the glories of filtered sidechain compression
sidechain compression has become a mainstay in production for its fun pumping effect, but of course it also serves a very functional purpose - allowing sounds to stay out of one another's way & be heard clearly. we can expand this functionality by working off only certain frequencies of the source & effected audio, which ableton makes it really easy to do with the eq features of the built-in compressor.
filtering a sidechain source
set up your sidechain as normal. in this case I have a kick feeding into the compressor. the kick is comically thick; not only does this get in the way of the bass & vice-versa, but take a look at the gain reduction on the compressor - you can see it has an awkwardly extended trough. kicks will often have a very sharp click in the high end, however, that we can take advantage: as soon as I enable the compressor's eq section & switch to the high-pass filter, I get the much cleaner & familiar sidechain curve I'm looking for.
the audio demonstration is doubtless a crude example, but you should be able to hear how the kick better retains its "fullness" when the sidechain enables in the second half.
filtering your sidechain source allows you to grab just what you need from the audio - it could be you want just the sharp attack of a kick, as above, or the low subs of a full drum kit to trigger based only on the bass drum. but what about filtering from the other end - if you only want the compressor to affect a certain frequency range?
applying sidechain to a specific frequency band
I had previously covered multiband racks in live, but the basic idea is to get a clean split of the audio spectrum by soloing bands of ableton's multiband compressor on separate chains of a device group. in doing so, any effect you place in one of the chains will affect only that frequency range that passes through it. this is a useful technique if your purpose for using sidechain compression is purely functional - you can keep sounds out of each other's way in the messy bass region, for example, while keeping the rest of the sound intact without pumping for more transparency.
it's pretty simple to build yourself, but if you prefer to just use a completed rack with macro parameters, you can download one I made here.
once you drop the rack in, you'll need to tell it what track you want to use as your sidechain source. you can also switch the eq to high pass if you want the click of a kick (as above). you'll find all the other controls you'll need in the macro - standard compression parameters, gain & frequency for the input, a crossover frequency for where the compression takes effect, & an output gain knob for any necessary leveling compensation.
here's an equally crude demonstration with a bass with more high end. you'll hear by the end, with the multiband engaged, the rough saturation in the mids is preserved without pumping while the subs still get out of the way of the kick to keep it full all the way through.
& by the way, if you want to brush up on sidechaining & compression in general, you should check this out.