In a stereo signal it is important that the two channels are in phase. This means that the loudspeakers push and pull the air in step with each other. If the connections to one of the loudspeakers were swapped, then they would be 'out of phase' and one would push as the other pulls, and vice versa.

Here is a 440 Hz sine wave tone that is in phase in both channels...

And here is the same tone, but one of the channels is inverted...

This effect never happens in nature; only in sound systems. The human hearing mechanism therefore has no means of interpreting this properly and it just sounds odd.

The same can apply to music. Here is a mono music track where the left and right channels are in phase...

And here is the same track, mono but out of phase...

An interesting thing happens if the out-of-phase signals are mixed. In this example the inverted channel is slowly faded up over the full duration of the track.

As the inverted channel is faded up, it cancels the other channel hence it gets quieter and quieter.

Eventually it cancels completely right at the end.

The same can be tried with stereo music. Here is the same music, in stereo and in phase...

And with one of the channels inverted...

The out-of-phase is not quite as pronounced as when the channels were identical (but one inverted), but it is clearly an undesirable effect.

Finally on this point, the out-of-phase stereo channels are mixed, once again the inverted channel is faded up gradually...

This is interesting because at the end it doesn't cancel completely. Only the elements that were identical in the two channels cancel. The things that were different remain. This can be an interesting way to gain insight into the sonic components of tracks.

Download the audio files for this tutorial...