Film-Tech Forum ARCHIVE: Polarity Checker Question

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Posted by John T. Moon II (Member # 1694) on 07-22-2003, 07:30 PM:

I was sent a Phase Checker called the "SMART-EZ POLARITY CHECKER" to check the wiring and installation of my speakers in the 14-Plex here in Las Vegas. So I complete the theater polarity checks and find the usual surround speaker not working here and there and a few speakers Out Of Phase .
My real question is the interesting thing I found in ALL theaters EXCEPT the 2 THX theaters. The Left, Center and Right Stage Horn on ALL theaters indicate the speaker is Out Of Phase . Could this be correct on 12 theaters? Or is this a monumental wiring screw-up? I can not seem to get an answer. We use a JBL 4675-8LF Stage Speakers.
I think all speakers should be In Phase like the THX theaters are indicating. Why are these different? Same speaker. And the reading was taken correctly. Only the Horn is reading different. [Confused]

Posted by Gordon McLeod (Member # 33) on 07-22-2003, 07:51 PM:

Is there a possiblity teh crossover is set incorectly and a phase reversal is occuring there

Posted by Darryl Spicer (Member # 711) on 07-22-2003, 08:19 PM:

if the speaker system is not bi-amped and the crossovers are speaker mounted reverse the possative and negative speaker wires on the horn and see if that clears things up.

Posted by Tim Reed (Member # 137) on 07-22-2003, 10:10 PM:

The diaphrams may have been wired incorrectly inside the housing. Wouldn't be the first time.

Posted by Dave Macaulay (Member # 813) on 07-22-2003, 10:55 PM:

If the sound is reasonably OK and you're not planning on doing a B-chain alignment I would suggest not changing anything. During the original EQ some compensation may have been done and correcting the wiring might now make things sound bad.
Basically the audible effect is around the crossover frequency if all 3 screen channels are the same, there won't be the strangeness that results when the speakers are hooked up with no thought of polarity (it happens).
It's also possible that the tuner reversed all the horns on putrpose. The horn and bass bin aren't always connected with the same pressure polarity (I think that your JBL should be though, if the horn is mounted properly). If I see a nasty spectrum bump in the screen channels at the crossover frequency, I try reversing the horns - although all drivers have been checked with the clicker and are supposedly correct. Sometimes that's the way it sounds (and looks on the RTA) best.

Posted by John T. Moon II (Member # 1694) on 07-23-2003, 01:03 AM:

The crossovers are in a DCM-1 and I plan to do a B-Chain as soon as the issue is resolved. I just find it hard to belive that JBL could mis-wire 36 speakers.

Posted by Darryl Spicer (Member # 711) on 07-23-2003, 01:15 AM:

I do recall something with JBL speakesr systems back in the mid 90's when we were installing speakers in a new theater here locally. We had to wire the polarities in revers because they had them labled backwards.