The release time is how long it takes the audio signal to revert from being affected to uncompressed. At low levels of gain reduction, fast release speeds sound the most natural. However, extreme compression with a fast release time can result in a pumping effect — or a sudden increase in volume — that can make the track sound uneven in some genre of music.
The knee is how quickly the compressor transitions from not compressing the signal to compressing it. A soft knee will sound smoother because the compression doesn’t begin as quickly, while a hard knee will sound more dramatic. “Bass guitar and vocals would sound better using a soft knee, but I would use a hard knee for any kind of percussive instrument, like the piano or kick drums,” says producer, engineer and mixing artist Gus Berry.
Make-up gain or output gain
The level of the signal coming out of a compressor won’t be as high as it was when it went in because the compressor is actively turning down parts of the signal. Make-up gain is used to turn the overall level of the signal back up. “The way you do this is to play the material while bypassing and activating the compressor, adjusting the make-up gain to where they sound more or less the same level,” advises Rodocker.
The five main types of compressors.
In addition to what controls you set, the type of compressor you choose will determine the overall effect on the sound.
Best for: Bass, drums and vocals.
The tube compressor is one of the oldest types of compression, used by the Beatles and Motown. “They usually have slower attack and release times and they have a specific sound that people often think of as ‘old school’ or vintage,” says Rodocker.
Best for: Aggressive material like drums, percussion, electric guitars, vocals.
An FET or “Field Effect Transistor,” emulates the tube sound with transistor circuits, making it a fast and reliable compressor that produces a clean sound.
Best for: Percussion, electric bass, vocals and live sound.
Parallel compression is achieved by mixing an unprocessed or lightly compressed signal with a heavily compressed version of the same signal. This reduces the dynamic range by bringing up the softest sounds.