As touched upon previously, some dynamics processors have filtering or EQ in their sidechain, causing them to react more (or less) to certain frequencies. This is known as frequency-conscious dynamics processing, and the most common form is the de-esser, a tool used to control high-frequency sibilance in vocals. However, the same concept is also found in general purpose frequency-conscious dynamics and dynamic EQs. .
For a de-esser or frequency-conscious compressor, the filtered sidechain signal feeds (aka keys) the detector, so gain reduction is caused predominantly by the focus frequencies. In the case of a dynamic EQ, the sidechain keys the gain of a filter circuit, and so it’s basically a specialised EQ and we therefore won’t be covering it in this article as we want to focus on the core dynamics processors (note, however, that some de-essers do use this technique of dynamically controlling EQ filters).