Stereo and Mono Sidechain Compressors for Tracktion

Appendix: Using Compressors

Compression has become such an integral part of mixing, that we thought an appendix might be called for.

Dynamic Range

To use a compressor effectively, it’s important to understand the concept of dynamic range. Dynamic range is the difference between the loudest and quietest signal levels passing through the system chain. The span between "barely audible" and "physical discomfort" is about 120 dB, so we consider this to be the dynamic range of human hearing. Anything below the threshold of hearing will be lost, as will anything above the threshold of pain. But how much dynamic range do we need in our record- ings, or, more importantly, how much can we really use?

Few people listen in a totally soundproofed room. A very well-designed studio control room has an ambient noise level 10 to 15 dB above the threshold of hearing. Since we want to keep ourselves safe from hearing damage, 95 dB or so is about all the dynamic range we can use. But consider this: A very quiet living room has an ambient noise level 25-30 dB above the threshold of hearing. The inside of an automobile at reasonable driving speed is upwards of 60 dB above the threshold of hearing, and with a window open, all bets are off.

Average consumer audio systems aren’t capable of producing painful sound pressure levels (with the exception of those cars that cruise by with the bass pumping loud enough to rattle the windows in your house), a typical listening environment can only support a dynamic range of 65 to 75 dB.

The electronics in our recording chain are much better than that, with digital recorders and analog or digital consoles capable of 105 to 115 dB of dynamic range. This means that the listening environment, not the gear, sets the practical limit on dynamic range. If you see a number greater than that on a spec sheet, it’s purely

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User’s Guide

 

theoretical – a 24-bit system is theoretically capable of 144 dB of dynamic range but practical things like electronic components stand in the way of actually achieving it in practice, at least today.

Even in the best of control rooms, we have to squeeze as much as 115 dB of obtainable dynamic range into a 95 dB box. For the rest of the world, though, we have to squeeze harder so soft passages don’t get lost when your neighbor starts up his lawnmower, or when listening to the car radio at 65 miles per hour with the top down. Therefore we can’t record all the dynamic range that’s available if we expect people to hear all the music we record.

What a Compressor does

A compressor reduces dynamic range. When used properly, it’s hard to detect the action of a good compressor, but a compressor can also be used creatively as a sound shaping tool. A compressor can be inserted into a single channel in the recording chain when recording or mixing a track, or compression can be applied to an entire mix or sub-mix.

Let’s look at applying compression to a vocal track. Hard consonants such as the letter ‘T’ create a high initial sound level before settling down, whereas most vowels tend to be more even. The average volume level of a word may be fairly low, but because of an initial loud con- sonant, we can only raise the word’s volume so far before running out of headroom. If there’s music playing under the voice, even when boosting the vocal level as high as possible without distorting the attack, a word (or a syllable) may be far enough below the level of the music to become lost or misunderstood.

If we reduce the gain momentarily during the loud attack, then bring it back up when the distortion -risking blast is over, we can now boost the average level of the word so that it can be understood clearly over the music. What we’re doing here is reducing the dynamic range of the word, the difference between the loudest and softest parts. By doing that, plus boosting the