Typically, different instruments and performances call for different compression ratios. For example, to compress a ballad’s near-perfect vocal track, a mild 2:1 ratio would probably suffice; at that ratio, and with the appropriate threshold dialed in, the compressor tightens up the performance enough to ensure quiet phrases are not lost in the mix and higher levels are not overbearing. At the other extreme, a bass guitar track that alternates between mellow finger-pad technique and aggressive pop ’n’ slap can easily have a huge dynamic range. To yield consistent levels from that type of performance, a higher ratio such as 10:1 may be in order.

Note that threshold and ratio work together to affect a signal’s output level. The lower the ratio, the less control the compressor has on the signal; the lower the threshold, the lower the signal level subject to compression. The relationship between the two controls affords flexibility and sonic variation. There are, for example, two different-sounding ways to get the same amount of gain reduction out of a compressor—low threshold and low ratio or high threshold and high ratio.

Attack time is how long it takes—measured in milliseconds (ms) or microseconds (µ)—for the compressor to kick in once the signal exceeds the threshold. A slow attack time lets inherently fast transient signals pass threshold before compressing the rest of the signal; a fast attack catches transients, but may diminish high-frequency content.

One thing worth noting is that manufacturers sometimes measure attack times differently. Some specify attack time as the time it takes for the compressor to react after the threshold is exceeded, and others specify attack time as how long it takes for the compressor to reach, say, 67 or 90 percent of the maximum gain-reduction level it will ultimately achieve. Fortunately, the exact definition is of little importance, as typically attack time is set by ear. Depending on what kind of effect you’re going for, simply decrease the attack time until unruly peaks are tamed or increase it until average levels are lowered and desirable peaks get through unscathed. If you’re having trouble hearing your settings’ effect, watching a downstream peak-level meter (that is, one that monitors the levels after the process—the compressor’s output-level meter, for example) will let you visually confirm what portion of the sound is attenuated.

Release time is how long—measured in seconds or hundredths of a second—it takes for the compressor to return the signal to unity gain (its unprocessed state) after the signal falls back below threshold. That is, once the release time passes, the compressor lets the signal pass through unaffected. In general, slower release times result in a more natural sound.

In general, set fast attack and release times when you want the compressor to do its job and get out of the way quickly—for instance, when you want to put a lid on transient guitar plucks but allow the ringing notes to pass through unaffected. Conversely, a moderate attack time coupled with a long release is perfect for those David Gilmour–esque guitar solos in which you want notes to sustain forever. At two seconds or longer, the extended release time causes the compressor to slowly restore compressed levels to their original (higher) gain, just as the sustained notes start to naturally die off, which counteracts the decay and makes the tails of the notes louder.

A compressor’s last control stage is its output level. That control is also known as make-up gain because it is used to make up for the gain reduction caused by the compressor. The usual approach is to increase the processed signal’s output level so it matches the unprocessed signal’s level. That creates unity gain between the two signals, which makes it easier to compare them using the bypass switch and ensures appropriate levels when recording or mixing.

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Reprinted with permission from Magazine, February, 2001
© 2001, Intertec Publishing, A Primedia Company All Rights Reserved