Subwoofer Phase

Can somone explain the phase control on a subwoofer.
How dose it work?
how do you know all your speakers are in time?
What do the numbers mean on the control knob?

I am just trying to learn about how to set this particular control. I would appreciate any help in this matter.

Thank You

The phase control on your subwoofer crossover is to help make sure that your subs and main speakers are "in phase"(obviously). That simply means that your sub & main drivers will be moving in the same direction at the same time - in and out.

Sounds obvious, but this isn't automatically true. Most times they will be moving in the opposite direction or some degree out of time. The result of "out of phase" drivers (relating to bass) is that the out of synch signals end up cancelling each other out(to some degree), or what audio techs call a "null". So, instead of getting bass enhancement from adding subwoofers, you end up with less bass. Sometimes, a lot less bass.

The calibration/markings on the phase control of a subwoofer crossover, often a "0-180" degree switch or a variable "0-360" dial" simple provide a repeatable setting. The value doesn't really mean anything more than that. What works, works, regardless of the number associated with it.

You normally can turn the phase dial while bass heavy music is playing and notice that at some point the bass will start to get louder and then past a point start to get softer. This is rudimentary way of hearing the effect of phase. But, it is no way to set it properly because what you hear next to the sub/crossover probably will not be the same at the listening position. There can be several areas with nulls and peaks within every room, some up to 30x louder of softer.

There are a myriad of ways to test and fine tune phase alignment, but a sure-fire, easy to do, method is(quoting a post on

1) reverse the red and black speaker leads going to your left and right speakers. Leave the wiring on your subwoofer alone.

2)Play a test tone at the crossover frequency (you can find tones on most test CDs) and have someone adjust the phase control while you sit in the listening position.

3)Turn the phase control until you hear the least amount of bass.

4)Return the speaker leads to their correct polarity and the sub is perfectly dialed-in.

By reversing the red and black leads on your speaker, you are putting them out of phase with the sub. When you hear the least bass, the main speakers and the sub are maximally out of phase. When you return the speaker leads to their correct position, the sub and main speakers are maximally in-phase.

It is much easier to dial-in a maximum null than a maximum peak.

Hope this helps and if I got something wrong, I hope someone with more experience will step in and correct it.
The degree of cancellation is dependent on the amount of bass your main speakers put out. If you have small drivers in your main speakers or have the bass rolled off, you may not be able to hear much if any difference in phase.

Thanks for the information I will try this set up.

Hi Rwwear,

Thanks for bringing up that very important point, mini-monitors and a subwoofer would be a perfect example.
Some subwoofers have a remote control for setting phase while at the seated position :)
Another great use for a spectrum analyser! Play the white noise signal and mess around with the phase until you get a deep sharp notch at the SW crossover frequency. Then set the phase 180 degrees from this setting.
Hey Eldartford,

I don't have a spectrum analyser, but I do have a couple of SPL meters along with white noise/test tone cd's, could I simply check for the notch that way?
Darkmoebius...Sure, but it won't be slam dunk easy as with the spectrum analyser. In general it is easier to find a minimum (notch) than a maximum.