GarageBand compressors don’t feature sidechain inputs, but you can recreate the effect with the built-in ducking used for podcasting.
Step 1: Launch project
Launch a new GarageBand project and set up two software instrument tracks by clicking the plus button in the lower left corner, then the ‘Create’ button.
Step 2: Set instruments
Click the first track’s header and assign it as Drum Kits>Dance Kit. You can then set the second as Synth Pads>Hovering Machine.
Step 3: Record kick drum
On the Dance Kit track, record in a simple four-beats- to-the-bar kick drum and quantize it to 4/4. This will be the trigger for the sidechaining effect.
Step 4: Record chords
On the second track, record in some long synth chords with the Hovering Machine sound. This is the part that will end up being processed.
Step 5: Reveal Master Track
In the Track Info pane, select the Master Track button at the top, then click on the Edit tab to reveal the Master Effects slots.
Step 6: Enable Ducker
Enable the Ducker plug-in by clicking on the small white button. Two new buttons, each containing a triangle, should now appear in each track’s header.
Step 7: Set up ducking
Click the upper triangle on the Dance Kit track to turn it yellow. This sets it as the ducking master track. The lower one on the keyboard track should turn blue.
Step 8: Numbers game
Click the Ducker plug-in’s control panel icon, next to its name in the track info pane, to reveal the controls for the effect.
Step 9: Adjust settings
Slide the Attack and Hold sliders all the way to the left, then raise the release control to get the effect. The required setting will depend on your track’s tempo.
Click the image to zoom in on the annotations.