Many compressors give you the option of either hard-knee or soft-knee compression. Still others (especially software compressors) give you a slider to adjust the response of the “knee.” What does this mean?
“Knee” refers to how quickly the compressor clamps down on the signal once it surpasses the threshold. It’s called “knee” because if you look at a gain-reduction graph, the line right after the compressor starts reducing the gain looks like a sharp angle, like the break of a knee. If the angle is very steep, that means the compressor clamps down on the signal very quickly and hard, and the “knee” in the graph looks very steep, almost like the knee of someone sitting upright. If the angle is very gentle, the compressor kicks in very gently as the signal crosses the threshold, and the “knee” is “soft”, like the knee of a person sitting in a recliner.
Both hard- and soft-knee compression have their uses; two examples: if you want to squash a signal’s transients quickly, you’ll want hard knee compression. If you want to use a compressor to gently glue a mix together by tightening up transients, you’ll want a soft-knee compressor.