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Polarity mystery: Can you help me solve it?

THE BACKGROUND: My speakers are Focal 1007be. They have a Linkwitz-Riley crossover with a 36 dB per octave slope. Because of that, the two drivers are wired with opposite polarity: the woofers are positive, the tweeters are negative.

WHAT I DID: At the advice of a friend with the same speakers, I inverted the polarity of the drivers, by simply reversing the red and black speaker wire leads to the terminals of both speakers, so that the speakers are still in phase with each other, but now the woofers are negative polarity and the tweeters are positive polarity.

WHAT HAPPENED: To my surprise, the sound improved! Specifically, image focus improved. The improvement can't be attributed to the preservation of the absolute phase of the recording, since the improvement was the same for many different recordings (some of which, presumably, preserve absolute phase, while others do not). And the improvement can't be attributed to the speakers being wired incorrectly at the factory, since the friend who suggested that I try this experiment owns the same speakers and experienced the exact same result. So I don't know what to attribute the improvement to.

Can anyone help with this mystery?
bryoncunningham
jea48
4,453 posts
 
One sure way to check the drivers, crossover, and internal wiring of the speaker is to place a
1 - 1/2 volt D cell battery across the external speaker box terminals. Disconnect the speaker cables from the speaker box terminals. Connect two test lead wires to the speaker box terminals long enough so you can view the drivers from the front. Tag the test lead that is connected to the positive speaker terminal. Use your fingers to hold the neg test wire on the neg post of the battery. Momentary touch the positive test wire to the positive post of the battery. The woofer cone should push forward, not suck back.
Not sure if you will see any movement in the other speaker driver.
onhwy61's avatar
onhwy61
5,229 posts
 
The fact that you think it sounds better doesn't mean it is performing better. It could be a problem with how the speaker mates with your room. Here are the Stereophile measurements.
almarg's avatar
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almarg
9,670 posts
 
Hi Bryon,

Gee, that's a tough one, assuming as you indicate that the effect is repeatable across a wide range of recordings, presumably ruling out the polarity of the recording as being a factor.

The one possibility that occurs to me is that the output of your power amp has some amount of dc offset present, due either to itself or to the preamp or processor that is feeding it, or the source component for that matter (if the entire signal path is dc coupled). Does your friend, who observed the same phenomenon, have similar electronics?

A dc offset would cause your woofers to have a rest position that is either slightly forward or slightly backward relative to their normal zero-signal rest position, the direction depending on the polarity of the offset (and therefore depending on the polarity with which the speakers are connected to the amplifier).

That would tend to bias the contribution of the woofers such that acoustic compressions or rarefactions in the speakers' outputs are slightly emphasized or deemphasized relative to one another, depending on the polarity of your connections but independent of the polarity of the recording. I think :)

That's just a wild guess, of course, but it's the only theory I can think of that seems to fit all the facts.

Best regards,
-- Al
eldartford
4,262 posts
 
Jea48...Some driver manufacturers (JBL for example) use the oposite polarity definition (cone pulls in).
jea48
4,453 posts
 
Some driver manufacturers (JBL for example) use the oposite polarity definition (cone pulls in).
Eldartford,

True..... My point was to verify the polarity was indeed reversed on the driver/s and with the test make sure both speaker box units are in phase with one another. http://recforums.prosoundweb.com/index.php/mv/msg/2661/29039/0#msg_29039
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