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The kink in a compressor's transfer curve graph is known as the compression knee, due to its distinctly bent shape. The knee sits at or around the threshold point and represents how much and at what amplitude level the compressor will attenuate the signal. Some compressors allow us to change this knee from what is known as hard knee, or the full ratio of compression, as soon as the signal passes the threshold to what is known as soft knee, a more gradual form of compression where the signal is eased into the compressor's ratio setting over a larger threshold range.
I say range because in a soft-knee compression curve, the threshold is no longer a single point on the graph, but begins and ends over a larger range of signal values centered around the original threshold setting, starting with a lighter ratio and working its way to the full amount as defined by the ratio control. Again, this concept is best represented by a transfer curve graph. Imagine a compressor with the ratio of 10:1 and a threshold at -20 dBFS.
In a soft-knee compression curve, the compressor would begin to attenuate the signal earlier, at around -30, with a ratio of 2:1. Maybe you're amping up to 4:1 at -25, 8:1 at -20, and finally reaching 10:1, just pass the threshold, at around -15 dBFS. On some compressors, the knee value will be a simple switch, allowing you to choose between a hard or soft compression curve, while on other compressors, this control may be defined in decibels, representing the threshold range over which the compression will ease into the full ratio.
Many compressors will not have this control at all and will either be fixed as hard-knee or soft-knee compressors or somewhere in between. For instance, the Dbx 160 doesn't have a knee control, but it's known for its over-easy soft-knee setting, which often sounds great on vocals and bass. As a general strategy, I like to use hard-knee compressors when I am working with the internal dynamics of a signal's envelope, especially on drums and percussion, like drawing out the attack of a snare or kick drum. The hard-knee transition at the threshold gives me tons of control over the signal's envelope.
Likewise, soft-knee compression is great on less percussive material like vocals, bass, and guitar, where you might not want to hear as much of an edge, but still have a nice firm signal sitting in the mix. Soft-knee compression can also work well on full mixes, where you're trying to subtly glue signals together without a lot of attitude from the compressor. So if you've never experimented with a compressor's knee, I think you'll find that it adds a lot of flexibility to dynamic control.
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