Guide to Compression in music production
Guide to Compression
Compression is a commonly used audio processing technique that reduces the dynamic range of audio signals by attenuating the volume of loud sounds and boosting the volume of quiet sounds. This allows audio engineers and producers to control the overall level of an audio track and improve its clarity, balance, and consistency.
Compression is achieved by applying a set of controls that adjust the ratio, threshold, attack, release, knee, and make-up gain of the compressor. Let's take a closer look at each of these controls:
The Controls
Ratio: The ratio control determines how much the compressor attenuates the volume of the audio signal above a certain threshold. For example, a 2:1 ratio means that for every 2 decibels (dB) of input above the threshold, the output level is reduced by 1 dB.
To better show this I have created a chart using fab filters C2 compressor, I have inputted a 1000hz/ 1Khz sine wave at -10db then set the threshold on the compressor to -20db giving us a reduction of 10db, with the different ratio settings shown in the chart you can see the gain reduction amounts.
C2 settings
Input sine-wave -10db
Threshold -20db
Attack time to 4ms
Release time to 10ms
Hard knee to 0db
Hold to 0ms
Style on Clean
| Ratio | Gain Reduction |
|---|---|
| 1.5:1 | 1.9 dB |
| 2:1 | 3.2 dB |
| 3:1 | 6.4 dB |
| 4:1 | 9.5 dB |
| 5:1 | 11.5 dB |
| 6:1 | 13.1 dB |
| 7:1 | 14.4 dB |
| 8:1 | 15.5 dB |
| 10:1 | 17.3 dB |
| 12:1 | 18.8 dB |
| 15:1 | 20.6 dB |
| 20:1 | 22.7 dB |
Threshold: The threshold control sets the level above which the compressor starts to attenuate the audio signal. For example, if the threshold is set to -20 dB and the input signal peaks at -15 dB, the compressor will reduce the volume by the amount specified by the ratio control.
Attack: The attack control sets how quickly the compressor responds to loud sounds above the threshold. A fast attack time can help prevent clipping and distortion, while a slower attack time can allow for more natural-sounding transients.
Release: The release control sets how quickly the compressor releases the attenuation after the input signal falls below the threshold. A fast release time can help maintain the track's dynamic range, while a slower release time can create a more sustained sound.
Knee: The knee control adjusts the curvature of the compression curve around the threshold. A hard knee means that the compressor applies the ratio immediately above the threshold, while a soft knee means that the compressor gradually applies the ratio as the signal approaches the threshold.
Hold time: Is a setting that determines how long the compressor will maintain its gain reduction after the signal falls below the threshold level. The gain reduction is maintained at a constant level, regardless of whether the input signal rises or falls. This can be useful for shaping the envelope of the compressed sound, and can help create a more sustained or "punchy" sound.
Lookahead: Is a setting that allows the compressor to "look ahead" to incoming audio and begin processing it before it reaches the threshold. This can help the compressor react more quickly and accurately to sudden peaks in the signal, resulting in smoother and more transparent compression. Essentially, it delays the audio signal slightly so that the compressor can process it before it actually hits the threshold level. The amount of lookahead can usually be adjusted in milliseconds, and the optimal setting will depend on the characteristics of the audio being compressed and the desired sound.
Make-up gain: The make-up gain control compensates for the overall volume loss caused by the compressor by increasing the level of the attenuated signal. This allows the track to maintain a consistent level throughout the mix.
Compression Types
There are several types of compressors used in today's modern world, including:
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Optical compressors: Optical compressors use a light-dependent resistor (LDR) to attenuate the output signal based on the level of the input signal. The LDR is driven by a light source, and the amount of light that falls on the LDR is determined by the level of the input signal. Optical compressors are known for their smooth and musical sound, and they are often used for vocal and bass tracks.
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FET compressors: FET (Field Effect Transistor) compressors use FET transistors as the gain-reducing element. They are known for their fast attack time and aggressive sound, and they are often used for drums and electric guitar tracks.
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VCA compressors: VCA (Voltage-Controlled Amplifier) compressors use VCA chips to control the level of the output signal. They are known for their transparent sound and versatility, and they are often used for a wide range of applications, from vocals and guitars to drums and bass.
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Vari-Mu compressors: Vari-Mu compressors use vacuum tubes as the gain-reducing element. They are known for their warm and smooth sound, and they are often used for mastering and mix-bus applications.
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Digital compressors: Digital compressors use digital signal processing (DSP) to control the level of the output signal. They offer a wide range of features and options, and they are often used for electronic music and sound design applications.
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Multiband / Spectral compressors: Multiband/ Spectral compressors divide the audio spectrum into multiple frequency bands and apply different compression settings to each band. They are often used for mastering and mix-bus applications, where precise control over the dynamic range is required.
Pictures shown links are Universal Audio, Fab Filter.
Compression has been used in audio production for decades, and the technology has come a long way since its early days. In the early days of compression, engineers used hardware compressors that were large and expensive. Today, digital signal processing has made it possible to achieve high-quality compression using software plugins and digital hardware units that are compact and affordable.
In conclusion, compression is a valuable tool for audio engineers and producers to control the dynamics of their tracks and achieve a consistent, balanced sound. The various controls of the compressor, such as ratio, threshold, attack, release, knee, and make-up gain, allow for precise control over the amount and character of compression applied. The different types of compressors available today each offer unique characteristics and are used in different genres of music. Understanding compression and its applications is essential for any aspiring audio professional
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