Ducking with commentary only when there is sound

but only when there is actually sound playing from the commentary track? S

You can do this by turning "Ducking" on. I am not sure, if GB '09 will automatically separate the movie soundtrack from the video, but GB '11 will add video to a different track than the soundtrack, if you drag a video to GarageBand.

Once you have a separate audio track for your movie,

• enable "Ducking" from the "Control" menu
• In the Track Head of the soundtrack click the downarrow to duck the track.
• In the Trackhead of the commentary click the up-arrow.

If your GB version does not separate the audio in a separate track, use Quicktime or iMovie to expoart the audio from your video as a separate track and import this into GarageBAnd.

Regards

Léonie

And if you don't like the way the ducking is acting (too slow/too fast/too much/too little), there's controls where you can modify it.

The commentary track will be the full length of the video, but there may be sections where no one is talking, even though the track is still playing. Is there a way to get the ducking to automatically find these sections of silence, and not reduce the videos volume during them?

Ducking will do exactly that - when the commentary is silent, you will here the video track instead.

My understanding of ducking is that it just quiets the second audio track whenever the primary track is present. Or am i incorrect?

That depend, on what you mean by "present". The background track (video) will play loudly, whenever the primary track is silent. This can be quite annoying, btw. A short phase of silence in the primary track may cause disturbing bursts of loudness of the ducked track.

It may give better results to put a volume automation onto the video track to gently fade in and out.

Regards

Léonie