June 23, 2011, 2:36 a.m.
posted by brevis
Mix Multiple Tracks
Multitrack editing [Hack #50] allows you to combine multiple sounds together into a single stereo mix. With Audacity and all the other editing programs, you can create an unlimited number of virtual tracks. Each track can contain one or more sounds that are positioned in the track at various points in time.
A multitrack sound project is not an audio file that can be played by a standard MP3 player. To do that, you need to perform a mix-down operation that flattens all the tracks into either a mono or stereo output file. This is the final stage of any project. You should always retain the multitrack project and consider the output mixed-down file as throwaway.
If you are familiar with Photoshop, Fireworks, or other image editing programs, you can think of a multitrack project the same way as you do an image file with multiple layers. A Photoshop file is the equivalent of a multitrack project with multiple layers. And the exported JPG or GIF file that has the flattened layers is the equivalent of the mixed-down MP3 file.
You can rename a track to something meaningful, such as voice, music, or ambience. Sometimes these are abbreviated as ax or acts for voice, mus for music, and abl for ambience. This will make it easier to manage your tracks.
A track can be mono, stereo left, or stereo right.
Stereo tracks can be located somewhere within the stereo field, from absolute left to dead center to absolute right. An interview that was recorded with two separate microphones might be mixed with one signal slightly left and the other slightly right to give a feel of separation between the interviewer and interviewee.
Some editing programs, such as Apple's GarageBand and Adobe's Audition, allow you to assign effects to each channel, including reverb, chorus, and delay. This is in addition to any effects [Hack #58] that you have applied to the signals within the channel.
Figure shows an example of a two-track interview project in Audacity.
An interview multitrack mix
The top track is the interviewee and the bottom track is the interviewer. The gain of the interviewee has been enveloped using Audacity's gain enveloping feature [Hack #56]. You can see the differences in the gray background of the track: where the gray pinches in, the sound level is reduced; where it reaches the top and bottom of the window, the signal is at full strength. Enveloping and the master gain of the track work together to control the overall gain of the signal.
Figure is a close-up of a stereo project. You can see the controls for stereo pan and gain in the sidebar of each channel.
Notice how the top channel is assigned to the left output channel, and the bottom channel to the right. You are free to have as many left, right, and mono channels as you like, and Audacity will handle mixing them properly during mix-down.
Stereo multitrack mix in Audacity
You can also mute the channel during playback with the Mute button. Listen to just one track by pressing the Solo button. Please note that the Mute button in recent versions of Audacity is ignored for mix-down.
A mono project, as shown in Figure, looks similar.
Two mono channels in Audacity
The two channels are marked as mono, with the master gain of each set to the appropriate levels.
You can test your mix and tweak it as you work by hitting the Play button on the document. Audacity does this mix to the left and right on the fly, and presents you with the sound of the finished product. If you want to remove a track from the mix, temporarily hit the Mute button on the track. Or if you want to check a particular track, hit the Solo button on the track.
At any time, you can export the project to MP3, which will flatten the mix down to what you hear on the left and right during playback. If you want to preview that in Audacity before you export to MP3, use the Quick Mix command to flatten the project down to a stereo or mono track.
"Mix Your Podcast in Hardware" [Hack #14]
"Choose the Right Audio Tools" [Hack #50]
"Maintain the Gain" [Hack #56]
"Set Up a Home Studio" [Hack #61]