Compressor

A compressor is a volume-based plug-in and one of the most useful tools in your DAW. Its primary purpose is to shrink the dynamic range of a track.

The dynamic range of a track is the difference between its highest and lowest recorded levels. A vocal track, for instance, will typically have a higher dynamic range, due to both the changes over time in the vocalist’s loudness and his or her distance from the microphone. A compressor monitors the level of a track over time and brings the highest peaks down while leaving the lower levels unaffected. A compressor is basically an automated volume fader.

A compressor can have many different parameters, but in the interest of brevity I’ll only cover the core parameters that you’ll find on most compressors: “threshold,” “ratio,” “gain,” “attack,” and “release.”

The threshold determines the level at which the compressor will start working. Anytime the level of the track exceeds this level, the compressor will lower the volume.

The ratio is what determines by how much the compressor will reduce the volume of the track above the chosen threshold. The ratio will always appear in the following form:

x:1, where “x” is a number equal to or higher than 1.

The significance of this ratio is that for every “x” number of decibels that the track’s level exceeds the threshold before the compressor’s effect, the track’s level will exceed the threshold after the compressor’s effect by only 1 decibel. That might seem complicated, but here’s an example:

Let’s say the threshold is set to -12 dB. So long as the track’s level never exceeds -12 dB, the compressor will have no effect whatsoever on the track’s volume.

Let’s say, however, that the track’s level does exceed -12 dB from time to time. Let’s also say that the ratio is set to 3:1.

If the track’s level exceeds the threshold (-12 dB) by 3 dB (-9 dB) before the compressor’s effect, the compressor will reduce the volume so that it’s level exceeds the threshold by only 1 dB (-11 dB). This is a reduction of 2 dB (from -9 dB to -11 dB).

Now let’s say the track’s level exceeds the threshold by 6 dB (-6 dB). Now the compressor will reduce the volume so that the track’s level exceeds the threshold by 2 dB (-10 dB), 1 dB for every 3 dB by which the track’s level originally exceeded the threshold. That’s a reduction of 4 dB (-6 dB to -10 dB).

The same two peaks, which were originally 3 dB apart (-9 dB and -6 dB), are now only 1 dB apart (-11 dB and -10 dB).

A ratio of 1:1 would have no effect on levels exceeding the threshold. The higher the ratio, the greater the reduction in volume.

The purpose of the gain parameter of a compressor to bring the reduced highest peaks of a track back up to their original volume, so that the overall track sounds about the same volume after the compressor’s effect as it did before, even though its dynamic range has been reduced. It does this simply by raising the volume of the compressed track as a whole, which brings the lowest levels (anything under the threshold) up by the chosen amount (the amount of greatest reduction) and closer to the original peaks.

The attack and release of a compressor determine how quickly or slowly the volume is reduced when the track reaches the threshold and how quickly or slowly it is returned to its original volume when it drops back below the threshold.

The reduced dynamic range of a track can give it “life” in a mix and allow it to be better heard in the context of a dense arrangement. A vocal track, for instance, can easily get lost in a mix before the use of a compressor. Compressing the signal and using the proper amount of make-up gain will bring out some of the quiet parts of a performance while leaving the loud parts near their original level, allowing the listener to more clearly hear the singer’s voice throughout the song.