Originally Posted by rcohen
We're actually talking about two different things.
When matching a sub to mains, you want to match the phase at the crossover frequency. You should tune that for the most bass, not the least. Also, you would need to use a test tone or other technique that will let you look at the level of the crossover frequency at the listening position.
When matching subs with each other, you want to match phase at all frequencies. This can be tricky with different subs, since the phase can shift in a frequency dependent way.
Basically, if sound is coming from two sources, you want the phase to match, not cancel. With a crossover, sound comes from two sources at the crossover frequency. With two subs, sound comes from two sources at all frequencies below the crossover.
If your left, center, and right speakers are different, you may have similar problems.
You are right I am interested in making sure the phase alignment between the subwoofer and the mains is dialed in correctly. I have created a CD with 70hz pure test tones. That is the crossover for the subwoofer. Do you agree with the following methodology for making sure the phase alignment is set correctly?
"There is a method for setting the phase control that guarantees perfect phase alignment between the subwoofer and main speakers. First, reverse the connections on your main loudspeakers so that the black speaker wire goes to the speaker’s red terminal, and the red speaker wire goes to the speaker’s black terminal. Do this with both speakers. Now, from a test CD that includes pure test tones, select the track whose frequency is the same as the subwoofer’s crossover frequency. Sit in the listening position and have a friend rotate the subwoofer’s phase control until you hear the least amount of bass. The subwoofer’s phase control is now set perfectly. Return your speaker connections to their previous (correct) positions: red to red, black to black.
Here’s what’s happening when you follow this procedure. By reversing the polarity of the main speakers, you’re putting them out of phase with the subwoofer. When you play a test signal whose frequency is the same as the subwoofer’s crossover point, both the sub and the main speakers will be reproducing that frequency. You’ll hear minimum bass when the waves from the main speakers and subwoofers are maximally out of phase. That is, when the main speaker’s cone is moving in, the subwoofer’s cone is moving out. The two out-of-phase waves cancel each other, producing very little bass. Now, when you return your loudspeakers to their proper connection (putting them back in-phase with the subwoofer), they will be maximally in-phase with the subwoofer. (It's much easier to hear the point of maximum cancellation.) This is the most accurate method of setting a subwoofer’s phase control. Unless you move the subwoofer or main speakers, you need to perform this exercise only once."