Compressor

What is a compressor

Compressor is an audio processor, which reduces the differences between the weakest and the strongest audio signal. Basically, it will help you to equalize your recording in a way that all parts will sounds approximately equally loud. Which means the loudest places are muted according to your choice of parameters, while the weak points are left untouched. You can use it on track for compressing individual instruments or voices, such as guitar, drums, vocals and also for the overall mix.

To understand the compressor it is first necessary to describe the basic types of settings.

Threshold

It is the limit of the signal at which the compressor is switched on. For example, it may be -10dB. The louder signal will be lowered by adjusting the ratio. The signal can be measured by its volume – Peak level. The human ear however perceives a signal in much more complex way. We call it its total acoustic power – RMS level. Some compressors can therefore be switched on this measurement. A special way of measuring is the so-called low- level, where the bass frequencies are considered for the final signal level.

Ratio

It is the ratio by which the signal will be muted (1: n ) or amplified ( n : 1). This ratio , however, may not be linear.

Knee (Waveform)

It is the shape of the curve. It can be linear (hard knee) and logarithmic (soft knee). Some compressors are also able to simulate the curve of the old tube compressors and thus simulate the old school sounds. Sometimes these compressors are referred to as vintage compressors.
The best way to describe it is in the picture below:

The X axis indicates the input signal level and the Y-axis output signal level. The curve shows how the sound will be muted. In our case, the compressor is set to a ratio of 1:2 and Threshold to -20 dB.

Attack

The time from the moment when the signal entered into the compressor until the compressor starts to operate. This will produce a more natural sound.

Release

It is the time at which the compressor stops compressing the sound after that the signal falls below the threshold. This is a very important setting as it affects how the recording will breathe.

Holdtime

It is a sort of extension of the compressor operation. It is used mainly for bass when the compressor operation in the classical setting is too fast and the sound is badly dyeing.

Before we describe the specific examples of using, there is necessary to explain what we shall avoid.

Change the color of sound

During strong compression, the signal will be more balanced, but loses its dynamics and the recording will not breathe properly. Because the loudest parts are usually the bass and treble sound, recording will be more centrally positioned, and especially the bass loses its dynamics.

Distortion

Distortion arises for example when we set a bad attack and release settings. If you set the attack time too short, the sound will sound unnatural. If the release is too long, it can change the sound. This could be seen for example on kick, where these values play an important role.

Using of the compressor

  1. Each time start with compressor off, or ratio of 1:1 and without threshold. It is important to first listen to the sound sample throughout its whole length and find out what we want to achieve.
  2. Set ratio according to the type of track. Set higher ratio for voice or live recording. To the contrary, for the mere deletion of records select a smaller ratio. You can experiment, in which case it is advisable to start from the ratio of 1:10.
  3. Set the threshold so that there is a small reduction in sound, but that it still sounds natural. Listen to the recording and set the appropriate value of the threshold.
  4. Set attack and release. It is a little bit harder, because it takes a lot of listening. Please note that the compressor with a shorter release moves the sound to the center strip. Use the equalizer after the compressor if necessary.
  5. Set knee – use soft knee for warmer and more natural sound.
  6. Repeat procedure until you are satisfied with the outcome.
  7. Do not be afraid to experiment and engage two different compressors to be consecutive. Each compressor is different and provides different sound which can be used. Or vice versa, using of an unsuitable compressor can drastically reduce the sound quality.

If you don’t want to spend time manually adjusting the compressor, use predefined presets, which almost every compressor has. At least as a base for further editing, presets are good and quick.

Basic settings for individual instruments

Kick
Ratio 5:1
Attack 50ms
Release 100ms

Snare
Ratio 4:1
Attack 38ms
Release 800ms

Contra bass
Ratio 3:1
Attack 60ms
Release 300ms

Acoustic guitar
Ratio 6:1
Attack 1ms
Release 100ms

Electric guitar
Ratio 5:1
Attack 60ms
Release 160ms

El. Piano
Ratio 2:1
Attack 23ms
Release 100ms

These settings are only indicative and I recommend it be used more as a basic guide. The final adjustment will be governed by sound quality, color of sound and overall dynamics. Threshold set by ear.

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