Ever wanted to get a more even tone when playing finger-style acoustic guitar? How about enhancing your percussive rhythm strumming when playing styles such as funk, reggae or ska? Compression for Guitars might be the answer. In simple terms, this effect produces a smoother, more evenly consistent sound by elevating the level of soft notes, whilst the louder signals are reduced. This has the effect of averaging out the overall audio signal. Think of it as an automatic control for adjusting the volume that is far more effective than what a human could do manually by manipulating a knob or fader.
When using compression for guitars, it is normal to place it first in an effects chain, as it gives any effects following it a smoother signal. Compression is very useful for lead guitar playing as it enhances sustain by boosting the signal when it would normally fade. There are many options for the guitarist, ranging from foot pedals through to digital rack-mounted units and in-built compression in some mixing desks. Depending on what unit you have access to, there are different levels of control available, but the basic settings are as follows.
Threshold
The threshold is basically the benchmark level above which the signal is not allowed to exceed – until it hits a pre-determined amount beyond. When the pre-determined level is reached, 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 commonly calibrated in dBs.
Ratio
The Ratio determines the amount of gain reduction that will be applied to the signal and is the crux of the whole idea behind compression. It’s usually expressed as:
6:1
4:1,
3:1,
infinity:1 and similar.
If you have a compression ratio of 3:1, this will mean that a signal must exceed the pre-set Threshold by 3dB to allow for a level 1dB above the Threshold. So, it is reduced by 2 dB from 3dB above, to 1dB above, hence the ratio 3:1. If you have a setting of infinity:1, this is more accurately described as a ‘Limiter’. This is because the output level is never permitted to rise beyond the pre-set threshold.
Attack
Usually measured between 1 and 20 milliseconds, the attack determines how long it takes for the compressor to kick in. How you use the attack parameter is dependent on the nature of the signal being effected. For example, a slow attack allows the percussive qualities of instruments to come through at the very start. The remainder of the signal is then gradually evened out. This is very handy for drums and rhythm guitars. A fast attack on the other hand will result in a consistently smooth sound and as a result, is ideally suited to vocals.
Release
The release determines how long it takes for the compressors gain to come back up to normal once the input signal has dropped back below the level of the threshold. You need to be careful with this setting as if it is set too fast, the result can be a pumping effect to the signal.
Hard Knee and Soft Knee
This setting will determine how aggressive the compression will be. A Soft Knee compressor allows the gain reduction to be brought in progressively, usually when the audio signal comes within 10 dB of the Threshold. This produces in a gentler, more natural sounding effect.With a hard Knee compressor, the full gain reduction will kick in when the signal hits the Threshold. So it’s a more abrupt compression.
Useful Compression Settings:
Electric Guitar
Attack: 2 -5 ms
Release: 0.5seconds/Auto
Ratio: 8:1
Knee: Hard
Acoustic Guitar
Attack: 5 -10 ms
Release: 0.5seconds/Auto
Ratio: 5 – 10:1
Knee: Soft.