HOW TO: Surgical Compression for Mastering

Use these meticulous—and improbable—techniques for transparent results
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You normally wouldn’t think of using sky-high compression ratios and a hard knee for delicate mastering applications. Those settings are used to slam tracks with heavy gain reduction, right? Yes, but sometimes they can be used for much more subtle and delicate effect. In this article, I’ll show you how to use unlikely compressor settings for surgical mastering.


Some mastering challenges may at first seem impossible to meet. Take for example a mix in which the lead vocal needs de-essing. Using a mid-side compressor, you can de-ess the mid channel—where the lead vocal lives—but it will be difficult to set a threshold that won’t also suck down every kick and snare drum hit and maybe center-panned guitars, too. The cure can be worse than the disease! That is, unless you use smart medicine.

For this task, nothing but the most full-featured compressor will get the job done without incurring audible collateral damage. You’ll need a multiband, mid-side compressor outfitted with either a freely adjustable sidechain filter or an external sidechain input. The compressor should also offer a range control, super-fast attack and release times, a very hard knee, sky-high ratio, sidechain monitor and very fine threshold control. FabFilter’s Pro-MB plug-in fits the bill perfectly.


Fig. 2: Detail of compressor controls: Use a hard knee and high ratio and threshold settings. Set up Pro-MB so that only the mid channel triggers compression and only the mid channel’s high frequencies—between roughly 5 and 10 kHz—will be compressed (see Fig. 1). Dial in an infinity:1 ratio and 2dB range to start. When de-essing a vocal embedded in a mix, the threshold typically must be set barely below the amplitude of the vocal sibilance. Any lower, and slightly quieter elements in the mid channel (such as electric guitars) will also spark gain reduction. You want sibilance that barely exceeds the threshold to reliably trigger significant compression (hence the high ratio), but no more than about 2 dB (delimited by the Range control); 2 dB is typically the max you can get away with when de-essing a full mix. Sibilance typically comes and goes very quickly, so set Pro-MB’s attack and release controls to their fastest settings to treat the problem and avoid collateral damage. You also don’t want the compressor to kick in as the audio signal approaches the threshold, so set Pro-MB’s Knee control to fashion the hardest possible knee.

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At this point, you’ll probably notice that some kick and snare hits are still triggering compression. In Pro-MB’s sidechain, narrow the range for the adjustable bandpass filter to whittle down the drum hits and pass only a thin slice of sibilant frequencies (narrower than the range that will be compressed in the audio path). (If your compressor doesn’t have a bandpass filter for its sidechain, route a bandpass-filtered copy of your mix to your compressor’s external sidechain input.) While listening to Pro-MB’s sidechain signal, fine-tune the bandpass filter’s range to find the sweet spot where vocal sibilance sounds louder than kick and snare hits. (On mixes in which the vocals are deeply buried, this might not be possible.) Even though you’re compressing in the audio path between roughly 5 and 10 kHz, the sidechain filter might only span, for example, 6.3 to 8 kHz—a much narrower slice.

After adjusting the sidechain filter, you’ll likely have to fine-tune Pro-MB’s threshold again to completely weed out the kick and snare. If the filtered sibilance barely exceeds the reset threshold (likely!), you may also need to increase Pro-MB’s Mix parameter to as much as 200% (to double the gain-reduction amount) and its range from 2 dB to 4 dB to achieve as much as 2 dB of gain reduction on peaks. (Because Pro-MB’s Range control slightly softens the knee at higher settings, sub-maximal gain reduction will also slightly increase.) With the correct setup, the vocals should be lightly and transparently de-essed. Mission accomplished!

Michael Cooper is a recording, mix, mastering and post-production engineer and a contributing editor for Mix magazine. You can reach Michael at and hear some of his mixes at