How to Use A Compressor/Limiter. Some of the most beloved processing is also the most potentially detrimental. In the hands of an artist, a compressor can be used to carve out a beautiful sonic sculpture, but in the hands of a novice it can completely ruin the music.
I will give you a rudimentary introduction to the operation of a compressor/limiter.
It is important to first understand the controls used in the design.
1) Threshold 2) Attack 3) Release 4) Compression ratio 5) Make-up gain
Threshold (occasionally called the ceiling) sets the point at which the automatic volume reduction kicks in. Anything below that volume will result in no compression. When the input goes above that level, the compressor reduces the volume automatically to keep the signal from getting louder.
The attack time determines how quickly the volume is reduced once the input exceeds the threshold. If too slow, a short burst of loud, unwanted music can escape. When using a compressor as a tool to prevent overload you want a very fast attack time. But when used on an electric bass, for instance, to obtain more punch, 20 to 50 milliseconds is often adequate because it allows the attack to penetrate before the volume is reduced. So each note has a little extra "definition" without the full length of the note being too loud. It reacts to the initial transient and then tapers off.
The release time determines how quickly the volume returns to the original position when the input is no longer above the threshold. If it's too fast you'll hear the volume as it goes up and down. That sound is called "pumping" or "breathing." This sound is often desirable for creating special effects, synthesizers, drums and other instruments. The appropriate setting depends on whether you intend to use the compressor as a dynamic tool or as an effect to create a unique sound, or even add more sustain to an instrument. If you would prefer more transparency for the compressor, set the release time relatively long--one second or more. If you want an aggressive, in-your-face sound use a shorter release time.
The compression ratio determines how much the level is reduced relative to how far beyond the threshold the signal occurs. A ratio of 1:1 does nothing. 2:1 means if the input rises to 2 dB above the threshold, the compressor will reduce the level by only 1 dB, so the output will now be 1 dB louder. 10:1 means the signal must be 10 dB above the threshold for the output to increase by 1 dB. When a compressor is used with a high ratio--say, 5:1 or greater--it is considered a limiter. In fact, the compression ratio is the only distinction between a compressor and a limiter. We often refer to limiters as compressors. One of the most revered is the Urei/Universal Audio 1176 Limiting Amplifier. I tend to apply it to practically every lead vocal. At higher ratios it works well at holding the vocal in place. It's very common for me to use the 8:1 or even 12:1 settings.
The makeup gain will compensate for the loss of volume relative to how much has been reduced by the compressor itself and return it to an adequate level.