The auto duck feature in Audacity is a very useful effect if you want to do voiceovers. Basically, this effect links two tracks together, a control track and a background track. The volume of the background track decreases if a certain dB level is reached in the control track. This allows you to run a background track throughout a piece, which can be louder when there is no over lapping audio, and softer when another track is being played. The auto duck feature saves time by automating the volume process, rather than the user going to each overlapping section and creating an envelope over one track. Auto ducking also creates uniform volume control, whereas enveloping individual sections to the same volume may be tricky.
How to Auto duck:
The first step is to place the control track BELOW the background track. For this example I used a song for the background (track 1) and a voice recording for track 2.
Select the background track, and make sure to DESELECT the control track.
Once the background track is highlighted, go to EFFECTS, Auto Duck
A prompt will come up with the parameters and a graph. The graph shows the fade in and fade out, as well as how many dB’s the volume will drop.
The fade in and out length can be adjusted in these parameters, as well as how much the volume will “duck.” The “Threshold” is used to detect volume in the control track.
After these parameters are set, click ok, and your tracks will change. While this is not a particularly pretty example, for long, sustained recordings over background music, the auto duck effect is extremely helpful.