How dynamic processors work

(I'm writing this article as part of the Introduction to Music Production course on Coursera.)

In my last blog article, I mentioned dynamic effects as one of the main categories of plugins, and explained that they function as a sort of automatic volume control. Today, we'll look at the four main working parts of dynamics processors: threshold, ratio, attack, and release.

The threshold setting describes when the plugin kicks in. It is usually given in dB units. In a downward compressor, when the audio signal reaches the threshold, the compressor begins reducing the signal's gain.

The black curve represents the original audio
signal, and the red line represents the threshold.

The ratio setting describes how much the sound is modified. A 2:1 compression will reduce the signal above the threshold by half. This means that if the input signal is 2 dB over the threshold, the compressor will reduce it to 1dB over the threshold.

The blue curve represents the compressed
audio,  reduced by a ratio of 2:1.

The attack and release parameters control how quickly the plugin reacts. In a downward compressor, the attack is how long it takes for the compressor to start reducing gain after the threshold is reached, and the release is how long it takes for the compressor to stop reducing the gain once the signal drops below the threshold.

Compressors, limiters, expanders, and gates all have these four parameters. Limiters function very similarly to compressors, but have extremely high ratios. Basically, they never allow the audio signal to get more than a little bit above the threshold. Expanders are sort of like the opposite of compressors. The reduce the signal when it drops below the threshold, which increases the dynamic range, making quiet signals quieter. Gates are basically extreme expanders. Whenever a signal drops below the threshold, the gate will allow no signal through.