Compression is a very important tool to a mix engineer. Unlike volume and EQ, however, compression can sometimes be difficult to hear. Where EQ adjusts the tone of the sound, compression adjusts the dynamics.
The simplest way to understand compression is as a process that automatically turns the volume down when the input sound gets too loud (and then turns it back up when the input sound gets quieter again). Basically, compression makes loud sounds quieter.
Typically, compressors will have four main controls:
- Threshold – This is the sound level which is considered ‘too loud’. When the input sound gets louder than this, it is turned down. When the input sound later drops below this level, it’s turned back up. The lower the threshold, the more compression will occur.
- Ratio – This is the amount by which the sound is turned down. It’s usually expressed as a ratio (e.g. 2:1) but you don’t need to understand the maths in order to use this. Quite simply, lower ratios (such as 2:1) mean the volume isn’t turned down much and higher ratios (such as 20:1) mean the volume is turned down a lot.
- Attack – This is the speed at which the volume is turned down. Normally this should be pretty fast (low numbers). If the attack is too fast, however, sometimes the sound can become too soft or even distorted. A slower attack can make the compression more gentle, but if the attack is too slow the compression will be ineffective.
- Release – This is the speed at which the volume is turned back up when the input sound level drops back below the threshold. Lower values (fast release) will make the compression more audible. High values (slow release) will make the compression smoother. Very high values will make the compression almost inaudible.
Compressors can be very versatile tools, and some have a distinctive sound (behaviour) of their own. As a starting point, try these approaches:
- First, not all sounds need compression. Try compression, but don’t be afraid to go without if it’s not actually improving the sound.
- For smooth compression on melodic instruments (such as vocals or other acoustic instruments), start with a
- low ratio and a threshold set so that the compressor is active most of the time. Set the attack as fast as you can without distortion and set the release to a medium speed. To make the compression stronger and tighter, raise the ratio. To make the compression smoother and gentler, increase the release time.
- For tight control, use a high ratio and a low threshold (similar to above – so that the compressor is active most of the time). Use the fastest attack and release times you can get away with (without getting distortion or other strange sounds).
- For punchy drums, use a longer attack time and medium release time. Make sure the threshold is set high enough that the drum hits well above the threshold but quickly drops below it. Higher ratios produce more extreme effects. Longer attack times will add more of the initial ‘thwack’ (the transient). The release time will have to be tuned by ear until it works with the length of the drum decay.