This is a rather hacky method of ducking the bass when the kick is hit. You need the bass and kick on separate channels (mono or stereo), but any stereo information on the bass will be lost.
- Add a stereo bus. Name it "bass sidechain" or something similar
- Add a mono send (remove the second output) to both the kick and bass. Feed the kick to the left channel of the sidechain bus and the bass to the right channel. Turn down the fader on the bass track, we're going to use the bus now. I suppose you could make the bass track mono and feed the output into the bus, but sends seem to work well.
- Add an SC4 compressor and open up that and the bass send. Set the attack and release of the compressor to their minimum for now. Lower the volume of the bass send until it hovers around around -25db in the left compressor meter. The kick should now cut right above the bass (to say -5db), increase the volume of the kick send if it doesn't.
- In the panning section of the sidechain bus, right click on the top (left) panner and tick mute. Put the second (right) panner in the centre.
- Now solo the bass track, sidechain bass bus, and solo and mute the kick track (so it still plays through the send).
- Play your project in somewhere with kick and bass, and open up the compressor. Set the attack to 2ms (double click to edit) and the release to 15ms. You can edit these later, but these values seem to work well for my short, punchy kick. Set the RMS/peak to 1 and the threshold to around 5. The bass should now be ducking when the muted kick is hit.
- Unmute the kick and set the compressor values to your taste. You'll probably need to raise the makeup gain in the compressor to bring the bass up to a sensible level.
The kick now leaps out of my mix, I'm very pleased. The bass is now completely mono, if you want to apply stereo effects to it, you would probably have to feed it through a second bus.