"As a songwriter I record my demos at a studio in Nashville on ADATs (later importing them into my DAW). Recently I went back to the same studio and changed a few lines of the vocal, as writers are known to do. When I got home I only liked part of the changes; so my plan was to cut the parts I wanted from the new .wav file and paste them into my original .wav file. When I pull up the old track and the new track together on my PC as .wav files I noticed the new sine wave was flipped upside-down from the old one. The new one was like // where the original was //. I flipped the new wave file over (which is an option in the software) and they matched. I merged the files, cutting pieces from the new one and inserting them into the original file. The final track is OK to my ear but I still wonder what happened. My guess is it's a phase thing but I really don't have a clue."

First we need to get squared away on our terminology. Unless the engineer recorded actual sine wave tones on to the ADAT your waveforms were not sine waves. Sine waves correspond to a pure tone. Music, on the other hand, is a complex waveform and wouldn't look at all like (or be referred to as) a sine wave. Since we don't know what was really recorded we'll leave that at that.

When recording actual sine waves into a DAW it is very easy to view the phase and polarity relationships because of how simple the waveform is. Normally complex music waveforms are so complex that polarity or phase is not easily observed. However, if one zooms in far enough it is sometimes possible to see these types of inconsistencies. We do not doubt what you say you saw, we're just providing a little background so other readers can get on the same page with us.

With the above caveats in mind, your assumption is basically correct. It probably has more to do with polarity than phase in this case. There is an important difference: polarity is a matter of having a couple of wires reversed, phase is a time delay. The "phase" switch on mixing boards is actually a polarity switch. It just reverses the wires. Our guess is the engineer had a cable or something in the signal path wired with the opposite polarity between your two visits. There are other ways this could happen, and it actually could be a phase (time) thing, but the chances of getting phase anywhere near 180 degrees off in this type of situation is pretty low.

By flipping the waveform over in your DAW you most likely corrected the polarity problem, and the sonic results you observed are consistent with that. It would be interesting to see if it sounded any different with the polarity left backwards. This begins to delve into the controversial world of relative versus absolute phase in recording, which is probably where we should leave this tip...