Multiband Compression is not something Standard.Most of the time it's a complex procedure but you have to keep in mind that you are doing this to a whole mix which means your dB reduction should be very careful or you might kill it.
That's a general guideline though , there are techniques that use aggressive/heavy compression.
I would recommend reading the booklet i share from Bob Katz not only the multiband and M/S multiband parts , but all of it !
I quote from Bob Katz's "The Secret of the Mastering Engineer"
The MS adventure begins
MS is another tool that reduces compromises and increases the
possibilities of mastering. The possibilities are only limited by
your imagination. The Finalizer, and especially the Finalizer
96K, allows you to manipulate stereo separation using MS
technique. Let's take a stereo recording with a weak, center-
channel vocalist. First we put in our MS encoder, which separates
the signal into M and S. Then we decrease the S level or increase
the M level. We then decode that signal back into L and R.
Presto, the vocal level comes up, as does the bass (usually) and
every other center instrument. In addition, the stereo width
narrows, which often isn't desirable. But at least we raised the
vocalist and saved the day! The Finalizer's built-in width control
does this job by changing the ratio of M to S.
But we can accomplish a lot more, often with no audible
compromise to the presentation, and make clients very happy.
Let's take our stereo recording, encode it into MS, and apply
separate equalization to the M and S channels. Here's the
traditional (pre-Finalizer) method: Feed the output of the MS
encoder to a dual-channel equalizer. Channel one of the equalizer
contains the M channel, which has most of the vocal. Channel
two contains the S channel, which has most of the ambience and
side instruments. With the M channel EQ, we can raise the vocal
slightly by raising (for example) the 250 Hz range, and perhaps
also the presence range (5 kHz, for example). This brings up the
center vocal with little effect on the other instruments, and lowers
the stereo separation almost imperceptibly.
The Finalizer 96K's Spectral Stereo Imager can also "remix" this
material, with a slightly different user interface. By raising the M
level (reducing the width) of the 250 Hz and/or 5 kHz range, we
bring up the center vocal very similarly to the traditional method,
and without seriously deteriorating the imaging of the other
instruments. In addition to this "remix" facility, the spectral
stereo imager has very creative width control, limited only by
your imagination. Spread the cymbals without losing the focus of
the snare, tighten the bass image without losing stereo separation
of other instruments, and so on.
Even More Advanced M-S Technique
Currently the Finalizer has a single threshold for both channels,
but other TC Electronics products can accomplish even more
sophisticated M-S mastering. You've all heard the mix that
sounds great, but the vocal is sometimes a bit buried when the
instruments get loud. We try compressing the overall mix, or
even narrow band compression of the vocal range, but it worsens
the great sound of the instruments. MS compression can help us
isolate the compression to the center chanel.... By only compressing
the M channel, we delicately bring up the center channel level
when signals get loud. Or, better yet, use multiband MS
compression, so, the bass (for example) is unaffected by our
compression. In other words, compress only the midrange
frequencies of only the M channel... A very selective and
powerful process, only available in today's digital world.
Useful GS Threads :
- Benefits of Mid/Side?
- M-S processing