Phase, part 1 http://www.music.columbia.edu/cmc/courses/g6630/Phase.html.

"Phase is a time relationship between two sounds, waveforms, or signals in a circuit. When the time relationship is coincident, the two signals or sounds are "in phase." And their amplitudes are additive. When the time relationship is not coincident, they are "out of phase" and their amplitudes are subtractive. This has ramifications both acoustically and electronically.

Example - Close miking a piano with two microphones. When you strike a key, the sound from the string will reach the closer mic sooner then the mic that is farther away, creating a slight timing or phase difference in the microphone signals. If those two signals are played on a stereo system the effect should be a stereo "image" of some sort. Our ears capture the same kind of timing and phase differences and the brain uses these differences for sound localization.

Now if the two signals from the two piano microphones are summed to mono, the phase difference between them could result in phase cancellation. In that case, certain frequencies, depending on the music content and the microphone distances, are reduced in volume as a result of the subtractive amplitude of the out of phase signals. (Conversely, if frequencies are in phase, the volume will increase.) The result could be a change in timbre of the piano, a decrease in volume of certain notes, or even a kind of distorted, dirty sound.

Whenever you are miking in stereo, especially close miking, it's a good idea to listen, then sum the two channels to mono and listen again. Then reverse the phase of one channel and listen again. This should tell you what frequencies are being canceled when you sum to mono. If you don't like what you hear, reconsider your microphone placement - phase is working against you.

ProTools has a Reverse Phase effect in the audio suite.

Sound reflections in a room can create similar cancellation effects, which recording engineers must consider in the room setup for a recording session."

Now if stereo patches sound so bad in mono, how come I don't notice how bad AM radio screws up a mix? Would a piano solo on mono AM radio sound bad?

Simple, because the phase problems than can be caused by mixing a stereo signal to mono is well known and almost all record producers check mono mixes of their stereo tracks to see if there are problems, then fix them if there are. Otherwise AM radio would really sound much worse than it does.

I learned early on, to test a mix in mono before the final master is made. If it sounds good in mono, it'll only improve in stereo..... that is, if you choose pleasing panning options.

Remember that dual mono approach the Beatles used? There are records that have only voice and drums on one side and guitars and bass on the other. Very distrete, and pretty weird sounding, unless you are in that one sweet spot that allows perfect bleding of the signals.
As much as I love the "Lads" ..... those early records had some anomolies that didn't impress me. Thankfully, the music always did !

By golly, this is the clearst, most common-sense explanation of phase cancelation I every read. Thank you everyone for sharing this. Now I know why my system always sounds so good in stereo.

Gary

[This message has been edited by travlin'easy (edited 02-13-2004).]