What is Phase?

Question

Hi,
Im studying Music Production and everyone keeps mentioning phase when recording. Im really unsure about what it is and what it does..... Someone said that it when the same signal is sent to the speaker and one tells the speaker to move back and forth and the other one does the opposite.
Can anyone explain what this means as i just dont understand and its proving to be at my disadvantage in recording sessions.
This isnt the only thing i dont understand, im trying to understand compression, distortion etc etc etc.
Has anyone got any advice on how to learn this?
I would greatly appreciate it,
Thanks, Ben

Answer 1

So in summary, I think a distinction exists between
out-of-phase signals that are time-coherent but
electronically manipulated to be out-of-phase, and,
signals that are acoustically captured but because of
differences in distances between those mics,
acoustically out-of-phase.
Maybe I wasn't clear: by definition, a signal that is out-of-phase must be out-of-phase COMPARED to the original waveform. You have to compare it or you wouldn't know that it's out-of-phase. That means that there is (somewhere, at least conceptually) the same signal which has not been delayed; hence no out-of-phase signals are time coherent to the original. It's really implicit in the concept.
A change in polarity simply reverses the positive and negative waveforms so that instead of beginning the cycle with a positive signal, you begin it with a negative signal. This can be electrical polarity or physical (pressure) polarity.
Delaying a signal 180 degrees and saying that you have now reversed the polarity is a bit of a misnomer. Yes, by delaying the signal you could put it 180 degrees out-of-phase, but that's not the same as a true polarity change - where no time shift has occurred but only an electrical or physical wavefront reversal.
With speakers, it's a simple polarity change from the positive and negative leads. This is sometimes referred to as "reversing the phase" or putting the speakers 180 degrees "out-of-phase" even though no phase shift is involved. Bass frequencies have the longest wavelength, so you'll notice the drop in low-end more easily when your speakers have their polarity reversed. At the higher frequencies, there is still a polarity reversal, but since the room interacts with it's own set of phase anomalies due to early reflections, etc, it's less noticeable.
When referring to subwoofers and "phase alignment", you are in fact delaying the LFE signal so that the wavefront from the low end aligns with the signal from the other speakers.
With microphones, the distance between the source and any two or more microphone elements will introduce phase anomalies as well. That's why time-aligning the phase of drum overhead mics with the BD and SN mics increases the "punch" in a set of drum mics: the attack transients would be in alignment creating more impact.
If you can come up with an example of an out-of-phase signal that is NOT time-dependant, then I might see where you're going with this. We may actually be saying the same thing.