I have some mono percussion loops on which I'd like to do some stereo processing. When converting to stereo in Sound Forge, I've noticed it offers me the opportunity to invert the polarity on either or both resulting L/R channels. My question is: Under what circumstances would inverting the polarity on one side be desirable? I've noticed it changes the timbre a bit, making it sound somehow more "hollow" (but still good) but beyond that...
The main reason for this would be to artificially expand the stereo field beyond the placement of the speakers.
For instance, if the channels are in-phase, panning from center to right will move the sound from the center position to the right speaker.
Now invert the left channel. As you pan from right towards the center, you will actually keep panning the sound beyond the right speaker. It works best in small amounts. As the pan pot approaches the center position, the effect is lost.
The best CD I've ever heard with this effect is Roger Waters' Amused to Death. It uses QSound technology, which goes beyond simple phase inversion, but it's mostly the same thing. The album was recoded and mixed on analog tape, and the fidelity is excellent. (Political Alert: the theme is anti-corporate, anti-war, anti-religion. It was written just after the Gulf War.) It's not Water's best material, but some songs are quite good. The engineering is absolutely outstanding.
If I invert the left channel, then the stereo track should be panned generously to the right to enjoy the full potential of the effect?
You've got it. If you can, try sitting in the sweet spot, while moving the pan pot slowly in real-time. The results will depend mostly on the ability of your monitors to image, as well as your room acoustics.