Compression – How much gain reduction to use? Pumping, how to avoid? Limiting versus compression?

Robert Katz Leave a Comment

From: Geoff Goacher 

 
Dear Bob:

I’ve really enjoyed reading your enlightening articles on the web site and Mix magazine. I have a great deal of respect for your opinions. > > I’m writing you with a couple of compression questions. First of all, is it true in mastering (or mixing for that matter) that signal compression/limiting should not exceed a gain reduction of 6 dB, unless you are going for a “noticeable compression effect?”

Hi, Geoff. Thanks for your comments.

Compression and limiting are so different that a generalization of “6 dB” in one sentence for both, and also for both mastering and for mixing and for all kinds of instruments is just that, too general. 6dB gain reduction on a vocal solo with a nicely-designed tube opto compressor may be barely noticeable, but 1 dB on an entire mix with the wrong unit can be severely degrading! So I stay away from such generalizations. Use your ears…

Second, is it true that pumping and breathing effects are really only a problem when a compressor/limiter is making gain reductions greater than about 10 to 12 dB?

Wrong. Pumping and breathing effects are matters of the attack and release times of the unit and how they relate to the tempo and frequency makeup of the particular music, as well as the signal to noise ratio of the music, for if there is no noise (e.g., tape or pre amp hiss) to “breathe,” you can get away with more compression. It is true that the more gain reduction, the more you will notice breathing and pumping, but in many instances, only a couple or 3 dB of compression will cause audible pumping or breathing effects. Multiband compression can reduce pumping by, for example, keeping the bass fraction from modulating the vocal, but I’m not a fan of multiband compression as it can produce a very unnatural sound, “balls to the wall,” but with no dynamics. It’s much too tempting… Read my article Secrets of the Mastering Engineer for more information on that.

Lastly, is it true that RMS level limiting (brick wall) provides the most natural-sounding mix compression? (Due to affecting only the highest peaks of the signal and making the majority of the signal louder, but uncompressed)

Not RMS, but peak limiting. Very short duration, very quick release time, very quick attack time, high ratio, 1 to 3 dB or so gain reduction can be very invisible. But invisible is not the same as “natural.” When you are talking about mix compression, are you solely talking about

a) trying to increase the loudness character by reducing the peak to average ratio but not affecting the sound, if that’s what you mean by “natural” or
b) are you talking about modifying the mix characterin a “pseudo-natural” manner?

In a), then limiting is the key, for it will be “invisible,” and what can be more “natural” than “invisible”

in b) some sort of compression rather than limiting may be the key, for you are looking to alter the sound in a “natural” way.

These are things that I’ve heard from various people but I wanted to get a more qualified opinion before I incorporated them into my working theory on using compression (or why you shouldn’t use it). Thanks for your help!

Geoff Goacher

Hope this helps!

Best wishes,

Bob Katz

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