Like most other people, engineers are creatures of habit and convention. When we think of frequency-conscious compression, the first thing that pops into our minds is de-essing vocals. Not surprising, as this is arguably the most common application for the technique. But there are many others, and if we think outside the box, we can discover ways to use frequency-conscious compression to tame boomy acoustic and electric bass guitars and cello, blaring vocals and ear-splitting wind instruments. Much of this is familiar to veteran engineers, but let’s look at some refinements that should at the very least jar your memory and tune up your chops.

First, a brief refresher for newcomers to the technique. To perform frequency-conscious compression, you’ll need to patch an equalizer into the sidechain of your compressor; of course, the track you want to process gets patched through the audio path via the I/O. While listening to the audio track, determine what frequencies you want to tame. Then simply boost those frequencies on your equalizer to make the compressor more sensitive to them. For example, you might boost 7 kHz to 13 kHz by 6 to 12 dB in order to de-ess a female vocalist. When searing sibilance rears its ugly head, the audio in the compressor’s sidechain—which is “hearing” an exaggerated response in this frequency band, courtesy of the EQ boost—will exceed the threshold and make the compressor reduce the level of the audio signal.

To be sure, the compressor attenuates not only the high frequencies, but the whole enchilada. The trick is to set up the compressor so that it releases and returns the track to unity gain immediately after the sibilance has ceased. Usually, an attack time of around 50 µsec (.05 ms) and a release time of 50 to 60 ms gets the compressor in and out fast enough to attenuate the sibilant portion, yet passing the rest of the vocal through unchanged.

The quality of the equalizer is not critical for this purpose, as it is not in the audio path. For de-essing male or female vocals, I typically start by boosting all frequencies above 5 kHz. Such a broad-stroke approach is usually okay, because there’s generally not enough energy above the sibilance band to unintentionally trigger the compressor, even with 12 dB of equalizer boost. This way, I don’t need to fuss with the high-frequency cutoff, and I know any and all sibilance will be nabbed. In addition, I usually cut all frequencies below 5 kHz to decrease the compressor’s sensitivity to those lower frequencies so that it does not kick in when the spectral balance is okay—such as during vowel sounds and nonfricative consonants.

As for compression ratios, I usually start with infinity:1 to really hear the effect and then back it off to where it sounds right. I might even go as low as 3:1 to perform subtle de-essing on a track that just needs a pinch of control.

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Reprinted with permission from Magazine, October, 2000
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