Describes the action of a compressor as the signal level crosses the threshold setting. A compressor functions by processing a signal above a certain threshold. Without a soft knee setting, the compressor would immediately begin processing the moment the threshold was reached. In fact this is how a hard knee setting works. But in certain circumstances this may be unmusical, so the soft knee setting allows compression to begin gradually and reach full processing only after the signal has reached a predetermined level above the threshold.
(See Compressor)