Compressors are often totally misunderstood and improperly used by most producers, especially those just starting out. Let’s get into this week going over what they actually do, and then how to best use them. Next week, once we know how they work, we can better understand how to break the rules a little bit.
A compressor makes a sound quieter. That’s all. There is a common misconception that compressors are used to increase volume, and I think this can be largely attributed to the default settings of most VST compressors and a lot of bad information in the online community. Compressors are, however, used in a chain of effects to allow a sound to be perceived with higher volume. Often times, this chain is built into one effect, using a Makeup Gain effect at the end.
Compressors make a sound quieter by “compressing” a signal when it comes in too loud. Originally this was to prevent overloading analogue circuits with an overpowered signal, using an electronic circuit to replace an engineer tasked with dropping a volume slider when a signal got too “hot.” With an analogue circuit or a VST model, “compression” is set as a ratio, from 1:1 through to ∞:1. The first number is the input, or how high the signal goes over a specified threshold, and the second is always 1, as a simplified fraction. 1:1 is no compression, 2:1 reduces the output to 1dB over the threshold for every 2dB the signal is over the threshold. Therefore a 10:1 compression ratio is 1dB over the threshold for each 10dB that the original signal is over by. This decreases the dynamic range, by quieting audio peaks and allowing quieter portions to pass through unaffected. Because of the way our ears perceive audio, and how headroom is processed in our modern digital audio systems, by decreasing the dynamic range of a sound with a compressor, we can actually make it seem louder afterwards. This is another place where confusion sets in.
Before we go over makeup gain, I will briefly cover the two other primary parameters: Attack and Release. These work very differently from the synthesiser envelopes you may be used to. In a compressor, the gain reduction takes place and is released almost instantaneously, from the moment the signal is over the threshold, to the moment it drops back down. Attack and release times tell the compressor how long to wait after the signal goes over and drops back under. So, a 10ms attack does not mean the compressor slowly increases gain reduction over the 10ms. Rather, it will wait for 10ms after the signal goes over the threshold volume, and then immediately reduce the output gain. The same is with the release time.
Makeup gain is the last phase and is the phase that can make the sound louder. Because sounds are made of waves, they have amplitudinal peaks. In a digital environment, clipping happens as soon as a signal exceeds 0dB, and in this instance, clipping is very destructive. Clipping is also a primary tool for distortion, but that is done in a more controlled way. With compression, we can decrease the volume of peaks, thus allowing us to increase the volume of the sound more before the risk of clipping. Then, we can increase the RMS volume while allowing the peaks to stay constant, or even fall from the original reference. RMS is more integral to how humans hear, so compressors will remain to be an incredibly valuable mixing and mastering tool.
Now that we all have a better understanding of compression as a technical tool, come back next week for some ways of using it as a more creative tool.