If you have an instrumental and original version of a pop song, it is sometimes possible to create an acapella (vocal only) edit by combining the two and using the invert filter in Audacity to cancel out the instruments. However this technique often produces low quality acapellas with plenty of noise and poor instrument cancellation.
This article acts as both an introduction to the idea of phase cancellation using the invert filter and also should assist anyone who is struggling with quality issues to improve their game. The same topic is covered in the Zynewave Podium tutorial showing how the same may be achieved, but with greater ease.
This tutorial uses Audacity version 1.3.12.
It's important to note that the normalization amount should always be kept identical for the full edit and the instrumental edit.
See below for more details on handling track gain.
August 2011: One of the criticisms that has been made of this video is that normalization is unimportant in avoiding clipping in the ripped track. A comment left by a Youtube user said: "05:55 if the cd clips, normalizing does nothing positive.. If it doesn't, normalizing is pointless." I have to admit I hadn't considered that CDs would be released onto the market with audio that is actually clipped. But not only does the audio clip on some CDs, I have found examples of extreme clipping on some very popular major label CDs. And of course, if the CD actually clips there is nothing that you can do to fix this. Amazing that the buying public is willing to pay for this. But then there's no accounting for taste. I can't completely agree with the criticism however, it may still be useful to normalize everything to 95% for instance to keep a consistent volume for tracks you rip.
It helps to reduce the gain on both tracks when previewing in Audacity, but be sure to reduce the gain by exactly the same amount. In some cases the instrumental may not simply be the full edit mastered sans vocals and in this case adjusting the gain on the tracks to different levels may lead to better cancellation.
Zero success is a fact of life with some attempts at cancellation. If at first you don't succeed then try one or twice more then give up. No point in continuing hitting your head against a brick wall... even if the everything seems to line up perfectly. There are several reasons why you might get zero cancellation: different arrangements on the tracks, different tempos, different mixes. If the maths doesn't add up, you simply won't get the phase cancellation occuring and for the maths to work the instrumental needs to be as close as possible to being the same edit as the vocal, but with just the vocals omitted.