Polarity test
A speaker wired in reverse works perfectly by every other measure: right side, right volume, no distortion. It just cancels the other one. If your system sounds thin and oddly wide with no bass in the middle, this is the test that finds it in fifteen seconds.
One of these two has the right speaker wired backwards. A sounds solid and centred; B sounds wide, hollow and thin in the bass.
Which one sounded fuller and more centred?
How to listen
Use speakers, not headphones, if the speakers are what you are testing — the cancellation happens in the air between them, and it is strongest when you sit midway. Play A, then B, then A again. The difference is not subtle once you know what to listen for.
A is the correct case: the same noise in both channels, in phase. It should sound like one solid block of sound in the middle, with weight at the bottom.
B inverts the right channel. It should sound wider but hollow, with the middle scooped out and much less bass. If the two sound identical, you are probably listening on a single speaker or with one channel silent — the channel test earlier in this run covers that.
What to do about it
On a passive speaker system: swap the two wires at one speaker, either at the speaker terminals or at the amplifier for that channel. One speaker only.
On powered monitors or a soundbar: the wiring is internal, so check any extension or adapter cable in the chain first. A single bad adapter is a more likely explanation than a manufacturing fault.
On headphones: if a plug or cable has been replaced, that is where to look. Otherwise it is rare enough to suspect the source instead.
Why it matters more than it sounds like it should
Polarity is not a volume problem, so nothing you measure catches it. Every level meter reads normal, every channel test passes, and the system still sounds wrong in a way people usually blame on the room or the speakers themselves. It is one of the few audio faults that is both common and completely free to fix.
Common questions
What does out of phase actually mean?
That one speaker cone moves outwards at the moment the other moves inwards. When both play the same sound, one is pushing air while the other pulls, and in the space between them the two largely cancel. Low frequencies suffer most because their wavelengths are long enough to cancel across the whole room.
How does it happen?
Almost always a wiring mistake: the two wires to one speaker connected the wrong way round, so its plus and minus are swapped relative to the other. It also happens inside cheap extension cables and adapters, and occasionally in a repair where a driver was rewired.
Can it happen with headphones?
Yes, though it is rarer since the wiring is inside the cable. It is more common with a repaired cable or a replacement plug. In headphones the effect is different but just as recognisable: the sound seems to come from outside your head rather than the middle, and the bass thins out.
Why does B sound wider if it is the faulty one?
Because cancelling the sound that is common to both channels removes the centre, and what is left is what differs between them. Some people initially find it impressive, which is why the test asks which sounded fuller and more centred rather than which sounded better. Anything mixed to the centre — most vocals, most bass — is what disappears.
How do I fix it?
Swap the two wires on one speaker only. On a passive system that is the two terminals at the back of one box, or at the amplifier end for that one channel. If both are swapped, everything is back in phase relative to each other and it sounds correct again, so if you swap one and nothing changes, check whether you have swapped both.