Understanding the knee setting on a compressor: hard knee & soft knee

Understanding the knee setting on a compressor

In this article, we’ll learn how to use the knee setting on a compressor. Alongside the other settings which control the way your compressor works, such as the threshold, ratio, attack, and release, the knee is a great setting to fine-tune the way your compressor applies compression to audio signals.

Some compressors allow you to switch between ‘hard knee’ and ‘soft knee’ settings. Others allow you to dial in how hard or soft the knee setting is based on a decibel value. Some compressors don’t allow you to alter their knee at all. Nevertheless, I think that the knee setting on a compressor is a really useful tool. That’s because the knee setting allows you to alter a characteristic of compression which the other settings do not.

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Hard knee compression

A hard knee setting on a compressor means that any parts of your audio signal which cross the threshold will be compressed to the compressor’s full ratio value. With a hard knee, the compressor does not apply any compression until the audio signal crosses the threshold.

Soft knee compression

A soft knee setting applies compression differently to a hard knee setting. A soft knee setting will apply the compression gradually until the full ratio amount is reached. This makes the transition from uncompressed to compressed smoother and more transparent. Also, soft knee settings do not wait until the threshold has been crossed to apply compression. Instead, the compressor will gradually apply compression to the audio signal as it approaches the threshold and will not reach the full ratio amount until a point past the threshold.

Dialing in the knee setting on a compressor

Instead of simply switching between hard knee and soft knee settings, some compressors, such as the compressor/limiter 3 plugin in Pro Tools First, allow you to dial in the hardness or softness of the knee by specifying a decibel value. This allows you to define the decibel range over which the compressor builds from no compression to the full ratio amount. The threshold usually sits at the center of this transition zone. In this case, the lower the decibel amount, the harder the knee will be. The higher the decibel amount, the softer the knee will be.

A hard knee setting on a compressor is great for instruments that have fast peaks, such as drums. Meanwhile, a soft knee setting can work well for things like vocals which benefit from a more gradual transition.

If your compressor allows you to toggle between hard knee and soft knee, try switching between them and see if you can hear the difference. Alternatively, if you can specify a decibel value for your knee, try different values and see how they sound.

How do you like to set your knee value for different instruments? Leave a comment below.