Before a mix session or after setting up new monitors: make sure left is left, right is right, both speakers push in the same direction, and the low end is really there.
Start with the volume low. Pure tones at high level can damage hearing and small speakers, especially high frequencies you can barely hear.
1. Channels: press Left, then Right. If they come from the wrong sides, the cables or the output assignment in your interface are swapped. 2. Phase: play In phase, then Out of phase. The out-of-phase signal is meant to sound vague and thin; if In phase sounds just as thin, one speaker's + and − wires are reversed. 3. Range: play the bass sweep and note where the low end appears; play the treble sweep to check tweeters and headphones.
Speakers wired out of phase cancel each other's bass and smear the centre image; you end up adding low end and widening sounds that are already fine, and the mix falls apart on every other system. Swapped channels put every pan move on the wrong side. Both problems are invisible on meters and obvious with these tests.
Why does 'out of phase' sound strange?
The right speaker plays an inverted copy of the left. Low frequencies cancel and the sound
seems to come from nowhere in particular. That is the effect you are listening for — it
tells you what a wiring mistake would sound like.
My laptop plays almost no bass in the sweep. Is it broken?
Probably not: small laptop and phone speakers rarely reproduce much below 100–150 Hz. Try
headphones to hear the full sweep.