Compression is a very useful effect that for the most part remains a bit of a mystery to those who use it. In very simple terms, it boosts the level of soft notes whilst the louder signals are reduced, effectively averaging out the overall audio signal, and resulting in a much smoother, more evenly consistent sound. In effect, it’s like an automatic control that adjusts the volume knob far more effectively than a human could do manually. It is normally placed first in an effects chain as it gives any effects following it a smoother signal.
For guitarists, compression has many applications. It can help finger picked acoustic passages sound smoother due to being able to compensate for the slightly inconsistent attack associated with that style. In the same way, it enhances percussive rhythm playing and is great for funk, reggae or ska playing. Compression can be also very useful for lead playing as it enhances sustain by subtlety boosting the signal when it would normally fade.
Settings used in Compression
This is a benchmark level and the signal is not allowed to exceed this until it hits a pre-determined amount beyond. When this happens, the compressor kicks in and reduces the signal by a second pre-determined amount. This is explained more in the paragraph about ratio. The Threshold is calibrated in dBs.
The Ratio determines the amount of gain reduction applied. It’s usually expressed as 4:1, 3:1, infinity :1 and similar. A compression ratio of 3:1 will mean that a signal must exceed the Threshold by 3dB to allow for a level 1dB above the Threshold – it is reduced by 2 dB from 3dB above, to 1dB above, hence the ratio 3:1. If it was 6:1 the signal must exceed the Threshold by 6 dB to allow a signal 1 dB above the Threshold – reducing it by 5 dB and so on…
A setting of infinity:1 is more accurately described as a “Limiter”. In this instance, the output level is never allowed to rise beyond the threshold.
Hard Knee and Soft Knee
With a hard Knee compressor, the full quota of gain reduction kicks in only when the signal hits the Threshold. With a Soft Knee compressor, the gain reduction is brought in progressively, usually when the signal comes within 10 dB of the Threshold. This results in generally a gentler, more natural sounding effect.
This determines how long it takes for the compressor to kick in and produce the gain reduction. It’s usually between 1 and 20 milliseconds. The application of this parameter is dependent on the nature of the signal being effected. As an example, a slow attack allows the percussive qualities of instruments such as drums and rhythm guitars to come through at the very start before the rest of the signal is evened out. For vocals, it is usual to use a fast attack to result in a consistently smooth sound.
This determines how long it takes the compressors gain to come back up to normal once the input signal has naturally dropped back below the level of the threshold. If it is set too fast, it can result in a ‘Pumping’ effect to the signal.
Useful Compression Settings:
Attack: 5 -10 ms
Ratio: 5 – 10:1
Attack: 2 -5 ms
Recommended Guitar Compression Units