Specifications

4.1.1 Why a Compressor ?
A compressor is designed to reduce the dynamic range of an audio signal. That means difference between the quietest and
the loudest parts of the sound. This prevents the sound to disappear into a mix by the masking of surrounding louder
elements. This also prevents the sound to totally cover other instruments during loud parts.
The traditional approach is to reduce the signal level in a certain ratio (2 to 1, 4 to 1, 20 to 1…) as soon as a threshold value
is reached (-5 dB, 0 dB, +10dB…)
This threshold point can be smoothed with the knee value. If the knee is set to hard, there is no smoothing. If the knee is set
soft, the application of the set ratio reaches progressively around the threshold point.
A latency can be introduced before this gain reduction. This Attack time can be set from few milliseconds to hundreds
milliseconds.
A latency must be introduced after this gain reduction. This Release time can be set from tens millisecond to seconds.
Depending on the original dynamic content, those two time related parameters must be set carefully. A too short release
time may produce a pumping effect. It can be deliberate but it’s often an unwanted artifact. A too long attack time may
produce a totally ineffective compression because of its too slow action.
The compressor action produces a gain reduction expressed in dB. Usually this value is displayed on the compressor.
Once compressed the sound level has been diminished since louder parts have been minimized. The level needs to be
manually increased except if the compressor features a Make Up function. Now the processed sound doesn’t disappear in
the mix because its average level has been increased, and doesn’t cover other instruments because the level of its louder
parts has been reduced.
The processed signal is easier to mix because its level is more constant and the sound is denser.
If a compressor is set with a 20 to 1 ratio, it acts as a limiter. The sound level never exceeds the threshold value more than 1
dB for 20 dB of variation.
During mastering the compression is used in a different way. Except a limited correction of mixing errors, the goal is usually
to increase the density by controlling the peak levels to magnify the mid levels where the essence of the sound lives. A tight
control of the dynamic can balance a rhythm section and lead vocal to closely serve the artistic purpose.
The compressor action (blue area) is on the right hand side of the threshold. It’s under the diagonal of the 1:1 ratio (gain
reduction).
4.1 Introduction to the Dynamic Why a Compressor ?
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