listeners know what NOT to google.

Today’s example was Dog Boogers. Do NOT google “dog boogers.”

It seems like a simple enough PSA. However, it’s basically Fireball whiskey whispers, temptation in your ear. You know you have to google “dog boogers” because…well…just in case. FOMO, baby!

I would also urge you NOT to google “YouTube speaker polarity test.” Don’t do it. I urge you…just…just don’t do it. Trust me on this.

http://lmgtfy.com/?q=youtube+speaker+polarity+test

(or you can just go here: https://www.youtube.com/watch?v=75tVu-qgZ_Q)

For the sake of argument, let’s pretend that you went there. Congratulations! You have stumbled across one of the more annoying videos on YouTube. Not only is it annoying, but like many videos on YouTube, it’s inaccurate. The person saying “In Phase. Out of Phase” really means “In Polarity. Out of Polarity.” Phase has to do with time differentials, as in a wave form arriving at your ear a few milliseconds after another waveform could have phase cancellation. Polarity has to do with being completely reversed, as in the “+” and “-“ on a loudspeaker output is swapped between the left and the right channels. One has the red conductor on “+,” and the other has the black conductor on “+.” So, while one loudspeaker is pushing air outwards, the other loudspeaker in pulling air inwards.

Ignoring the inaccuracies, it is still a great video to find. It clearly demonstrates what it sounds like when loudspeakers are wired out of polarity. If you close your eyes and listen to it, when the person says “In Phase,” the sound psycho-acoustically comes from the middle of your screen. It is properly localized. When the person says “Out of Phase,” it gets weird. It feels weird. I usually describe it as the sound psycho-acoustically coming from around the screen and almost from behind me. It makes me feel icky. However you describe it, the localization is not correct.

Loudspeakers wired out of polarity sound icky with complex waveforms like speech. If you were seated perfectly equidistant, or centered, in between two loudspeakers and they played a simple waveform (like a sine wave) while out of polarity, you might not hear anything at all. If the loudspeakers are out of polarity, one pushes out while the other draws in…and with a simple wave form, they can effectively cancel out each other’s sound pressure wave. As you move away from that perfect spot in between them, you’ll pick up some audio because you might be slightly closer to one loudspeaker. Its sound arrives slightly earlier, and NOW we are talking phase cancellation. It will still sound much different than if they were wired in polarity.

This is the reason we test for polarity. It is important in program loudspeakers where localization is crucial. It is still very important in ceiling loudspeakers. The loudspeakers should be working in concert with one another, not fighting one another because someone swapped the red and black conductors at the amplifier. As AV professionals, we should avoid creating “cheap seats” (as in, seats under ceiling loudspeakers wired out of polarity) where ever we can.

You can certainly test this with a polarity analyzer and test signal. The results can get a little flaky as you move further away from the loudspeaker, so testing ceiling loudspeakers is a bit tricky. I find the easiest way to test for polarity is to play a known signal, close my eyes, and just make sure the sound is “coming from” the middle of the loudspeakers, and not sneaking up behind me. We don’t want people feeling icky.

And speaking of icky, our friends at Good Mythical Morning also recommend NOT googling ballerina feet. Just…just don’t do it.

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