Subwoofer phase is a pretty simple concept in speaker/sub integration, but it is often overlooked by people starting out. Typically a subwoofer has either a phase switch (often labeled with either 0/180 or normal/reverse settings) or a phase knob (which allows more granular control of the phase).
This is sometimes misunderstood because the idea of a crossover point is viewed as a ‘brickwall’ or an immediate hand-off from the speakers to the subwoofer at that frequency. In reality, there is a fairly wide range where the speakers and sub overlap.
The basic concept is for the speakers and subwoofer(s) to be working together in this overlap area instead of fighting each other (constructive versus destructive interference to use physics terms). This is accomplished by adjusting the phase (and possibly position) of the subwoofer to get it in sync with the main speakers (in a purely home theater setup, an argument can be made that coordination with the center speaker is more important).
Below are two graphs showing the frequency response in my home theater with my Hsu VTF-2 sub and Ascend Acoustics CBM-170 mains using the 0 degree and 180 degree phase settings. In this example, the 180 degree setting results in a pretty flat response (close to +/-2db from 20Hz to 130Hz), but the 0 degree setting has a very large drop (greater than 15db) in the crossover range.
180 degree phase setting (click to enlarge):
0 degree phase setting (click to enlarge):
Measurements using standard setup at position 3 in room diagram.
