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

Understanding the knee setting on a compressor hard knee vs soft kneeUnderstanding the knee setting on a compressor

In this article, we’ll learn how to use the knee setting on a compressor. Along side 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. Never the less, I think that the knee setting on a compressor is a really useful tool. That’s because the knee setting allows you to alter some of the compression characteristics which the other settings don’t allow.

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 ratio value. The compressor’s attack setting controls how quickly the compressor turns the signal down once it has crossed the threshold. 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 is reached. This makes the transition from uncompressed to compressed audio smoother and less abrupt. 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 somewhere past the threshold.

Dialing in the knee setting on a compressor

Instead of simply switching between hard knee or 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. 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 which have fast peaks, such as drums. A soft knee setting is better for instruments such as piano or vocals which benefit from a gradual transition into compression.

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.

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