The best bet for finding instrumental tracks completely free of human vocals is to look for instrumental cover versions through Amazon.com, iTunes or other mass marketers. If tracks with background vocals are acceptable or desired, karaoke tracks are widely available in a variety of formats.
If an instrumental version is unavailable, several programs for removing vocals from music tracks are available online. Most work on the principle that the vocals in a recording have been mixed to the center of the stereo image. The idea is to negate the vocal frequencies in the track by changing the polarity of one of the stereo channels and then combining it with the unaltered one. This does not always work because of various music files' differences in encoding and compression. Also, the sounds of some of the instruments may end up distorted or reduced because they lie close to the vocals in the stereo image.
The most common way to find instrumental versions of popular songs is to look for tracks marketed for karaoke, which are widely available as MP3 tracks, DVDs or compact discs. The majority come with backup vocals, and some formats offer the option of switching the lead vocals on and off.