The simplest and most common reference to "phase" concerns speakers. If a speaker system is "in phase", it means that the drivers of both speakers are moving forward at the same time -- i.e., the drivers in both speakers in a stereo pair are moving forward simultatenously, and moving backward simultaneously. A speaker system that is out of phase, however, has one speaker's set of drivers moving forward (toward the listener), and the other speaker's drivers are moving backward (away from the listener). It is fairly easy to tell when speakers are in phase -- the sound you hear is coherent and seems to be centered more or less between the two speakers. Speakers that are out of phase, however, sound diffuse and rather directionless -- like they are disembodied, and maybe even floating in the room. To have your system deliver accurate stereo imaging, the speakers must be "in phase". If your speakers are out-of-phase, all you need to do is reverse the signal wires on ONE of the speakers (i.e., reverse the positive wire so it is connected to the negative post, and connect the negative wire to the positive post). There are a number of test CD's that you can buy that have phase tests on them.
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