Compression can be confusing. It is not always an obvious effect, but it can easily be abused and if not set correctly the results can be a bit of a disaster.
So what is it? Compression acts like an automatic volume control, boosting the lowest parts of the audio signal and attenuating (lowering) the loudest signal peaks. This delivers a more consistent volume across the signal.
Compressors in the form of a guitar pedal might only have a couple of controls (though some actual only have an on or off switch!), though some have considerably more than that.
The MXR Dyna Comp Mini we demo in the video below has an output knob to set the overall level of the pedal and a sensitivity knob to set the level of compression. This pedal also features an attack knob, which switches the knob from slow to fast attack; which determines how quickly the effect kicks in.
The MXR Studio Compressor for comparison also has a ratio control, and seperate attack and release knobs. This makes the pedal more versatile but also more confusing for the novice when it comes to using the pedal, and not as easy to adjust on the fly at a gig.
So what can it do for my sound?
A compressor can make the volume more even, make the guitar tone smoother, thicken the sound, make it tighter, and also even out the dynamics. Compressors are great for funk and slide playing in particular, though some players like to use them as a subtle set and forget effect for everything they do.
Compression can also give the note more ‘snap’, so heavier settings are often used for country guitar (and similar to Mark Knopfler’s guitar tone on the early Dire Straits albums) and increase sustain. David Gilmour (Pink Floyd) uses a Dyna Comp compressor for this effect. The compressor can add sustain and ‘weight’ to the tone without adding in extra overdrive or distortion.
And of course they are not just used with clean sounds. Some players will use a compressor with an already driven amp to act as a booster to hit the front end of the amp even harder, or for gain stacking with pedals.