Polarity and Phase

Sound is air being pushed and pulled. When two sources push at the same moment, their pushes add up. When one pushes while the other pulls, they cancel. Polarity and phase are two ways that can go wrong.

Polarity is simply which way the wires go. Every sub must have the amp’s plus going to its plus and minus to minus, through every bit of wiring in between. If one sub is wired backwards, its cone moves in while the others move out. In the bass, where the subs sit close together compared to the length of the sound wave, that sub cancels a big share of another one’s output.

It is an easy mistake with dual voice coils and series or parallel wiring, and it doesn’t always sound obviously wrong. It just sounds weak.

Check every sub with the battery test from the stuffing and damping page: disconnect it from the amp, touch a single 1.5 volt battery plus to plus for a moment, and make sure the cone jumps outward. Mark the wires once you know they are right.

Watch out for subs mounted the other way round, magnet out or facing the opposite way. To push air with the others, that sub has to be wired backwards on purpose. The battery test only checks a sub against its own terminals, so on a flipped sub the cone should pull in, not jump out.

Even with the wiring right, two sounds can still fight if they reach the listener, or the meter, at different times. One wave lands a little behind the other, so its push lines up with part of the other’s pull. The further out of step, the more they cancel. At the one note where the delay is exactly half a cycle, it cancels like a sub wired backwards. A reversed wire cancels at every note.

A little out of step costs almost nothing. A quarter of a cycle (90°) costs 3 dB, the same as throwing away half your amplifier power. Half a cycle (180°) wipes that note out.

• Distance. Sound needs about one millisecond to travel a foot. At 40 Hz one cycle is 25 milliseconds, so a few feet of difference barely matters in the deep bass but matters more as notes get higher.
• Subs facing different ways in the same box or the same car. The sound from each takes a different path to the listener.
• The front speakers and the subs overlapping in the frequencies where both play. That is what the phase switch or knob on an amp, and time delay on a processor, are there to line up.
• Cone and port. In a ported box the port and the cone play together near tuning, and the sound coming from each reaches the listener from a different spot. Box layout and which way the port faces change how they add up in the car.

Play a steady tone in the range you care about and flip one thing at a time: a phase switch, one sub’s wiring at the amp, the delay setting. Keep the version that reads louder. For an SPL run, check it with the meter’s microphone where it will be on the day, because the spot where the sound is measured is the spot where everything needs to add up.

Phase also explains why one vehicle can have a loud spot and a dead spot just a few inches apart. Sound bouncing around the cabin arrives from more than one direction and adds or cancels by location.