Isolating the vocal track from a multitrack recording is a common practice, usually undertaken to produce what is often referred to as an "a cappella" (though spellings vary). A cappellas are used in many contexts, most notably by remix artists producing remixed versions of a given song. There are several methods for producing a cappellas, only one of which is possible in Audacity, a free audio editing tool for Windows and Mac OS X. The ability to perform this procedure is limited by the fact that it requires an instrumentals-only version of the song from which the vocals will be isolated.
Launch Audacity. When its interface appears, click on the File menu and select Open. A file selection dialog will appear. Navigate to the version of the song that includes the vocal track you want to isolate and double-click it. Audacity will place the song on a track in its arrangement area.
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Click on the Project menu and select Import Audio. Find the instrumentals-only version of the song in the file browser and double-click it. The instrumentals-only version will be instantiated on a second track below the first.
Use the Zoom In tool to zoom in on the tracks at any point in the timeline until the waveforms associated with them are reduced to a single, wavy line running across the track. The waveforms should appear nearly identical. They must be aligned precisely on the timeline in order for the vocal isolation process to work.
Select a peak in the waveforms that is identical in shape on both tracks. This will be particularly easy at those points where the vocalist does not sing.
Use the Time-Shift tool to align the tracks as precisely as possible. The Time-Shift tool is accessed by clicking the double-ended arrow icon in the toolbar. Click-hold and drag one of the tracks left or right in small increments until the waveforms are aligned. If they are not aligned precisely, the vocal isolation process will not work correctly.
Click on the track header for the instrumentals-only track to make it the focus of the application, and choose Select All from the Edit menu. The entire instrumentals-only track will be highlighted.
Click on the Effects menu and select the Invert function. The waveform for the instrumentals-only track will be flipped vertically.
Click the Play button. Because the instrumentals-only track is aligned with the first track and inverted, it will mute all identical sound information in the first track completely, leaving only that sound information that the two tracks don't share. As a result, the vocal track will be almost perfectly isolated from the track.
Save the project by clicking the File menu and selecting Save. You will now have a usable a cappella for use in future projects.