For well over a decade now manufacturers have been selling multiband compressors as a panacea for your mix. Hardware units like the TC Finalizer and later on plugins. The problem is that while judicious use can yield spectacular results, it's difficult to resist temptation. Compression sounds good right? So why wouldn't more compression sound better?
Because if you can suck the life out of the project by compressing the full spectrum, you can multiply the ill effects by doing it four times over.
Now that said, being able to treat the lows like the kick and the bass separately from mids and highs like vocals and guitars can be a godsend. But it's so easy to tip over the edge and just murder a mix, particularly when the makeup gain is set to auto. Every little bit you pull the threshold down raises the output gain up by an amount determined by the ratio of the compressor (usually). You can get a mix jacked all to heck before the compressors even start to kick in.
Rather than describe in horrifying detail how to kill a mix, let's get on to how to use one of these things the right way.
First of all, they can be handy when used a little bit differently than their intended purpose. If I'm moving quickly in a session I find that it's faster to slap a multiband compressor on a track as an insert plugin, turn off three of the bands and just use one as a de-esser. For me it's quicker than setting up a side chain and I find that I have better control than with some of the de-esser plugins. It's also possible to use it more like an EQ, treating just the low mids of a guitar track for instance.
That brings me to going about setting up a multiband on a master bus. The first thing to do is carefully determine how wide the bands will be. Treating the lows below 100 Hz might work great on a death metal track but totally murder a jazz track. The same goes for the mids, if not more so. The safest bet is to try to divide it up to treat instruments and vocals separately.
The next thing to do is turn off the auto setting on the makeup gain. It's never a good idea to let a machine do your thinking for you so try and decide for yourself how much gain to put back on after you've finished messing with the rest of the settings.
If your compressor lets you mute the bands you're not working on that will probably help get things dialed in as far as threshold and ratio. Just make sure you keep switching back to listen to the whole thing once in a while. Remember that everything has to fit together when you're done. Keep in mind that the attack and release times will probably need to be different for each band. What works to keep transients in the kick while fattening it up might not work for the keys and guitars up higher.
At this point a delicate touch is all you need. Strapping a limiter on the two mix and slamming the daylights out of it won't work for every style of music. The same goes but more so when you're doing it three, four, or more times. Not that you should be mixing primarily with your eyes, but if you're seeing six dB of reduction in each band you've probably overdone it. Think of this as a chance to make each section of the spectrum dance a little on its own, while helping to tie the whole mix together.
Once all the settings are dialed in then you can go back and start to add the makeup gain. Just apply carefully and keep an eye on the meters. You may even want to have a regular limiter down stream just as a safety measure.
Just take it easy. That's the key. Ignore the presets. Ignore the urge to keep squeezing and squeezing. It's not getting better. It's just not. Take deep breaths. Just tickle the settings. Nudge them. Do it nicely and your tracks can come alive.