Compression, short for dynamic compression, is a topic that confuses pros and amateurs alike. In fact, it can take some people years to thoroughly understand how it is affecting our audio signal and to fully utilize the many applications of dynamic compression. I believe the confusing aspect of this effect is that the difference in sound can be quite subtle, making it difficult to associate the turning of dials with the change in sound.
Here’s a step by step of what happens to a signal going through a compressor. First, a volume threshold is set. The audio signal needs to go past this threshold in order to activate the compressor. You also need to understand that a compressor will only affect the part of the audio that crosses this threshold. You can also control how fast the compressor reacts to the signal going above and below this threshold by controlling the “attack” and “release” setting. These controls are important in targeting the portion of the signal you want affected/unaffected. Once the compressor is triggered, the signal is reduced by a set ratio (oftentimes 2:1, 4:1, 8:1). Say for example, a signal that is 6dB over the threshold will now only be 3dB loud with a ratio of 2:1.
The threshold is the minimum amount of volume needed to engage the compressor effect. Therefore, if the signal you are dealing with is too quiet and below the set threshold, the compressor will never engage. The compressor releases after the signal falls below threshold. On many stompbox compressors, the input gain is adjusted relative to a fixed threshold. In other words, you control how much signal goes past the threshold by adjust the volume of the signal, instead of adjusting the threshold itself.
As explained earlier, this is the ratio at which the original signal is compressed. The greater the ratio, the more dynamic range (difference between loud and soft parts) is lost. For guitar pedals, this ratio tends to be fixed but generally set around 4:1 or 8:1. When the ratio is 20:1 or greater, it is considered to be functioning as a limiter and has other uses associated with it. Lower ratios will have a much more transparent effect on the sound.
The attack and release parameters may also be fixed on certain guitar pedal compressors. These controls adjust the speed at which the compressor engages and releases when crossing the threshold, and is typically measured in milliseconds. It’s important to adjust attack so that it is affecting the part of the sound you want to, namely the transient or the tail. The transient is the “attack” of the guitar string when being picked. If you were to look at a visual representation of a guitar note in a DAW, you will see the attack as the loudest portion of the sound and the tail of the note as a steadily decaying waveform. Generally speaking, you want to set the attack at a speed which avoids compressing the transient, and engages on the tail. The release setting will be more dependant on the style of guitar you are playing. A funk rhythm guitar will likely have a quick release so that it doesn’t affect the next strum, where as sustained lead notes will have a slow release to keep notes from dying out too soon. Some digital compressors will have an automatic release control that responds intelligently to your playing.
A compressor, by definition, will reduce the overall volume of a sound. The make-up gain is there to “make-up” for any lost volume by adding some gain after the effect has taken place. It is meant as a corrective tool, so all you need to do with it is to set it so that the processed sound is at the same volume as the original signal.
If your guitar tends to have loud/soft spots, or you’re still having problems controlling the volume of your picking or strumming, there is a lot to be gained by using a compressor. What you may lose in dynamic range, you will gain in dynamic consistency which is an important part of genres such as funk. Furthermore, it can also be a way of protecting your precious equipment by reducing sudden spikes in volume that can harm speakers and overload power amps. Tonally speaking, it’s a great way to add punch and warmth to your guitar sound in a subtle way and might just be the thing you need to take your sound to a “professional” level. Have fun!