This is a relatively simple trick which allows you to split the frequencies of a particular sound in order to process the different frequencies separately. This comes in very hand when trying to tame the dynamics of the lower end of the bass without squashing the entire sound. Thus you get a nice solid low end, but a relatively “unprocessed” sound.
To begin with, choose the track that you want to do Multi-Band processing on and duplicate it. On the first insert slot of the original track, insert a low pass filter with a steepest curve you can (12dB, 18dB or 24dB) in order to cut all frequencies that you feel are slightly above all the bass energy. I typically put this around 200Hz to start and play with it from there. Solo this track to make sure that only the lows are coming through.
Now, copy/paste that same insert plugin into the duplicate channel. Open up the plugin and change it to a high pass filter, and set the frequency to the same setting as on the first track. This will allow only the high frequencies through. Solo this track and make sure that it is working properly.
Now you should have two bass tracks. One which includes high end content and one with low end content. This is now a great opportunity to compress or side-chain compress that low bass track without affecting any of the transients and articulation of the high bass track. You can often get away with rather heavy compression which will help solidify the low end energy. The high bass can be treated separately, perhaps with a touch of overdrive/ distortion or chorus to liven it up. Maybe some stereo enhancement. Well, hopefully you get the picture about what sorts of things you can do from this point.
This was a quick little technique that I showed my students in regard to Mastering, yet it does really apply in any situation where you feel it could be useful. For instance when you add broadband distortion (insert a distortion plugin on the bass track), the bass frequencies often get reduced. With this technique, you can just add distortion to the high end bass track and get the grit, but also not lose the lows. It is cool for processing stereo keyboard tracks, you can split the highs and lows, pan the lows more center and the highs you can add a touch of delay on. Get creative with it.