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Chapter 4 Dynamics 39
Using the Compressor
The following sections provide information on using each of the main Compressor
parameters.
Threshold and Ratio
The most important Compressor parameters are Threshold and Ratio. The Threshold is
the level (in decibels) above which the signal is reduced by the amount set as the Ratio.
Because the Ratio is a percentage of the overall level, the more the signal exceeds the
threshold, the more it is reduced. For example, with the Threshold set at –6 dB and the
Ratio set to 4:1, a –2 dB peak in the signal (4 dB louder than the threshold) is reduced
by 3 dB so that it is just 1 dB above the threshold, while a +6 dB peak (12 dB above the
threshold) is reduced by 9 dB so that it is 3 dB above the threshold. The scale of
dynamics is preserved, but the differences between the peaks are evened out.
Attack and Release
After Threshold and Ratio, the most important parameters are Attack and Release. You
use the Attack and Release parameters to shape the dynamic response of the
Compressor. The Attack parameter sets the amount of time after the audio exceeds the
threshold before the Compressor starts reducing the signal. For many sounds, including
voices and musical instruments, the initial attack is important for defining the sound,
and setting the Attack higher ensures that the original attack is not altered. To
maximize the level of an overall mix, setting the Attack lower ensures that the
Compressor starts reducing the signal right away.
Similarly, the Release parameters controls how quickly the Compressor stops reducing
the signal once it falls below the threshold. Setting the Release higher makes the
difference in dynamics smoother, while setting it lower can make the difference more
abrupt. Adjusting the Attack and Release parameters properly can help avoid pumping,
a common side effect of compression.
Knee
The Knee parameter smooths the effect of the Compressor by controlling whether the
signal is slightly compressed as it approaches the threshold. Setting the Knee close to 0
(zero) means that levels just below the threshold are not compressed at all (1:1 ratio),
while levels at the threshold are compressed by the full Ratio amount (4:1, 10:1, or
more). This is what audio engineers call hard knee compression, which can cause the
transition to be abrupt as the signal reaches the threshold. Increasing the value of the
Knee parameter applies some compression to the signal as it approaches the threshold,
creating a smoother transition. This is called soft knee compression. Setting the Knee
parameter controls the shape of compression around the threshold, while the
Threshold and Ratio parameters control its intensity.