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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
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almarg
9,670 posts
 
How would I know if my system had a DC offset?
Hi Bryon,

You would need a good multimeter, preferably a digital one that can measure dc voltage with a resolution adequate to read say a few millivolts (mv), or ten's of mv at most.

You would power up the entire system, let it warm up for a few minutes, and set the multimeter to measure dc voltage. With no music playing you would then place the two leads of the multimeter between the red and black speaker terminals of one channel, or between the corresponding terminals on the amplifier. Repeat for the other channel.

I don't particularly have a feel for what the upper limit of acceptability would be for the reading. Perhaps someone else reading this will. But I would feel fairly comfortable with a reading of say 60db below the voltage that, were it ac, would result in the 88db spl that your speakers are rated to produce, at 1 meter, in response to an ac input of 2.83 volts. 60db below 2.83 volts is 2.83 millivolts.

Hope that helps,
-- Al
bryoncunningham OP
890 posts
 
Al - I believe I have a digital multimeter around here somewhere. I will look for it and report back my findings. Thanks!
bob_bundus
1,834 posts
 
expect a typical offset under 30mv = 0.030 volts DC

This is obviously a longshot. Two completely different setups with the same exact 'issue' is highly unlikely.
bryoncunningham OP
890 posts
 
Al/Bob - I could not locate my meter, so I asked my friend with the same speakers (and same observations) to test his speakers. His result: 2mV. So I guess that's not the solution to the mystery.
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almarg
9,670 posts
 
Hi Bryon,

OK, so much for that theory. Well, I'm left at a total loss in trying to suggest an explanation.

I can't envision how your observations could be related to interaction with room effects, as has been suggested. The effects of inverting the signal polarity into the speakers would be undone by either the polarity inversion or the lack of polarity inversion of some of the recordings you listened to, relative to the effects on other recordings that presumably have the opposite polarities. And add to that the fact that it is meaningless to speak of absolute phase preservation or polarity inversion on many recordings, because they comprise a combination of various sounds and instruments that may be mixed together with random phasing. All of which makes the consistency of your observations across a variety of recordings befuddling.

Best regards,
-- Al
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