Don’t know what a compressor is yet? You will soon. If you want to learn how to mix hip-hop, and/or learn how to mix rap, you will use compressors often. For a basic intro to compressors, read on.
Compressors Control Dynamics
Compressors are used to control a sound’s dynamics. Think of them as a smart fader. Compressors make the “louder” parts softer by a certain ratio (more on this shortly). You can use a compressor to reign in abnormally loud sections of sound. You can also use a compressor to even out the volume of a sound over time.
Parameters Make A Difference
You decide what the compressor does to your sound by manipulating the compressor’s parameters. The most important compression parameter is gain ratio. Gain ratio relates the level going in to the compressor to the level coming out of the compressor. In less technical terms, it’s how much the sound gets “squished” (OK, that might not have been technical at all.). A ratio of 3:1 means for every 3dB (dB stands for decibel, which for our purposes is a measure of volume) that comes in to the compressor, only 1 dB will come out. Likewise, a ratio of 8:1 means for every 8dB that comes in to the compressor, only 1dB will come out.
Side note: Ratios greater than 10:1 are generally considered limiting.
You determine the level the compressor begins working by setting the threshold level. Sounds above the threshold will be compressed. Sounds below the threshold won’t be. The higher a sound is above the threshold, the more its volume is reduced, hence the term gain ratio.
In addition to controlling how much the compressor compresses the signal (through gain ratio) and controlling the level the compressor begins working (through the threshold), you can control how quickly the compressor acts on a sound and when it lets go through the attack and release parameters respectively. Set a fast attack to get the compressor reduce the level quickly. Set a slow attack to get the compressor to wait to begin compression. Likewise, set a fast release to get the compressor to stop squishing the sound as soon as it returns below the threshold. Set a slow release to let the compressor gradually let go of the sound.
Fast attack and fast release is good for lowering brief sections of loudness, sometimes called transients, without affecting the rest of the sound.
Fast attack and slow release is good for increasing the volume consistency of a sound.
Slow attack and slow release is good for reducing the tail end of a sound.
Slow attack and fast release is stupid, just playing. I just haven’t found a purpose for it. Maybe you will.
Make-up gain is the last, but important parameter we’ll go over. It’s really simple. Make-up gain is how much you boost the compressor’s output signal. If you make the loud parts softer and turn up the resulting signal using make-up gain, you essentially turned the soft parts up.
In summary, compressors control dynamics by turning down loud parts (defined by the threshold) by a set ratio (called the gain ratio). Like we said at the beginning, compressors are like a smart fader.
If you learned something new, please leave a note in the comments. If something was missed, confusing, correct, or misleading, please leave a note in the comments.
One more thing: For a different introductory take on dynamics processing, including compressors, check out the Mysteries of Dynamics Processing Revealed post over at The Pro Audio Files