The Knee function appears in some compressors and has two parameters the Soft Knee, and Hard Knee.
The Hard Knee makes the compressor stricter, and starts compressing from the point where the sound exceeds the Threshold at the ratio we have specified, from beginning to end. That is, the Knee is related to the intensity of the Ratio during the beginning of the compression (in the first moments before crossing the Threshold).
What a Soft Knee does is to create a smoother transition between the uncompressed signal and the compressed signal. The Hard Knee helps us keep the Transients and make the sound rougher and the compression faster for fast sounds (Drums for example). The Soft Knee could be used for more subtle sounds, like vocals or a violin.
Become an expert in compression with the recommended book “ Compression in Music Production” (click on the image to go to the link):
