Multiband Compression

Setting Multiband Cutoffs

In general, you want to try to split your mix so that each region captures a prominent section of your mix. For example, the strategy behind the default band cutoffs is as follows…

Band 1: This band is set from 0 to 120 Hz, to focus on the "meat" of the bass instruments and kick drum.
Band 2: Band 2 extends from 120 Hz to 2.00 kHz. This region usually represents the fundamentals of the vocals and most midrange instruments, and can represent the "warmth" region of the mix.
Band 3: Band 3 extends from 2.00 kHz to 10 kHz, which usually can contain the cymbals, upper harmonics of instruments, and the sibilance or "sss" sounds from vocals. This is the region that people usually hear as "treble".
Band 4: Band 4 is the absolute upper frequency range, extending from 10 kHz to 20 kHz. This is usually perceived as "air".

Keeping in mind that instruments have harmonics that can extend over several octaves, the goal is to try to partition your mix into bands. Play your mix, and click on the "M" button on each of the bands. This mutes the output of that band. Now you can hear exactly which frequencies are contained in each band. Try adjusting the band cutoffs by dragging them with the mouse.

If you can hear the "parts" of your mix captured in each of the bands you're in good shape. If you don't know exactly where to set them, don't worry. Once you start applying processing to each of the bands you'll begin to develop an intuition for where they should be set. The main ideas at this point are simply:

Multiband effects are applied independently on four separate bands
Each band should represent a musical region of your mix (bass, warmth/vocals, air, etc.)
You can adjust the cutoffs of each of these bands
You can mute the output of the bands to hear exactly what is passing through the remaining bands.

Overall Compression Strategy

Here's a sequence of steps that you could use to get started using a compressor effectively.

Set your ratio. Depending on what you're trying to compress, here are some starting points to try.
Full mix: Try 1.1 to 2.0 ratios
Bass, kick: Try 3.0 to 5.0. Depending on the sound you're shooting for, you can even go all the way to 10...
Vocals: Try 2.0 to 3.0.
Bring down your Threshold.
Turn up the gain as you see fit to boost the compressed signal.
Experiment with attack and release timings. No good single tip here, but remember that shorter attacks will level off more of the transients but possibly cause distortion. So you could shoot for the lowest attack possible before hearing any artifacts.

Expander

The Expander section affects the lowest level signals; those signals below its threshold (as opposed to compressors and limiters which work on signals above their thresholds) There are three main uses of the expander section…

Just as you could add more volume by compressing the lower signals, and bringing up the overall gain of the output of the compressor, you can also add more volume (or at least fullness and body) by bringing up just the softest parts of the mix. This is done with the expander. To achieve this "upward compression" set the expander ratio less than 1.0.
You can use an expander as a noise gate, where any signals that are below the Threshold are effectively turned down completely. As a gate, use a high Ratio (8 to 10 or so) and set the Threshold down at the point of the noise floor.
If necessary, you can use the expander as an "uncompressor" to expand an overly compressed signal. Of course, it's better not too over compress to begin with, but sometimes you have a mix that has already been destructively compressed. As an "uncompressor", try Thresholds around –20 to –30 to start and a ratio of 1.1 to 2.0 (with higher ratios providing more "uncompression").

Bass Boost

A multiband compressor is the key to get more bass out of a mix (as opposed to trying to boost the gain of the low end with an EQ). In addition to just getting "more bass", a multiband compressor can simply even out the dynamics of a kick or bass to give a more professional consistent sound. For this application, you can think of a mulitband compressor as an EQ that understands dynamics. With a ratio of 1 for the expander/compressor/limiter, you can just turn the gain of a band up and down and it behaves like an EQ. As you add compression (adjusting ratios and thresholds) it will still boost (or cut) the gain of the band, but only when the level exceeds certain ranges. For bass boost, you're obviously going to be focused on the lowest band, Band 1. A cutoff of 125 Hz or so will contain most of the "meat" of the bass, kick, etc., although the attack and harmonics of these instruments will be found in upper bands. For general bass compression, you could start with a ratio of 4 on the compressor but feel free to try extremes. Bring down the Threshold and bring up the Gain until you've got the punch that you want.
Assuming you have both bass and kick drum in your mix, it's relatively easy to guess how the attack time will affect the sound. A kick drum typically has a faster attack than a bass guitar or synth. So with longer (slower) attack times you'll hear more of the kick drum, as that has a higher "attack" level that will pass unaffected through a compressor with a slow attack time. Conversely, a fast attack time will soften the attack of the kick drum and tend bring out more of the bass guitar or synth.

Warmth

Another application of multiband compression is to bring out character in the mix such as warmth in vocals (or any number of additional characteristics). Treating a multiband compressor as a "dynamic EQ", the general tips regarding equalization and frequency ranges also apply to a multiband compressor. For example, Band 2 can be used to bring out the warmth in the body of a vocal. For this application, try setting Band 2's low cutoff at 125 and the high one at 500. (note that boosting EQ in this range could just add "mud"). Set a ratio of 2 to 3 on the compressor and bring down the Threshold (and bring up the Gain). Adding a little "upward expansion" with the expander can also work very well in this situation.

Vocal Treatment

A multiband compressor is great for controlling unwanted noises in vocals, including pops, "sss" sounds, etc.

The low band (120 Hz and below) can be used to limit and compress "pops" and related sounds. Use a relatively fast attack to capture the transient "pop".
For controlling "sss" sounds (as a de-esser) start with the mid-high band ranging from 3 kHz to 10 kHz and a ratio of anywhere from 5 to 8. Bring down the threshold just to the point that the peaks of the sss's are compressed: typically around 2-6 dB, with a fast attack.
You can add "presence" to the vocals by compressing and bringing up the gain slightly for the region from 4 to 8 kHz. Use light ratios (1.5 to 2.5).
The breath or airy region is in the 10 kHz and up region. You can use Band 4 to either bring out or bring down this range. Using a combination expander/compressor, you can also use this band to gate hiss and other noise while still allowing the mid and high levels to pass through.

Noise Reduction

Using a multiband expander allows you to control the noise across multiple bands. Your best strategy is to loop a section of the mix that has the noise, most likely at the beginning of the mix before the real music kicks in or at some quiet point during the song. Watch the EQ's FFT analysis and see where the noise is. Adjust the bands so that the segments of noise are roughly contained within a couple bands, set a high Ratio, and bring up the Threshold of the Expander section until the noise is affected. Note that too high a threshold can cause gating of the quiet sections of music. While it takes no time at all to get a rough setting, it can require some adjustment to balance between gating the noise and gating the music. Don't become so obsessed with cutting out the noise that the mix just sounds sterile.

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